WO1995020770A1

WO1995020770A1 - Method and apparatus for seismic exploration

Info

Publication number: WO1995020770A1
Application number: PCT/GB1995/000154
Authority: WO
Inventors: Steven Holmes
Original assignee: Solution Management Limited
Priority date: 1994-01-26
Filing date: 1995-01-25
Publication date: 1995-08-03
Also published as: GB9615558D0; GB9401424D0; AU1462895A; GB2300709A

Abstract

An acoustic and vibrational noise shielding device for use in association with a geophone (1) in a method of seismic exploration comprises a cover element (7) adapted to substantially surround the geophone when the geophone is located in an operating position and means (10) for retaining the geophone when the geophone is removed from the operating position, the arrangement being such that when the geophone is located in the operating position, the geophone is substantially isolated from the shield device.

Description

Method and apparatus for seismic exploration

This invention relates to a method and apparatus for seismic exploration, and to the use of geophones and geophonic assemblies generally.

Examples of seismic exploration methods using geophones and geophonic assemblies are disclosed in US 4,497,045, CA 967,678, GB 1,599, 146, Re 31,559 of US 4,146,872, US 3,923,121, US 4,078,223, US 3,987,405, US 3,921,755 and US 3,934,218, relating to both surface planted geophones and towed geophone cables, henceforth referred to as conventional methods.

There is disclosed in US 4,838,379 a geophone implanting and positioning apparatus in which a receptacle for receiving and retaining a geophone assembly is implanted by means of spikes and provision is made for controlling the orientation and level of the implanted assembly.

A particular problem that arises in relation to seismic exploration concerns the signal- to-noise ratio ("SNR") in respect of the signal generated by the one or more geophone assemblies in relation to the inevitable noise signal likewise generated by those assemblies due to a variety of factors including but not exclusively phenomena such as wind, rainfall, hail, snow, dust, air, air motion and like factors causing the generation of "noise" signals. A more complete disclosure of the factors relating to the SNR is given below.

I have identified shortcomings in relation to prior proposals such as the disclosure in the aforementioned prior patent specifications and like conventional systems for the use of geophones in relation to the signal-to-noise ratio or SNR. In short, I have discovered that the conventional mode of using geophones as disclosed in the aforementioned prior patent specifications and in like conventional techniques inherently gives rise to the production of a level of noise which is susceptible to significant reduction, and an object of the present invention is to provide a method and apparatus offering improvements in one or more of these respects, or generally. According to the present invention there is provided a shield device for use in association with a geophone in a method of seismic exploration, the shield device comprising a cover element adapted to substantially surround the geophone when the geophone is located in an operating position and means for retaining the geophone within the shield device when the geophone is removed from the operating position, the arrangement being such that when the geophone is located in the operating position, the geophone is isolated from the shield device. The geophone and the shield are isolated from one another because there is substantially no mechanical connection between them when the geophone is located in the operating position. The shield device protects the geophone from undesired sources of noise, thereby improving the SNR.

The term "geophone" as used herein is intended to cover all devices, such as movement and vibration transducers, that are used to detect signals transmitted through the ground in methods of seismic exploration.

Advantageously, the cover element is open on one side and is so adapted that, when the shield device is located over die geophone in the operating position with the open side towards the ground, the sides and top of the geophone are enclosed by the cover element. The shield device thus protects the geophone against direct sources of noise, as defined below.

According to the present invention there is further provided a shield device adapted for use in association with a geophone and a signal transmitting device, the shield device including means for isolating the signal transmitting device from the geophone, thereby protecting the geophone against indirect sources of noise as defined below. The signal transmitting device may, for example, be an electrical cable or an optical fibre cable.

The shield device may include means for connecting the signal transmitting device to the geophone, said connecting means being adapted to prevent noise signals being transmitted from the signal transmitting device to the geophone. The shield device may include a cover element and means for engaging the signal transmitting device, as described in the preceding paragraphs.

Advantageously, means are provided for securing the shield device to the ground.

Advantageously, the retaining means comprises a releasable clamp that is operable to engage the geophone. When the clamp is in the released condition, the shield device is mechanically isolated from the geophone.

Advantageously, the retaining means comprises a surface that is adapted to engage a mating surface of the geophone and to disengage the mating surface when the geophone is located in the operating position, thereby isolating the shield device from the geophone.

Advantageously, the retaining means comprises at least one flexible element for connecting the geophone to the shield device. The flexible element may comprise a chain, a rope or cable or a flexible membrane, and prevents noise being transmitted to the geophone when it is located in the operating position.

The shield device may include means for urging the geophone into engagement with the ground. Advantageously, die cover element includes a resilient portion, said cover portion being deformable to allow the geophone to be urged into engagement with the ground. Alternatively, the clamp means may be moveable relative to the cover element to allow the geophone to be urged into engagement with the ground.

The shield device may include means for providing a non-structural connection between the shield device and the geophone casing. The non-structural connection may take the form of a diaphragm.

According to the present invention there is further provided a shield device for use in association with a longitudinal array of geophones in a method of seismic exploration, the shield device comprising a cover element that is adapted to substantially surround the longitudinal array of geophones, and to be substantially isolated from the array, when the array is located in the operating position. The shield device may be in the form of a tunnel that substantially encloses the array.

The shield device is advantageously adapted to be moveable witii the array of geophones to the operating position. Such an arrangement is suitable for use with a towed array, and may be towed with the array to the operating position.

Advantageously, die shield device is manufactured in situ at the operating position. Such a structure may be immoveable, and may be manufactured as die array is towed to die operating position.

The shield may be manufactured from local materials in the vicinity of the operating position, thereby avoiding the need to transport the construction materials to the site.

According to die present invention there is further provided a metiiod of seismic exploration, the method comprising locating at least one geophone in an operating position and positioning a shield device so as to substantially surround die geophone, the geophone being substantially isolated from die shield device when die geophone is located in die operating position.

Advantageously, the shield device includes means for retaining the geophone when the geophone is removed from die operating position, and die method includes die step of placing die shield device on die ground to locate die geophone in the operating position.

