WO2015150813A1

WO2015150813A1 - A receiver apparatus and method

Info

Publication number: WO2015150813A1
Application number: PCT/GB2015/051027
Authority: WO
Inventors: Antoni Marjan Ziolkowski; David Allan Wright
Original assignee: The University Court Of The University Of Edinburgh
Priority date: 2014-04-02
Filing date: 2015-04-01
Publication date: 2015-10-08
Also published as: GB201405977D0

Abstract

An electromagnetic receiver apparatus comprises a cable and a plurality of sensors spaced along the cable, wherein at least one of the sensors comprises a plurality of electrode pairs, each electrode pair being connected via a respective length of the cable and configured to receive a respective response signal.

Description

A Receiver Apparatus and Method

Field

The present invention relates to a receiver apparatus and method, for example to an electromagnetic receiver apparatus in the form of a cable comprising a piurality of sensors distributed along the cable and configured for towing as a streamer behind a vessel in surveying for the presence of sub-surface hydrocarbon deposits, or configured for stationary use on land, or on the sea floor.

Background

Porous sub-surface rock formations are usually saturated with fluids. The fluids may be water, gas, or oil, or a mixture of all three. Detection of formations in which oil or gas, as well as or instead of water, is present is of particular interest.

It is known that electrical resistivities of rock formations are affected by the saturating fluids that are present in pore spaces of the rock formations, as well as by other properties of the rock formations, for example the fractional volume of pore spaces with respect to the total rock volume and the spatial configuration of the pore spaces. Brine-saturated porous rocks are much less resistive than the same rocks saturated with hydrocarbons. By measuring the resistivity of geological formations, hydrocarbons can be detected.

Various techniques for measuring the resistivity of geological formations are known, for example time domain electromagnetic techniques, as described in US 6,914,433 B2, the contents of which are incorporated herein by reference. Conventionally, controlled source eiectromagnetic investigations use a transmitter and one or more receivers. The transmitter may be an electric source, that is, a grounded bipoie, or a magnetic source, that is, a current in a wire loop or multi-loop. The receivers may be grounded bipoles for measuring potential differences, or wire loops or multi-loops or magnetometers for measuring magnetic fields and/or the time derivatives of magnetic fields. The transmitted signal is often formed by a step change in current in either an electric or magnetic source, but any suitable transient signal may be used. Measurements can be made on land or in an underwater environment. In the case of measurements made in an underwater environment, it is known to perform towed controlled source electro-magnetic (CSEM) measurements in which a transmitter cable is towed behind a survey vessel and used to transmit a time-varying electromagnetic field into rock formations beneath the sea-bed by passing transient currents through the transmitter cable (see, for example, Anderson, C. and J. Mattsson (2010) An integrated approach to marine electromagnetic surveying using a towed streamer and source: First Break, Vol. 28, pp 71-75). An associated receiver cable is also towed behind the survey vessel, or behind another, associated vessel. The receiver cable usually comprises an array of in-line dipole receivers distributed along the cable, each dipole receiver comprising a pair of electrodes separated by a length of the cable. The length of each dipole receiver is around 100m in some known configurations, and each dipole receiver is configured to measure the time-varying potential difference (V(t)) between its electrodes.

As described in US 6,914,433 B2, both the signal transmitted by the transmitter and the time-varying signals received by the dipole receivers are measured and processed to obtain the response from the sub-surface rock formations to the transmitted electro-magnetic signal, from which information concerning the electrical properties of sub-seabed formations can be obtained. That information can be used, for example, to determine the presence or absence of a target hydrocarbon, or the presence or absence of sequestered carbon dioxide or other substance of interest that affects the electrical properties of the sub-seabed formations. US 6,914,433 B2 applies equally to land and marine surveys.

The signaf-to-noise ratio of CSEM data is determined by the signal strength, which depends on the strength of the electromagnetic source and the path between the source and the receiver, and by ambient electromagnetic noise. The electromagnetic noise in a receiver streamer array towed by a vessel is generally found to be larger than the noise measured by a similar stationary streamer lying on the sea floor, known as an ocean bottom cable (OBC). In one case, receiver cable noise measurements in the North Sea indicate that towed receiver cable noise may be around 20dB larger than noise measured with a stationary ocean bottom cabie on the sea floor 100m beneath the surface of the sea.

The characteristics of OBC electromagnetic noise in shallow water (for example, less than 500m deep) are known and are dominated by magnetotel!uric variations that increase in amplitude with decreasing frequency and are correlated over distances of several kilometres, in the frequency bandwidth of interest to CSEM (for example, 0.001 Hz to 1.0 Hz) this is the dominant noise in OBC data. Several mechanisms have been proposed for the increased noise observed from towed CSE measurements in comparison to OBC measurements, including motionally-induced inductive noise generated according to Faraday's law of electromagnetic induction as the receiver cable moves relative to the Earth's magnetic field. Such mechanisms would result in noise that is correlated over distances significantly greater than the length of the receiver cable used for the measurements. The Earth's magnetic field is also changing with respect to time, causing the magnetote!luric signals that are seen in both the OBC and CSEM measurements.

Summary

In a first, independent aspect of the invention there is provided a receiver apparatus, comprising a plurality of sensors spaced along the cable, wherein at least one of the sensors comprises a plurality of electrode pairs, each electrode pair being connected via a respective length of the cable and configured to receive a respective response signal. The cable may comprise an aquatic cable or a cable for use on land. The cable may be for use in controlled source electromagnetic (CSEM) measurements.

By providing a plurality of electrode pairs for each sensor, the response signals from the electrode pairs may be summed, averaged or otherwise combined to provide an increase in signal to noise ratio for the sensor. This is particularly significant in the context of towed streamer CSEM measurements, where noise is a significant factor, as it is suggested herein that at least some of the noise in such measurements may be uncorrelated at relatively short electrode separations. If the different pairs of the electrodes have a suitable separation in comparison to a noise correlation distance, then summing, averaging or otherwise combining the signals can provide a particularly significant increase in signal to noise ratio. The noise correiation distance, also referred to as a noise coherence distance, may, for example, be of the order of a few cm to a few metres and may be frequency dependent.

An increase in signal to noise ratio may make towed streamer CSEM measurements more effective or desirable in certain circumstances, and may lead to more regular or wider adoption of such measurements. For example, it may allow CSEM or multi-transient electromagnetic (MTEM) measurements to be used in a wider range of ocean depths and/or to obtain better 3D imaging of subsurface structures of interest to hydrocarbon exploration. In some cases, it may also allow CSEM or multi-transient electromagnetic (MTEM) measurements to be more regularly conducted simultaneously with seismic surveys. The aquatic cable may be a cabie suitable for performing measurements in a body of water, for example the sea or a lake or river.

Each electrode pair and the length of cable between the electrodes of the pair may be or form part of a respective dipole receiver, such that each of said at least one of the sensors comprises a plurality of dipole receivers.

By stacking signals obtained by several dipoles bundled together, the system signal to noise ratio can be significantly improved in the presence of uncorrelated noise.

Each of said at least one of the sensors may comprise an overlapping plurality of the dipole receivers.

The apparatus may further comprise, for each of said at least one of the sensors, means for combining the response signals from the electrode pairs to obtain a combined response signal for that sensor, thereby to increase a signal to noise ratio for the sensor.

The combined response signal may comprise or represents a sum of the response signals from the electrode pairs. For example, the combined signal may be a sum or average of the response signals.

For each of said at least one of the sensors, the electrode pairs for that sensor may be interconnected thereby to provide a signal at an output of the sensor that comprises or represents a sum of the response signals from that sensor. For example, for each of said at least one of the sensors, the electrode pairs for that sensor may be connected in series. Interconnection of one or more electrodes of the electrode pairs, such that there is an electrically conducting path via the interconnection(s) between the electrode pairs may provide a particularly robust and simple way of obtaining the combined signal for the sensor.

For each of said at least one of the sensors, each electrode may be offset from a corresponding electrode of at least one other of the electrode pairs of that sensor by at least an offset distance. For each of said at least one of the sensors, each electrode may be offset from each other of the electrodes by at least the offset distance. For each of said at least one of the sensors, each electrode may be offset from its adjacent electrode(s) by the offset distance. The electrodes or electrode pairs may be the electrodes or electrode pairs whose response signals are combined to obtain the combined response signals

The offset distance may be greater than a noise coherence distance. The noise coherence distance may, for example, be a minimum separation at which at least one noise component, for example a turbulence-related noise component, for a pair of electrodes is substantially uncorrelated.