According to the present invention tiiere is further provided a metiiod of seismic exploration, the method comprising locating at least one geophone in an operating position, connecting a signal transmitting device to die geophone so as to transmit signals generated in operation by die geophone, and isolating the geophone substantially from the signal transmitting device. The geophone may be isolated from die signal transmitting device by mechanically engaging die signal transmitting device witii the ground.

Advantageously, die shield device includes means for urging the geophone into engagement witii the ground, and die metiiod includes the step of operating said urging means to locate die geophone in the operating position.

Advantageously, the shield device includes means for securing the shield device to the ground, and die method includes die step of securing the shield device to the ground in die operating position.

The method may include die steps of locating an array of geophones in the operating position and positioning shield devices to substantially surround each geophone.

According to me present invention there is further provided a method of seismic exploration, the method comprising locating a longitudinal array of geophones in an operating position and positioning a shield device so as to substantially surround die array, die array being substantially isolated from the shield device when die array is located in me operating position.

The method may include the step of moving the shield device to die operating position with the array.

Advantageously, die metiiod comprises manufacturing the shield device in situ at die operating position.

Advantageously, the shield device is manufactured in die form of a tunnel.

The shield device may be manufactured as the array is moved to the operating position.

The method advantageously comprises preparing the surface of the ground prior to placing the array in the operating position.

The shield device is advantageously manufactured from local materials in e vicinity of the operating position.

The invention relates to a method and apparatus employing geophones or like sensory equipment and is applicable to seismic exploration and like techniques. In accordance witii the invention, means is provided to shield and/or isolate die geophone apparatus, whereby to eliminate or reduce the effect of the direct and/or indirect routes for the generation of noise by the geophone apparatus. Direct and indirect routes are explained below.

In an embodiment, shield means is provided for a geophone. The shield means acts as a cover to intercept potentially impinging materials such as rain, hail, snow, dust or otiier airborne matter, and likewise to shield the geophone from the direct impact of significant air currents. In this way, the direct generation of noise by wind or airborne matter is greatly reduced. Moreover, die geophone is arranged to be mechanically isolated from die shield whereby vibrations and otiier transmissible phenomena are thereby rendered incapable or less capable of causing the direct generation of noise in the geophone assembly.

In the embodiment, it is arranged that there is little or no structural connection between the shield and die geophone when the latter is in its in-use and operating condition. It may be provided tiiat prior to installation in its working condition, die geophone is indeed structurally connected to its shield means, for convenience of assembly or storage or transport or otherwise.

In accordance witii a second aspect of the invention there is provided a metiiod and apparatus whereby, for use in relation to a geophone assembly as discussed above, means is provided for shielding and/or isolating the cable, conductor or other signal transmission means used to connect die geophone to signal sensing and/or recording apparatus and/or adjacent geophones. This aspect of the invention is based upon my discovery that a significant or appreciable proportion of the noise generated in use by geophone assemblies can be directly attributed to noise resulting from the presence of a cable or like conduit connected to die geophone for die purpose aforesaid, and which itself serves to transmit to the geophone in a mechanical fashion the vibrations or related phenomena resulting from the air movements or other airborne factors or phenomena, as described above. Accordingly, it can now be seen that the selfsame cable assembly which serves to transmit away from the geophone assembly the signals generated also serves to transmit to the geophone assembly vibrations and die like, which give rise to the generation of unnecessary and undesirable noise that is thereby mixed witii the desired seismic data.

Thus, in accordance witii this aspect of die present invention, the provision of shielding and/or isolating means in relation to the cable or the like also, does tiiereby serve to reduce or eliminate this further source of noise, whereby the SNR is improved.

For me avoidance of doubt, I make clear that I accept of course that prior proposals in relation to the use of geophones and tiieir associated cables have not necessarily excluded situations in which the geophone and/or its connected cable are covered (e.g. as in US 4,838,379) or in which the cable itself may be covered by material such as earth or sand during use. It is to be understood that such prior proposals are not of direct relevance to the present invention since such an arrangement does not serve to isolate in any significant way the geophone from the effect of the noise-generating factors and phenomena discussed above. In the case of coverings used for positioning the geophones (e.g. as in US 4,838,379) there is no isolation offered during die geophone in-use and operating condition. In the case where a cable or a geophone is simply covered or buried, die covering or burying matter or material itself is well capable of transmitting noise to the geophone, and tiierefore the SNR is not significantly improved. Furthermore, the covering or burying matter or material does not provide efficient isolation of the cable or like because of the nature of the structural characteristics of the material, which is generally unconsolidated and δ compliant. In some conditions, the airborne phenomena as described above can dislodge die covering or burying matter or material, uncovering the geophone, cable or the like.

The present invention provides an improvement in the signal to noise ratio ("SNR") of data measured in seismology and seismic exploration methods utilising any number of motion sensors placed on or substantially on the earth's surface, and more particularly to the method of seismic exploration using motion sensors, for example geophones, on surfaces including snow, ice, sand, mud, rock, soil, clay, gravel, vegetation, underwater surfaces, and any otiier forms of earth covering where access permits geophones to be utilised ("die surface").

Without loss of generality the case of SNR enhancement of seismic exploration data measured witii geophones will be described. It is already known to provide a spread of geophones ("die spread") consisting of a plurality of individual geophone groups, each group containing a number of individual geophone assemblies, each assembly consisting of one or more individual geophone sensing elements within a casing. Single geophone sensing elements within a casing are most common, although by way of example only, three individual geophone sensing elements within a casing can be used witii the sensing axes of the individual elements in some predefined relative orientation, e.g. mutually orthogonal. The individual geophone sensing elements housed within a casing may for example be oriented to have tiieir maximum sensitivity substantially vertical, horizontal or other position defined by die application. Henceforth 'geophone' will be taken to mean the assembly of one or more geophone sensing elements in a casing. The number of geophones in a group may be by way of example 12. The grouping of geophones provides a mechanism to attenuate certain components of noise which is dependent on die spatial correlation of the noise components over the geophone spread (page 157, Section 11.2.2, Field Geophysics, Milson J, ISBN 0-471-93248-5, John Wiley & Son; page 148-153, Basic Exploration Geophysics, Robinson ES, Caruh C, TN 269.R54.1988, John Wiley & Son; page 53- 55, Section 4.5.2, An Introduction to Geophysical Exploration, 2nd Edition, Kearey P, Brook M, ISBN 0-632-0292-1, BlackweU Scientific Publications; page 14-17, Handbook of Exploration Geophysics, Chapel PA, ISBN 90-5410-206-3, A A Balkema/Rotterdam). In effect the noise components at the individual geophones are summed witiiin the group, and depending on the spatial correlation over the group, cancellation or augmentation will occur. The spread is arranged in an areal pattern, which may for example be substantially a straight line, and each geophone having a data communication connection to processing and recording equipment. The areal pattern of the spread may typically have its major dimension up to several kilometres with thousands of geophones either manually planted to die surface ("die planted spread") or attached to, or integral within, a carrier structure ("the towed spread"). The entire survey site is incrementally covered by repositioning the spread, which is achieved manually with a planted spread or by towing the carrier structure across the land surface for a towed spread. The spreads are used in gathering data, which when interpreted yields information on the subsurface geological formations and structure, such as in seismic surveys of potential hydrocarbon and mineral deposits, including oil and gas or any other valuable commodity deposit. In seismic exploration, each group produces a record of geophone group voltage output versus time. The spread is used to detect motions of the earth's surface caused by seismic waves reflected and refracted from subsurface geological features, where such waves originate from a controlled remote source, which may for example be a controlled variable frequency vibrator such as Vibroseis (RTM), explosive, dropped weight, hammer blow or some other energy input into the earth. The output from the whole spread in a particular location is a series of group output voltages versus time, which when displayed with time on one axis, typically vertical, and position along the spread on d e other axis, typically horizontal, is known as a "seismic section". Since the time axis relates to the travel time from the controlled remote source to the geophone group, it can be equated to depth from the earth's surface, and since die output voltage corresponds to reflections and refractions from the geological formations, then the seismic section is actually an image of the underlying geological formation, identifying the location of such geological formations. The controlled source may be an appendage to a vehicle, for instance as used in die Vibroseis (RTM) system. Sources such as explosive cause a particular airborne phenomena, the shock wave, sometimes referred to as air blast. In the case of seismology the source is that emanating from natural activity witiiin the earth which is not controlled, and in this case the required data relates to die activity itself.

It is one disadvantage of die spread of geophones that they are extremely sensitive to airborne phenomena, which add a spurious signal to the detected received signal from the seismic wave. The phenomena include, but not exclusively; wind, rainfall, sleet, snowfall, sand storm, dust storm and any otiier motion of the air, airborne suspended particle, insect or vegetation in the vicinity which may impinge on the geophone spread. In the case of geophones used on surfaces with a covering of water or other liquid, d e phenomena may include, but not exclusively, hydrodynamic flow and tidal effects, fluid borne suspended particle or vegetation in the vicinity, which may impinge on the geophone spread. The spurious signal is in the form of unwanted added "noise", and therefore the SNR of the measurement process is reduced. Reduced SNR is severely detrimental to the interpretation of the seismic data in that it becomes increasingly difficult, in the presence of noise, to identify the true signal which when interpreted reveals die geological information. It is standard practice to have an independent Quality Assurance Representative at the site of the seismic survey, who decides when die SNR is sufficiently high, corresponding to a low noise condition, so as to allow the controlled remote source to be activated and data acquired. This limit on acceptable noise equates by way of example only to a wind speed of around 15 mph in certain environments. Any level of airborne or liquid borne noise (henceforth "fluid borne noise" refers to either airborne of liquid borne noise) causes a degradation in data quality by reducing SNR. It is one trend of oil and gas exploration to undertake seismic surveys in hostile terrains and weather conditions, for example Arctic conditions, where wind speed may be classified as high, perhaps 60% of the time and too high to take data perhaps 30% of the time. Since the seismic survey "time window" can be seasonal in die hostile conditions, the fluid borne noise problem can drastically reduce the area seismically surveyed in a particular allotted time, and tiierefore has a direct impact on the hydrocarbon or mineral production effort.

By way of example, geophones which have a measurement axis substantially parallel to the surface are particularly prone to the reduced SNR in that axis due to fluid borne phenomena propagating substantially parallel to the surface thereby causing unwanted noise levels.

By way of further example, seismic surveys which utilise Vibroseis (RTM) or other energy sources which have a relatively long signal duration and low energy per unit time characteristics are particularly prone to fluid borne phenomena reducing the SNR. This arises due to die low energy per unit time characteristics producing low measured signal levels, prior to signal processing, which by implication reduces SNR.

By way of further example, geophone spreads which are substantially perpendicular to die propagation direction of the fluid borne phenomena in the plane containing the surface to which the spread is in contact, are particularly prone to reduced SNR due to fluid borne phenomena. In this case fluid borne phenomena are perpendicular to die spread and minimum attenuation of noise caused by tiiose phenomena occurs because noise components are spatially correlated across die spread, and die individual noise signals are consequently additive, and the group noise may often be increased.

By way of further example, seismic surveys in regions where a low proportion of the source energy is returned ("the return") to the spread due to geological conditions, are susceptible to low SNR because of the reduced signal.

It is a feature of geophone spreads which comprise of geophone groups with some of the geophone sensing elements substantially horizontal that tiiey are particularly sensitive to horizontally propagating phenomena, for instance but not exclusively wind. Furthermore, it is a feature of geophone spreads which are subject to fluid borne phenomena, for instance wind, tiiat they are particularly sensitive to those phenomena which propagate in a direction substantially at right angles to die spread.

Heretofore none of die previously known land seismic exploration methods using spreads have overcome die fluid borne noise limitation and are tiierefore inefficient in terms of the rate at which seismic surveys can be carried out, and die quality of the seismic data defined by SNR.