The offset distance may be greater than or equal to 0.1m, optionally greater than or equal to 0.5m, optionally greater than or equal to 1m. The offset distance may be less than or equal to 10m, optionally less than or equal to 5m, optionally less than or equal to 5m.

The offset distance may be in a range between 0.1 m and 10m, optionally between 0.1 m and 5m, optionally between 1 m and 5m.

Each electrode pair may be offset from an adjacent electrode pair by the offset distance.

The offset distance may be the same for each electrode or electrode pair of the sensor, or may be different for different ones of the electrodes or electrode pairs.

For each of said at least one of the sensors, the electrode pairs for that sensor may be provided in an interleaved arrangement.

For each of said at least one of the sensors, each electrode pair may comprise a respective first electrode towards a first end of the sensor and a respective second electrode towards a second end of the sensor, and the electrode pairs may be arranged such that for at least one of the electrode pairs, the first electrode of that pair is located between the first and second electrodes of at least one other of the electrode pairs, and the second electrode pair is not located between the first and second electrodes of said at least one other of the electrode pairs.

For each of said at least one of the sensors, the spacing between the electrodes may be substantially the same for each electrode pair.

The apparatus may comprise means for selecting electrodes from a plurality of electrodes arranged along the cable, thereby to form the electrode pairs. The cable may, for example, be formed from at least one of stainless steei or copper. Each sensor may have a length of between 50m and 500m.

The cable may have a length, between a first one of the electrode pairs and a last one of the electrode pairs, of between 500m and 10,000m.

The cable may comprise or form part of a streamer for towing behind a vessel. Each response signal may comprise a potential difference signal.

In a further aspect of the invention, which may be provided independently, there is provided a method of processing response signals from a receiver apparatus as claimed or described herein, comprising, for at least one of the sensors combining the response signals from a plurality of the electrode pairs for that sensor to obtain a combined response signal for the sensor, thereby to increase a signai to noise ratio for the sensor.

The combined response signal for the sensor comprises may comprise or represents a sum of the response signals from the electrode pairs.

The method may comprise at least one offer said at least one of the sensors processing the combined response signal of the sensor to obtain an impulse response for the sensor representative of at least one electrical property of at least one formation beneath the bed of a body of water;

for said at least one of the sensors, deconvolving the combined response signal of the sensor with a system response.

The deconvolved response signals and/or the determined impulse response signals from different sensors and/or obtained from different measurements at different positions may subsequently be processed together to determine a model or image of electrical properties of sub-surface formations, for example a 3D model or image of electrical properties of subsurface formations.

The method may further comprise performing a noise determination process to determine a noise-dependent receiver or electrode separation threshold, selecting dipole receivers or electrode pairs that are separated by at least the threshold separation, and performing measurements using the selected dipole receivers or electrode pairs and/or processing response signals obtained from the selected dipole receivers or electrode pairs to obtain the combined response signal.

In a further aspect of the invention, which may be provided independently , there is provided a system for performing controlled source electromagnetic (CSE ) measurements comprising a transmitter cable for towing behind a vessel, a receiver cable as claimed or described herein, and a processing resource configured to process, for each of at least one of the sensors of the receiver cable, response signals from the electrode pairs of that sensor to obtain a combined response signal for the sensor, thereby to increase a signal to noise ratio for the sensor.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. For example, apparatus features may be applied as method features and vice versa.

Brief Description of the Drawings

Embodiments are now described, by way of non-limiting example only, and are illustrated in the following figures, in which:

Figure 1 is a schematic illustration of a system for performing towed controlled source electro-magnetic (CSEM) measurements;

Figure 2 is a schematic illustration showing the relative position of in-line dipole receivers of one sensor of the system of Figure 1 , in an exploded view in which the in-line dipole receivers are shown one below the other, for clarity; and

Figure 3 is a schematic illustration showing the relative position of in-line dipole receivers of one sensor of a CSEM system of an alternative embodiment, in an exploded view in which the in-line dipole receivers are shown one below the other, for clarity.