Accordingly it is a primary objective of this invention to provide a method by which the spread is made substantially insensitive to fluid borne noise phenomena as previously described, to a far greater extent than currently possible. For example, 20 to 30dB improvement in SNR may be achieved in some conditions.

It is another objective of this invention to filter the noise at the geophone spread rather than in the recorded data, tiiereby greatly reducing the need to identify the noise sources in the measured data for subsequent processing.

It is a further objective of the invention by filtering noise at the geophone spread, to reduce and simplify die required level of processing of the measured data, which is normally a highly skilled, time consuming method which is prone to operator judgement and requiring sophisticated processing and recording equipment with high dynamic range to identify weak signals in high background noise levels.

It is another objective of this invention to facilitate acceptable SNR for a greater proportion of the on site time in hostile conditions, and consequently increase the rate at which seismic surveys can be carried out.

It is anotiier objective of this invention to reduce die on site time necessary for a given survey in hostile conditions.

It is anotiier objective of this invention to achieve die said benefits in SNR enhancement and increased rate of undertaking seismic surveys witiiout increasing the manpower and die associated logistic support costs. Accordingly the mass and volume of die shield plus geophone is as far as possible equal to or less than that of a conventional geophone assembly.

It is another objective of this invention to increase the deptii to which SNR is sufficiently high in seismic sections. It is anotiier objective of this invention to offer the option of reducing the number of geophones in a group, whilst maintaining acceptable levels of SNR.

It is another objective of this invention to have application to all future and current geophone spreads, including but not exclusively tiiose spreads which are already deployed in seismic surveys, those which are being transported to survey sites and tiiose which are currently being manufactured or designed.

It is a further objective of this invention to be applicable to single and multiple sensing elements housed within a single casing.

It is a further objective of this invention to be in either a removable or permanent attachment embodiment.

According to an embodiment of the invention, each geophone in the spread is made substantially insensitive to the aforementioned noise sources arising from fluid borne phenomena, by isolating the geophone from those phenomena, thereby increasing SNR. The noise effect in the measured data arising from e aforementioned sources is attenuated due to the isolation, which is achieved by substantially eliminating direct and indirect routes for the aforementioned fluid borne noise phenomena to arrive at the geophone ("the direct route" and "die indirect route"). The direct noise route includes impingement on the geophone or geophone casing structure by any of the aforementioned fluid borne phenomena. The indirect route includes impingement by any of die aforementioned fluid borne phenomena on any item, such as the cable, which is connected to the geophone or geophone casing structure, which by virtue of connection to die geophone or geophone casing structure provides a potential route. The indirect route may include by way of example only impingement on die ground in die geophone vicinity, to which by necessity the geophone must be connected.

According to an embodiment of die invention, the isolation is passive in that the noise phenomena are substantially filtered out prior to arrival at the geophone; a process which requires no prior knowledge of die characteristics of the noise in order to function, and reduces die level of identification and processing required for tiiose noise components in the measured data.

According to an embodiment of d e invention, seismic survey times are reduced by facilitating acceptable SNR for a greater proportion of the time, thereby reducing on site time.

According to an embodiment of the invention, it is fully compatible with current practices and utilisation of the invention causes no increase in manpower or logistic support costs. Furthermore, since seismic survey times are reduced, commensurate reductions in logistic support costs are expected.

According to an embodiment of die invention, it is applicable to single and multiple sensing elements housed within a single casing.

According to an embodiment of die invention, the deptii to which acceptable SNR is achieved in seismic sections is increased. The signal portion of geophone group output voltage diminishes witii time in any particular record, due to mechanisms which dimmish the source energy witii deptii, for instance but not exclusively reflection or refraction, and consequently the SNR decreases witii time and tiierefore deptii. In normal circumstances, fluid borne noise effects are essentially constant and independent of deptii on the seismic section. By reducing die fluid borne noise, die SNR is generally increased, and will be higher than the minimum acceptable level to a greater deptii.

According to an embodiment of d e invention, current levels of acceptable SNR are achievable with reduced number of geophones per group.

According to an embodiment of the invention, a means is provided to prevent ingress of surface material inside die shield, for example gravel, sand, soil, dust and snow.

According to an embodiment of die invention, it is applicable to any geophone or motion sensor spread, whetiier current, planned or future, and available either as a removable or permanent attachment embodiment, and attached eitiier at the survey site or at any facility or site prior to the survey.

Embodiments of die invention as applied to planted spreads are now described.

In the preferred embodiment for a planted spread of geophones, die individual geophone is totally shielded from any fluid borne direct impingement route by an auxiliary structure, the shield, which has minimal direct structural connection, ideally none, witii the geophone or geophone casing, and has such sfructural properties, principally but not solely stiffness, mass and structural damping so as not to transfer any of the direct route fluid borne noise from die shield to the geophone. Furthermore the shield is connected to, and forms a seal with, the surface preventing ingress inside die shield of die fluid borne direct impingement.

Preferably, die structural properties of the shield are such that its natural frequencies are outside die frequency range of die fluid borne noise travelling along the direct and indirect routes, which ensures there is no dynamic amplification of noise arising from botii direct impingement on the shield nor of indirect preferential coupling to the shield. The performance of the shield in this respect may be enhanced by incorporating optional damping treatments and vibration isolators. Furthermore, die said natural frequencies are preferably outside die frequency range of the measured data.

In addition the shield preferably has structural connections with any item, such as the cable, tiiat forms part of the indirect route and has a proportion of its volume outside die space envelope of the shield. The structural properties of the shield are such that die indirect route fluid borne noise preferentially couples through the external sections of those items, for example the cable section outside die shield, to the shield itself rather than the section of those items internal to the shield. The preferential coupling to the shield effectively isolates the internal from the external sections of those items, and tiiereby substantially eliminates the indirect route along those items. In addition the shield preferably eliminates the indirect route whereby fluid borne phenomena impinge on the ground immediately adjacent to die geophone by virtue of the shield dimensions.

Preferably the shield is on all direct and indirect fluid borne noise routes to die geophone and performs as a noise filter ("d e shield filter effect"), substantially attenuating noise phenomena travelling along these routes prior to them producing a noise signal at die geophone.

Preferably in the embodiment for the permanent attachment, any item which forms part of the indirect route, and has sections both internal and external to die shield, has die sections of those items internal to the shield specifically configured to afford die greatest degree of isolation whilst still remaining functional. For example, the cable section internal to the shield has a reduced cross sectional area whilst maintaining electrical connection through the cable to the geophone. Alternatively, the items internal to the shield may remain unchanged for other considerations, including spares availability and manufacturing cost.

Preferably in the permanent attachment embodiment, a structural connection exists between the shield and die geophone casing in all circumstances, including but not exclusively during handling, planting, removal, replanting, packaging, transportation, except when the spread has been planted and is being used or is to be used witiiout further handling for measuring data. The condition dependent structural connection

("the connection") allows the shield and the geophone to be handled as a single item, whilst ensuring no structural connection between the shield and die geophone during data measurement. The connection may be realised, for instance but not exclusively, by a lockable clamp which releases on completion of die shield and geophone planting process, and reconnects on commencing any repositioning, lifting or removal of the geophone and shield. This ensures that no additional time or manpower is required to undertake die complete process of a seismic survey over a prescribed area, requiring the spread including die geophones and shields to be repositioned and replanted, and also ensures that the method using the invention is compatible with current practices. Alternatively the connection may be realised by mating surfaces on the geophone casing and die inside of d e shield, which are in contact at all times other than when the spread including the shields has been planted and is being used or is to be used without further handling for measuring data. Optionally, the lockable clamp and the mating surface connections may be augmented by otiier devices, by way of example only, springs.

Furthermore the connection ensures the relative positions of die shield and die geophone are always consistently maintained not only during occasions of handling but also during die process of measuring data when the connection is in the released state. By way of example only, the geophone and shield during data recording should not be in contact and their vertical axes should preferably be consistently and repeatably coaxial.

Preferably a non-structural connection exists between the shield and the geophone casing, preventing ingress of surface materials such as sand, dust, snow, gravel. By way of example only, the non-structural connection may take the form of a diaphragm.

Preferably in the removable or detachable attachment embodiment, die items internal to the shield as previously described are not modified and remain unchanged. In addition die connection as previously described is utilised in this embodiment.

In an alternative embodiment of the removable attachment, the shields may be totally separate from the spread, and positioned relative to the planted geophones at die survey site. In this embodiment the geophone spread and the shields are planted separately, but in such a way as to ensure the direct and indirect routes of fluid borne noise are addressed, requiring that the relative positions of the geophones and shields in die planted condition during data measurement are as previously described for die ot er embodiments .

In a preferred embodiment of die removable attachment, die shields may be manufactured remotely from the survey site. In an alternative embodiment of the removable attachment, the shields may be produced at die survey site from materials either carried to die survey site or from materials found at die survey site, by way of example only sand, earth, snow, ice, mud, vegetation or combinations thereof.

In an alternative embodiment, only die direct route is addressed.

In an alternative embodiment, only die indirect route is addressed.

Embodiments of die invention applied to towed spreads are now described.

In die preferred embodiment applied to towed spreads comprising all geophones and cables housed witiiin or on a carrier structure as described for example in Patent Nos. GB 1599146, US 3923121, US 4078223, US 3987405, US 3921755, US 3934218, die shield takes the form of a continuous tunnel ("the tunnel shield") enclosing the whole spread. The tunnel shield has no direct physical connection to the towed spread. The tunnel shield is connected to, and forms a seal with, the surface.

The tunnel shield may be of substantially the same length as the towed spread, and die spread longitudinal and the tunnel shield axis coaxial. The relative positions of the tunnel shield and die spread axes are maintained, by way of example only, by means of a surface preparation device or tool attached to die towing vehicle. The towing vehicle repositions the spread and die tunnel shield by simultaneously towing them to the new position. The surface preparation may be by way of example only, a ploughed or gouged trough in the surface locating the tunnel shield and the spread in a fixed relative position along the whole length of the spread. The tunnel shield is made from a material which ensures the necessary articulation required for botii transporting to the site and functional use on the site.

Preferably the structural properties of the tunnel shield are such that fluid borne noise phenomena are not transferred from die tunnel shield to die spread, nor does any dynamic amplification of noise arising from fluid borne phenomena occur, as described for the planted spread embodiments. Optional damping treatments and dynamic absorbers may be incorporated.

In an alternative embodiment, the tunnel shield may be a continuous stationary structure made from naturally occurring materials found on or in the vicinity of the survey site, or transported to the site. The forming process utilises an attachment to the towing vehicle which ingests the materials and forms the tunnel shield around the spread as it tows the spread. The tunnel shield is not moved, but new sections are formed as die spread is towed by die towing vehicle. The spread is effectively towed dirough the tunnel shield, and as previously described die axes of die spread and die tunnel shield are coaxial. The length of the tunnel shield at any one time is approximately equal to the distance travelled by the towing vehicle during die survey. In this embodiment the tunnel shield is biodegradable, being made from materials such as snow, ice, earth, vegetation plus optional organic binders.

The tunnel shield eliminates the route by which fluid borne noise impinges on the ground immediately adjacent to the spread by virtue of the tunnel shield dimensions.

Preferably the tunnel shield is on all fluid borne noise routes to the geophone spread, and performs as a noise filter ("the tunnel shield filter effect"), substantially attenuating noise phenomena travelling along these routes prior to them producing a noise signal at the geophones.

Preferably in embodiments utilising towed spreads, die position of the whole of the towed spread is known by utilising GPS on the towing vehicle to measure the precise route of the towing vehicle within the tolerance of the GPS system. The location of the spread is known since it follows the route of the towing vehicle, by way of example only, due to the geometric constraint of the tunnel shield and/or die surface preparation.