Detailed Description

Figure 1 is a schematic illustration of a system for performing towed controlled source electro-magnetic (CSEM) measurements, in which a transmitter cable 4 is attached to a survey vessel 2 such that the survey vessel 2 is able to tow the transmitter cable 4 through a body of water, for example through the ocean or a lake. The transmitter cable 4 in this embodiment is formed of copper with a rubber sheath and includes electrodes 6, 8 formed of copper or stainless steel via which current can be applied to the transmitter cabie. In 1027

8 alternative embodiments, the transmitter cable and electrodes may be formed of any other suitable material.

A receiver cable 10 is also attached to the survey vessel 2, and is formed of stainless steel or copper. A series of electrodes each comprising a respective silver-silver chloride sensor element are distributed along the receiver cable and form an in-line series of dipole receivers. In alternative embodiment, the receiver cable and electrodes may be formed of any other suitable material. The dipole receivers are arranged such that the receiver cable provides a plurality of sensors spaced along the cable, each sensor comprising a plurality of dipole receivers. The arrangement of the electrodes is discussed in more detail below in relation to Figures 2 and 3.

In the embodiment of Figure 1 , the transmitter cable has a length of around 500m, and the receiver cable has a length of around 5,000m, and a diameter of around 5cm. An active part of the receiver cable, between the first and last electrodes, has a length of around 2,500m. Each sensor has a length of around 100m and comprises 50 dipole receivers. Each sensor is separated from the next sensor by around 50m and the active part of the receiver cable comprises 16 sensors. Cables with any suitable dimensions, and any desired number of sensors and receivers can be provided in alternative embodiments.

A sensor processing resource (not shown) is installed on the vessel 2, in the form of a data logger unit interfaced to a PC or other user terminal, for receiving and logging response signals received from the dipole receivers of the receiver cable 10, and for logging corresponding signals transmitted by the transmitter cable 4. A transmission control unit {not shown) comprising a current source for providing electrical current to the transmitter cable 4 is also installed on the vessel 2. In the embodiment of Figure 1 , the transmission control unit is interfaced to the PC or other user terminal such that in operation current is applied to the transmitter cable, and consequent transmitter and receiver signals are logged, in response to instructions provided by a user via the PC or other user terminal and/or in response to a control program running at the PC or other user terminal. The resulting logged data can be processed locally at the PC or other user terminal, or can be provided to a remote processing resource, for example a remote server, for processing. in alternative embodiments, any other suitable arrangement of hardware and/or software components may be used to control application of current to the transmitter cable and to log the resulting transmitter and receiver signals. In operation the system response at the source (e.g. the transmitter cable) can be determined, either using a dipo!e receiver positioned close to the source (for example within a few cm) and having closely separate electrodes (for example, a separation of a few cm) or by measuring input current to the transmitter cable directly, as described for example in US 6,914,433 B2The system response can subsequently be used in processing the signals from the sensors, for example using techniques as described in US 6,914,433 B2.

Figure 2 showing the relative position of in-line dipole receivers 20, 20', 20", 20"' of one sensor 18 of the receiver cable 10 of Figure 1 , in an exploded view in which the in-line dipole receivers are shown one below the other, for clarity. Each dipole receiver comprises a respective pair of electrodes 22a, 22b separated by a respective portion of the receiver cable. Although Figure 2 appears to show separate cables for each dipole receiver, positioned one below the other, in fact in the embodiment of Figure 2 there is a single cable, with the dipole receivers 20, 20', 20", 20'" interleaved along the cable, such that a part of the cable included one dipole receiver (e.g. 20) also forms part of a neighbouring, overlapping dipole receiver (e.g. 20').

Only four of the pairs of electrodes, each pair forming part of a dipole receiver, are shown in Figure 2 but the sensor includes fifty such pairs of electrodes distributed over the 100m length of the sensor 18.

In operation, a transmitter signal is transmitted by the transmitter cable under control of the transmission control unit, whilst the transmitter cable is being towed by the vessel 2. The vessel 2 is able to tow the transmitter and receiver cables at any suitable speed, for example around 4 knots. Any suitable transmitter signal can be used, for example that resulting from a step change in current applied across the electrodes, or any other suitable transient signal, for example a signal representing a pseudo random binary sequence. The transmitter signal passes through the water and to the formations beneath the bed of the body of water, and response signals are subsequently received at the dipole receivers of the sensors of the receiver cable 10. As water, in particular sea water, is a conductor of electricity, contact of the water with the electrodes of the dipole receivers provides electrical coupling between the electrodes and the formations beneath the bed of the body of water.