In an alternative embodiment, the individual geophones of die towed spread use individual shields as previously described for die embodiment for planted spreads utilising the removable shield attachments which are totally separate from the spread. In this embodiment for the towed spread, die spread is positioned by towing and surface preparation, and then individual shields are positioned over die geophones, ensuring no direct contact between geophone locations in the spread and the shield.

In an alternative application of the tunnel shield, the tunnel shield embodiments as previously described are used to shield a planted spread of geophones, in which the direct and indirect routes are addressed.

Reference is now directed to die accompanying and following pages of text and drawings disclosing additional material witii respect to the background and general factors concerning the present invention, including specific embodiments thereof. It is to be understood that in the present application, or any application claiming priority herefrom, the broad or broadest aspects of the invention are not to be understood to be limited by any detailed disclosure in the accompanying description, but individual features therefrom may be taken individually tiierefrom for use in combination with the concepts disclosed above in relation to the claiming of the broader aspects of the present invention.

Embodiments of the invention will now be described, by way of example only, witii reference to the accompanying drawings, of which:

Figure 1 is a perspective view of a typical geophone assembly;

Figure 2 shows a portion of a conventional geophone group;

Figures 3 and 4 are sections through a shield according to die present invention, showing a geophone in raised and planted positions respectively;

Figure 5 shows a portion of a geophone group with shields in position;

Figures 6 to 10 are sections through a shield according to a second embodiment of the present invention, showing a geophone in various positions;

Figure 11 shows a modified form of a part the second embodiment of die shield;

Figures 12 to 14 are sections through shield according to a third, fourth and fifth embodiments of die present invention, respectively;

Figure 15 is a sections through a sixth embodiment of die present invention;

Figure 16 is a perspective view of a conventional towed geophone spread;

Figure 17 is a side view of a conventional towed geophone configuration;

Figure 18 is a side view of a towed geophone configuration according to an embodiment of die present invention, and

Figure 19 is a cross-section on line a-a of figure 18, showing a section through a tunnel shield according to die present invention.

Figure 1 shows a typical geophone assembly, which consists of a geophone casing 1 which houses the geophone sensor and a means of attaching to the ground, in this case a ground spike 2. Other methods of ground connection can be accommodated by die invention.

Figure 2 shows a section of a conventional geophone group, which itself is part of a geophone spread. The group consists of a plurality of geophone assemblies 3 as described above witii reference to Figure 1, connected to die surface 5. The ground spikes are not visible in this figure. The geophones are connected by die cable 4 which connects the geophone group to the recording and processing equipment 6.

Figure 3 shows a section through a permanent attachment embodiment of die invention, utilising a clamp. A geophone casing 1 and ground spike 2 are shown in the handling position, that is not planted, firmly attached to the shield 7 by means of the structural connection previously described. The structural connection is achieved by jaws 10 which clamp onto the geophone casing by means of a mechanism 11 which is activated by lifting the plunger 8. The jaws 10, mechanism 11 and plunger 8 are locked in position by a sprung loaded ball 14 mating with a hole in the shield 7. The geophone cable 4 is structurally connected 12 to die shield 7, which avoids damage to die non structural portion of the cable 9 which is internal to the shield 7 and provides further isolation by virtue of the reduced section. The cable is slack in this condition to avoid undue stress in die cable 9. The shield has ground coupling devices, in this case ground spikes 13. From this state, the shield and geophone and assemblage of all components shown in the Figure can be handled as a single entity.

Figure 4 shows the geophone and shield are planted to die surface by pressing the ground spikes 2,13 to die ground, until the shield and geophone are planted and the mechanism 11 releases the jaws 10 from the geophone casing. The mechanism is so designed as to facilitate a first condition in which there is a structural connection between the shield and geophone, and also a second condition in which there is no structural connection between the shield and geophone, tiiese conditions corresponding to the unplanted and planted cases respectively. Any mechanism so designed is covered by die present invention. In the planted case die internal cable 9 is slack, which minimises any noise transmitted along tiiis cable section to the geophone casing 2.

Figure 5 schematically shows a section of a geophone group, witii the shields 7 in position. Shield 7a is a part section of a shield showing the internal geophone. The shields 7 could be any of die previously described shields, whetiier permanent attachment or removable.

Figure 6 shows, in part section, an alternative embodiment of a permanent attachment embodiment of die invention, utilising the mating surfaces. A redesigned geophone casing 17 and ground spike 2 are shown in the handling position, that is not planted, connected to die stiff lower shield 15 by means of mating surfaces on the geophone casing 17 and lower shield 15. The connection is achieved by die relative dimensions on die lower shield 15 and geophone casing 17 such that in normal operation the geophone casing 17 cannot be removed from die total assembly. The cable sections 4, 9 are attached to the lower shield 15 as described in previous embodiments. The flexible upper shield 16 is structurally joined to the lower shield 15, such that the joint carries all structural loads during handling and any otiier operational activity. Furthermore, the joint is such that tiiere is no ingress route between the upper 16 and lower 15 shields. A pressure pad 18 is attached to the upper shield 16, witii the vertical axis coaxial with the geophone casing 17 vertical axis.

Figure 7 shows the assembly ready for planting, positioned adjacent to die ground 5. The shield assembly rests on the geophone casing 17, in a predefined orientation with respect to the geophone casing 17, that is substantially coaxial. The predefined orientation is fixed by die mating surfaces on the geophone casing 17 which are in contact with adjacent mating surfaces on the pressure pad 18 and die lower shield 15. The geometry of the assembly and all component parts is such that die geophone ground spike 2 extends below the lower shield ground spikes 13. In die planting action applying pressure or load substantially vertically on the upper shield causes die geophone ground spike to penetrate the earth to an initial depth. Continuing the load application causes the geophone ground spike 2 to penetrate progressively deeper, and whilst the lower shield ground spikes 13 are not in contact with the surface, the upper flexible shield 16 remains substantially undefoπned except for local deformation on the load application area. Without substantial deformation of the upper shield 16 die overall dimensions of d e assembly are preserved, and in particular the relative positions of the geophone casing 17, geophone ground spike 2, lower shield 15 lower shield ground spikes 13, upper shield 16 and pressure pad 18 are maintained. On continuing the load application, the lower shield ground spikes 13 make contact with die ground in such a position that the geophone casing 17, the lower shield 15 and upper shield 16 have coaxial vertical axes, and on further load application they penetrate the ground maintaining this relative position of the upper and lower shields 15, 16 and die geophone casing 17. Figure 8 shows the lower shield 15 planted, which occurs prior to the geophone by virtue of the tailored structural deformation characteristics of the upper shield 16 and the pressure pad 18, and the relative dimensions of the geophone ground spike 2 and the lower shield ground spikes 13. By way of example only, substantial deformation of the upper shield 16 might only occur once die lower shield 15 has been fully planted, and continuing the load application in this example would be undertaken to further penetrate the geophone spike 2. At this point the coaxial position of the geophone casing 17, the upper shield 16 and die lower shield 15 vertical axes is maintained, although the mating surfaces on the geophone casing 17 and the lower shield 15 have separated.