It is a feature of the receiver arrangement of the embodiment of Figures 1 and 2, that the response signals from the dipole receivers for a sensor can be combined to provide a significant increase in the signal to noise ratio for that sensor. That is because it has been found that the additional noise component for towed measurements includes a significant component that is uncorrelated over short distances.

It is hereby suggested that the noise in question may be caused by variation in the potential difference between the receiver electrodes and the water, caused by turbulence in the water. This turbulence has been studied extensively in other contexts and is known to have a correlation distance of about 0.1 m for a seismic frequency of 1 Hz, the correlation distance decreasing with increasing frequency (e.g. Eiboth et al., Investigation of flow and flow noise around a seismic streamer cable, Geophysics (2010) Volume: 75(1), pages: Q1-Q9) Frequencies of interest in marine CSE are generally in the range 0.001 - 10.0 Hz.

In the following analysis we posit that the correlation distances for electrical conductivity are substantially the same as the correlation distances for pressure. In particular, we assume the noise is local: it occurs at the electrode.

Consider a measurement V(t) between two receiver electrodes L m apart, where L is of the order of 100. The measurement consists of signal s(t) and noise n(t) :

V{t) = s{t) + n(t) . (1 )

The electric field is the gradient of the potential. Because the signal changes only slightly over a distance L , the measurement can be expressed as

V(t) = L.Es{t) (2) in which Es(t) = s(t) I L is the electric field signal. It is clear from equation (2) that the signai-to-noise ratio should increase as the length L increases. The noise n(t) is local to the electrodes. If

L > D , (3) where D is the correlation distance, the noise is independent of L . Consider now the arrangement of n electric dipole receivers 20, 20', 20", 20"' shown in Figure 2. Each one is of length L/2 , and consists of two electrodes connected by a telluric cable. The voltage across the first one is

V_J(f) = (L/2).Es(t) + n_i (t) (4)

The second dipoie 20' is offset to the right relative to the first one 20, such that the distance between the e!ectrodes 22a, 22b at each end of the first dipoie and those 22a', 22b' at each end of the second dipoie 20' is

X > D , (5) where X is 1 m in the embodiment of Figures 1 and 2. The voltage across the second dipole is

K₂(0 = (£/2)£J(/) + ¾( , (6) in which the signal (L/2)Es(t) is the same as in the first dipoie 20, because it is correlated over large distances. If X ~ 1 and L = 100 , then 50 dipoles can be fitted into a length of 100 m. Adding up the signal from N dipoles (20, 20', 20", 20"' ... etc) gives fX (r) = N(L/2)Es(t) + f , (0 . (7)

<t=i *=1

If the noise is random with zero mean and standard deviation σ , its variance is σ². The variance is additive, so the root mean square value of the noise in equation 7 is

Thus the coherent signal amplitude increases by a factor N , while the incoherent noise amplitude increases by a factor N , giving an increase in signal-to-noise ratio of N . If N = 50 , this is an increase of 17 dB.

In the embodiment of Figure 2, for each sensor 18, the response signals from the dipoles for that sensor are recorded separately and then summed by computer, for example by the PC or other user terminal interfaced to the data logger unit, or by a remote processing resource, to obtain an output signal from the sensor.

The resulting, combined response signal from the sensor, comprising a variation in potential difference as a function of time, can then be taken as representing the response signal obtained at the position of the sensor 18, and can be used in subsequent analysis of the electrical properties of formations below the bed of the body of water. For example, each combined response signal from a sensor (obtaining by summing responses from the different dipole receivers of the sensor) can be used to determine an earth impulse response and/or the combined response signal can be deconvolved for a known system response, for example using techniques such as those described in US 6,914,433 B2.

It will be understood that the response signals may be subject to other processes either before or after being combined (e.g. summed) to obtain the combined response signal. For example, the signals may be subject to suitable filtering processes and/or removal of outlier response signals. Furthermore, in some cases the combined response signal obtained for a sensor in response to a transmission signal may be summed or averaged with at least one further combined response signal for the sensor, each of the further combined response signals for the sensor being obtained in response to repetition of the transmission signal.