Figure 9 shows the case where further application of load has fully planted die geophone ground spike 2.

Figure 10 shows the planting load has been released, allowing die flexible upper shield 16 to recover its undeformed shape. In the planted case tiiere is no structural contact between the geophone casing 17 and die upper 16 or lower shield 15.

Lifting or unplanting the assembly requires a lifting force on the upper 16 or lower shield 15, which may be achieved directiy or through an auxiliary attachment or through the external structural cable 4. The lower shield ground spikes 13 , retract from the ground first whilst the geophone ground spike 2 remains planted until the mating surfaces of the lower shield 15 and the geophone casing 17 contact. At this point further lifting force moves the whole assembly as a single unit as shown in Figure 6.

In an alternative embodiment (not shown in the drawings), a non-structural connection exists between the shield and d e geophone casing, preventing ingress of surface materials such as sand, dust, snow, gravel. For example, die non-structural connection may take the form of a diaphragm tiiat extends across the open side of die lower shield 15 of die embodiment shown in Figs 6 to 10 and has an aperture for the ground spike 2 of the geophone. A non-structural connection in the form of a diaphragm may similarly be added to die embodiment shown in Figs. 3 and 4.

In an alternative embodiment of a permanent attachment embodiment of the invention, utilising the mating surfaces, the upper shield 16 flexibility may achieved by a stiff upper portion 16a and a flexible or spring loaded lower portion 16b as shown in Figure 11.

Figure 12 shows an alternative embodiment which addresses only die direct route. The cable 4 is not attached to die lower shield 15, and is free to move independently of die lower shield 15, and by necessity the section of the cable 9 internal to the shield 15 has die same structural properties as the external cable 4, and is actually an identical continuation of the external cable 4. The cable 4, 9 is free to move relative to the shield 15.

Figure 13 shows an alternative embodiment in which die shield 15 offers little or no attenuation of the direct route. The shield 15 is an open structure, by way of example only an annular ring. The cable section internal to the shield 9 has a reduced cross sectional area and is a continuation of the external section of the cable 4 which is attached to the shield 15.

Figure 14 shows an alternative embodiment in which the shield offers littie or no attenuation of the direct route. The indirect route along die cable 4 is attenuated by an isolation mechanism which ensures that, by way of example only, cable excitation is not transmitted to the geophone 17. In this embodiment die relative geometry of die articulated structural connection 28, the shield 15 and the geophone 17 is such that in a planted condition the geophone 17 and die shield 15 cannot contact, and under no circumstances do tiiey contact. The nature of the articulated structural connection 28 is such that it is not stressed or otiierwise loaded in the planted condition thereby acting as an isolator. By way of example only, the articulated structural connection 28 may be a fine chain. The relative geometry of the cable internal to die shield 9 and die articulated structural connection 28 means that in any circumstances, including planting, repositioning and lifting, the cable internal to the shield 9 is not stressed or otherwise loaded.

Figure 15 shows an alternative embodiment in which the shield is dispensed with, and there is no attenuation of the direct route. Attenuation of the indirect route is achieved by two U shaped clamps 29, which anchor the cable 4 to the ground 5. In this embodiment, the cable 4 has constant cross sectional properties.

Figure 16 shows a section of a typical towed geophone spread. The geophone is housed in a casing 19 which in turn is housed witiiin, or on, a carrier structure 20. The carrier structure houses all geophone cables, and also structural members required to support the towing load.

Figure 17 shows a typical configuration. The towed geophone spread 20 is connected to a recording sled 23 by means of a structural coupling 21. The recording sled accommodates recording and processing equipment 24 connected to the towed spread by die data cable 22. The data cable 22 supports none of the towing load by virtue of slack in its design. The recording sled 23 is connected to die towing vehicle 25 by a structural coupling 21. The geophone spread 20 couples to the ground 5 by virtue of its self weight.

Figure 18 shows a preferred embodiment of die invention as applied to a towed geophone configuration, with an attachment 27 to form the tunnel shield 26. The towed geophone spread 20 is connected to the attachment 27 by a structural coupling 21, which in turn is connected to die recording sled 23 by a structural coupling 21. The data cable 22 connects from the towed geophone spread 20 to die recording equipment 24. Preferably the data cable 24 may be routed tiirough the attachment 27 or otherwise isolated from d e fluid borne phenomena. On forward motion of the towing vehicle 25 when repositioning the towed geophone spread 20, die attachment ingests surface material and forms a stationary tunnel shield 26 around die towed geophone spread 20 which passes tiirough the tunnel as it is towed. Over survey distances longer than the towed geophone spread 20, the formed tunnel 26 will be longer than the geophone spread, and tiierefore produces the desired tunnel shield filter effect. It may be desirable to drive the towing vehicle 25 over a lead in section to the survey site to ensure that the tunnel shield 26 is formed all along die length of the towed geophone spread 20 at die first data collection position. Preferably the attachment 27 may also prepare the surface 5 to allow good connection of the tunnel shield 26 to die surface 5 to eliminate fluid borne phenomena ingress, and to provide good coupling between the towed geophone spread 20 and die surface 5. The surface preparation, by way of example only, may include scraping to produce a flat surface, ploughing or gouging to provide a recess, or any otiier surface preparation with the purpose of providing isolation of the towed geophone spread 20 from the fluid borne noise phenomena, improved attachment between the shield 26 and die surface 5 and improved coupling of the towed geophone spread 20 and die surface.