In the embodiment of Figure 2, for each sensor the summing of response signals from the receivers of that sensor is performed by computer, in alternative embodiments, for each sensor the receiver electrodes for that sensor are interconnected so as to provide a sum of the electromagnetic response signals from the receivers at an output of the sensor. For example, in the embodiment of Figure 3, a series connection is provided between the dipole receivers 30, 30', 30", 30'" of the sensor 28.

In some embodiments, the sensor processing resource includes a switching arrangement so that, for each sensor, potential differences can be measured between any two electrodes of the sensor. In some embodiments, the sensor processing resource can be used to select, for each sensor, the pairs of electrodes that will be used as dipole receivers for a particular measurement, and some of the pairs of electrodes may be selected not to be used as dipole receivers for a particular measurement. Alternatively or additionally, the sensor processing resource can be used to record response signals for each of the dipole receivers for a particular sensor, and select response signals from only some of the dipo!e receivers to be combined (e.g. summed) to obtained the combined response signal for the sensor. In some embodiments, the sensor processing resource is configured to perform an initial noise determination process in which the response signals from each of the dipole receivers are measured over time without a signal being applied by the transmitter, whilst the receiver cable is being towed by the vessel at an operational speed (e.g. at a speed at which transmitter and receiver measurements are to be performed). The sensor processing resource then analyses the resulting measured noise signals to determine a threshold separation, in this case representing the minimum receiver or electrode separation at which the noise becomes substantially uncorrelated. The analysis can include performing a respective cross-correlation process between selected pairs of dipole receivers, or between each pair of dipole receivers. The processing resource is then able, for example, to select those dipole receivers or electrode pairs that are separated by at least the threshold receiver or electrode separation determined in the noise determination process. Measurements may then be selectively performed by the selected dipole receivers or electrode pairs, or response signals obtained from the selected dipole receivers or electrode pairs may be selectively used to obtain the combined response signals. The processing resource may ensure that measurements are not performed or recorded in respect of dipole receivers or electrode pairs that are not selected, or that response signals from dipole receivers or electrode pairs that are not selected are not used to obtain the combined sensor response signals.

In alternative embodiments, other arrangements of receivers and electrodes can be provided. As the noise may be correlated over short distances compared with the signal, in various embodiments each dipole may be as long as possible to maximize signal amplitude, and the dipoles may be offset from each other sufficiently that the noise at any two electrodes is largely uncorrelated.

!n some embodiments, it is possible for two or more dipole receivers to have an electrode in common. For example, in a variant of the embodiment of Figure 2, electrode 22a may be the first electrode for two different electrode pairs each forming a respective dipole receiver, with each of the dipole receivers having a different second electrode to the other. However, as the turbulence-related noise is expected to be local to each electrode the noise from such common electrodes would be reinforced in the summed sensor signal, and such embodiments would be expected not to show as significant an increase in signal to noise ratio as embodiments such as those of Figures 2 and 3, in which each electrode is an electrode for only a single one of the dipole receivers. In the embodiments of Figures 2 and 3, the receiver each have only two efectrodes. In alternative embodiments, each receiver may comprise more than two electrodes.

In the embodiments of Figures 1 to 3, the transmitter and receiver cables are aquatic cables forming part of a system for performing towed controlled source electro-magnetic (CSEM) measurements. Embodiments are not limited to performing aquatic or towed-source measurements and in some embodiments the receiver cable is for use on land, for example for use in a system for performing stationary controlled source electro-magnetic measurements. In such embodiments the source may be a stationary source and may comprise a transmitter cable adapted for use on land or any other suitable source. The receiver cable may be laid on the ground surface in a substantially straight line or in any other suitable arrangement relative to the source.

As discussed, it may be possible to exploit a new understanding of the sources of noise generation to obtain a receiver electrode layout that minimises or reduces noise.

It will be understood that the present invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. A receiver apparatus, comprising a cable and a plurality of sensors spaced along the cable, wherein at least one of the sensors comprises a plurality of electrode pairs, each electrode pair being connected via a respective length of the cable and configured to receive a respective response signal.

2. A receiver apparatus according to Claim 1 , wherein the cab!e comprises an aquatic cable for use in controlled source electromagnetic (CSEM) measurements,

3. A receiver apparatus according to Claim 1 or 2, wherein each electrode pair and the length of cable between the electrodes of the pair comprises or forms part of a respective dipo!e receiver, such that each of said at least one of the sensors comprises a plurality of dipoie receivers,

4. A receiver apparatus according to any preceding claim, further comprising, for each of said at least one of the sensors, means for combining the response signals from the electrode pairs to obtain a combined response signal for that sensor, thereby to increase a signal to noise ratio for the sensor.