Figure 19 shows a sectional view corresponding to section a-a in Figure 18. The surface 5 in this case is flat within the tunnel shield internal volume at this particular position.

In an alternative embodiment of die tunnel shield configuration, the tunnel shield 26 is pre-manufactured unit, either manufactured at die site or transported to the site, and is substantially the same length as the towed geophone spread 20 and fully encloses the towed geophone spread 20 providing isolation against the fluid borne phenomena along the full length of the towed geophone spread 20 at all times. In this case the tunnel shield 26 and the towed geophone spread 20 are towed simultaneously and their relative positions transverse to die longitudinal axis are maintained, tiiat is they are not in contact at any time, by the previously described surface preparation.

Claims

1. A shield device for use in association with a geophone in a method of seismic exploration, the shield device comprising a cover element (7) adapted to substantially surround die geophone (1) when die geophone is located in an operating position and means (10) for retaining the geophone when the geophone is removed from the operating position, the arrangement being such that when the geophone is located in die operating position, the geophone is substantially isolated from die shield device.

2. A shield device according to claim 1, in which die cover element (7) is open on one side and is so adapted that, when the shield device is located over die geophone in the operating position with die open side towards die ground, die sides and top of die geophone (1) are enclosed by die cover element.

3. A shield device for use in association with a geophone (1) and a signal transmitting device (4) in a metiiod of seismic exploration, in which the shield device includes means for isolating the signal transmitting device from die geophone.

4. A shield device according to claim 3, in which the shield device includes means (9) for connecting the signal transmitting device to die geophone, said connecting means being adapted to prevent noise signals being transmitted from die signal transmitting device to the geophone.

5. A shield device according to claim 3 or 4, when dependent on claim 1 or 2.

6. A shield device according to any one of the preceding claims, including means (13) for securing the shield device to the ground.

7. A shield device according to any one of die preceding claims, in which the retaining means comprises a clamp (10) that is operable to engage the geophone.

8. A shield device according to any one of claims 1 to 6, in which the retaining means comprises a surface (15) that is adapted to engage a mating surface of the geophone and to disengage die mating surface when the geophone is located in the operating position.

9. A shield device according to any one of claims 1 to 6, in which the retaining means comprises at least one flexible element (28) for connecting the geophone to the shield device.

10. A shield device according to any one of the preceding claims, including means (8) for urging the geophone into engagement with the ground.

11. A shield device according to claim 10, in which the cover element includes a resilient portion (16), said cover portion being deformable to allow d e geophone to be urged into engagement witii the ground.

12. A shield device according to claim 10 when dependent on claim 7, in which the clamp means (10) is moveable relative to the cover element to allow the geophone to be urged into engagement witii the ground.

13. A shield device according to any one of die preceding claims, including a non- structural connection means between the shield device and the geophone casing.

14. A shield device for use in association with a longitudinal array (20) of geophones in a method of seismic exploration, the shield device comprising a cover element (26) tiiat is adapted to substantially surround die longitudinal array of geophones, and to be substantially isolated from die array, when die array is located in die operating position.

15. A shield device according to claim 14, said shield device (26) being adapted to be moveable witii die array of geophones to the operating position.

16. A shield device according to claim 14, said shield device being manufactured in situ at the operating position.

17. A shield device according to claim 16, said shield device being manufactured from local materials in die vicinity of the operating position.

18. A method of seismic exploration, the method comprising locating at least one geophone (1) in an operating position and positioning a shield device (7) so as to substantially surround die geophone, the geophone being substantially isolated from the shield device when die geophone is located in the operating position.

19. A method according to claim 18, wherein die shield device includes means (10) for retaining the geophone when the geophone is removed from me operating position, the method including die step of placing the shield device on die ground to locate the geophone in the operating position.

20. A method according to claim 18 or claim 19, wherein die shield device includes means (8) for urging the geophone into engagement with the ground, the method including die step of operating said urging means to locate the geophone in the operating position.

21. A method according to any one of claims 18 to 20, wherein die shield device includes means (13) for securing the shield device to die ground, die method including die step of securing die shield device to die ground in die operating position.

22. A method according to any one of claims 18 to 21, including die steps of locating an plurality of geophones in operating positions and positioning shield devices (7) to substantially surround each geophone.

23. A method of seismic exploration, the method comprising locating a longitudinal array (20) of geophones in an operating position and positioning a shield device (26) so as to substantially surround the array, the array being substantially isolated from die shield device when die array is located in die operating position.

24. A method according claim 23, including the step of moving the shield device (26) to die operating position with the array.

25. A method according to claim 23 , including die step of manufacturing the shield device in situ at the operating position.

26. A method according to claim 25, wherein die shield device (26) is manufactured in die form of a tunnel.

27. A method according to claim 26, wherein die shield device (26) is manufactured by processing local materials and forming them into a tunnel around die array as die array is moved to die operating position.

28. A method according to any one of claim 23 to 27, including die step of preparing die surface of the ground prior to placing die array (20) in die operating position.

29. A method according to any one of claims 18 to 28, wherein die shield device is manufactured from local materials in die vicinity of the operating position.

30. A method of seismic exploration, the method comprising locating at least one geophone (1) in an operating position, connecting a signal transmitting device (4) to die geophone so as to transmit signals generated in operation by the geophone, and isolating the geophone substantially from die signal transmitting device.

31. A method according to claim 30 when dependent on one of claims 18 to 29, in which die geophone is isolated substantially from the signal transmitting device by die mechanical engagement of the signal transmitting device witii the ground.

32. A metiiod according to claim 30 when dependent on one of claims 18 to 29, in which die geophone is isolated substantially from the signal transmitting device by die mechanical engagement of the signal transmitting device witii the shield device.