5. A receiver apparatus according to Claim 4, wherein the combined response signal comprises or represents a sum of the response signals from the electrode pairs.

6. A receiver apparatus according to any preceding claim, wherein for each of said at least one of the sensors, the electrode pairs for that sensor are interconnected thereby to provide a signal at an output of the sensor that comprises or represents a sum of the response signals from that sensor.

7. A receiver apparatus according to Claim 6, wherein for each of said at least one of the sensors, the electrode pairs for that sensor are connected in series.

8. A receiver apparatus according to any preceding claim, wherein for each of said at least one of the sensors, each electrode is offset from a corresponding electrode of at least one other of the electrode pairs of that sensor by at least an offset distance.

9. A receiver apparatus according to Claim 8, wherein for each of said at least one of the sensors, each electrode is offset from each other of the electrodes by at least the offset distance.

10. A receiver apparatus according to Claim 8 or 9, wherein for each of said at least one of the sensors, each electrode is offset from its adjacent e!ectrode(s) by the offset distance.

11. A receiver apparatus according to any of Claims 8 to 10, wherein the offset distance is greater than a noise coherence distance.

12. A receiver apparatus according to any of Claims 8 to 11 , wherein the offset distance is greater than or equal to 0. m.

13. A receiver apparatus according to any of Claims 8 to 12, wherein the offset distance is in a range between 0.1 m and 10m, optionally between 0.5m and 5m, optionally between 1 m and 2m.

14. A receiver apparatus according to any preceding claim, wherein for each of said at least one of the sensors, the electrode pairs for that sensor are provided in an interleaved arrangement.

15. A receiver apparatus according to any preceding claim, wherein for each of said at least one of the sensors, the spacing between the electrodes is substantially the same for each electrode pair.

16. A receiver apparatus according to any preceding claim, further comprising means for selecting electrodes from a piuraiity of electrodes arranged along the cable, thereby to form the electrode pairs.

17. A receiver apparatus according to any preceding claim, wherein the cable is formed from at least one of stainless steel or copper.

18. A receiver apparatus according to any preceding claim, wherein each sensor has a length of between 50m and 500m, optionally between 70m and 120m, optionally between 90m and 1 10m,

19. A receiver apparatus according to any preceding claim, wherein the cable has a length, between a first one of the electrode pairs and a iast one of the electrode pairs, of between 500m and 10,000m.

20. A receiver apparatus according to any preceding claim, wherein the cable comprises or forms part of a streamer for towing behind a vessel.

21. A receiver apparatus according to any preceding claim, wherein each electromagnetic response signal comprises a potential difference signal.

22. A method of processing electromagnetic response signals from a receiver apparatus according to any of Claims 1 to 21 , comprising, for at least one of the sensors combining response signals from a plurality of the electrode pairs for that sensor to obtain a combined response signal for the sensor, thereby to increase a signal to noise ratio for the sensor.

23. A method according to Claim 22, wherein the combined response signal for the sensor comprises or represents a sum of response signals from the electrode pairs.

24. A method according to Claim 22 or 23 comprising at least one of:- for said at feast one of the sensors processing the combined response signal of the sensor to obtain an impulse response for the sensor representative of at least one electrical property of at least one formation beneath the bed of a body of water;

25. A method according to any of Claims 22 to 24, further comprising performing a noise determination process to determine a noise-dependent receiver or electrode separation threshold, selecting dipole receivers or electrode pairs that are separated by at least the threshold separation, and performing measurements using the selected dipoie receivers or electrode pairs andVor processing response signals obtained from the selected dipole receivers or electrode pairs to obtain the combined response signal.

26. A system for performing controlled source electromagnetic (CSEM) measurements comprising a transmitter cable for towing behind a vessel, a receiver apparatus according to any of Claims 1 to 21 , and a processing resource configured to process, for each of at least one of the sensors of the cable of the receiver apparatus, electromagnetic response signals from the electrode pairs of that sensor to obtain a combined response signal for the sensor, thereby to increase a signal to noise ratio for the sensor.

27. A. receiver apparatus, system or method substantially as described herein with reference to the accompanying drawings.