GB2236392A

GB2236392A - Method for improving cross-borehole seismic surveys

Info

Publication number: GB2236392A
Application number: GB9019728A
Authority: GB
Inventors: Edwinus Nicolaas Maria Quint
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1989-09-11
Filing date: 1990-09-10
Publication date: 1991-04-03
Also published as: GB8920504D0; GB9019728D0

Abstract

In a method for determining the location of a layer interface 10, which is comprised within a structure of subsurface formation layers 2, 3, which functions as a seismic boundary, and which has at least a substantial lateral extent between at least two boreholes 4,5 extending through said structure, one borehole 4 is used for arranging a seismic generator G generating seismic waves, and the other for a seismic receiver R directionally detecting arrival times and amplitudes of said waves. The method further comprises, generating seismic P waves at least at one position in the one borehole, and detecting seismic waves at least at one position and in two directions, the first being the axial direction and the second the radial direction within the other borehole, thereby obtaining layer interface related structure parameters. <IMAGE>

Description

METHOD FOR IMPROVING CROSS-BOREHOLE SURVEYS The present invention relates to a method for improving cross-borehole surveys.

Particularly the invention concerns a method for determining the location of a layer interface, which is comprised within a structure of subsurface formation layers, which functions as a seismic boundary, and which has at least a substantial lateral extent between two boreholes used in order to carry out a cross-borehole survey of said intermediate structure.

It is known from U.S. patent specification No. 4,711,303 to use a method for determining the subsurface position of a blowing well with respect to a relief well. In said method sound waves emanating from che blowing well, particularly from a "source" within said well, are detected and analyzed, in that within the relief well firstly by means of hydrophones axially arrival time differences of said waves and secondly by means of accelerometers acoustic intensities are detected at a plurality of tangentially spaced locations alongside the borehole wall. Thereby respectively the magnitude and the azimuth of the distance vector, which is indicated as the vector from the point of closest approach between said source and the axis of the relief well, is determined.

Although triaxial accelerometers for detecting axial, radial and tangential components of said seismic waves are employed only the direction of said vector within a plane substantially normal to the blowing well is determined.

Thus it is an object of the invention to determine arrival directions of seismic waves from cross-borehole surveys of structures of subsurface formation layers.

It is another object of the invention to determine the structure of subsurface formation layers.

The invention therefore provides a method for determining the location of a layer interface, comprised within a structure of subsurface formation layers and functioning as a seismic boundary, said interface having at least a substantial lateral extent between at least two boreholes extending through said structure, the one borehole being used for arranging a seismic generator generating seismic waves, and the other for a seismic receiver directionally detecting arrival times and amplitudes of said waves, the method further comprising, generating seismic waves at least at one position in the one borehole, and detecting seismic waves at least at one position and in two directions, the first being the axial direction and the second the radial direction within the other borehole, thereby obtaining layer interface related structure parameters.

Furthermore detecting of said seismic waves in a third direction being the tangential direction normal to the axial and radial directions is provided.

Advantageously from said detection of seismic waves reflecting, critically refracting and direct seismic waves can be distinguished from each other.

Moreover the invention provides a method for arranging the receiver for detecting seismic waves within the other borehole.

Both arranging said receiver at a single position near the wall and additionally at least at one subsurface level on a plurality of positions alongside the wall of said other borehole is provided.

The method in accordance with the invention further comprises detecting and generating said seismic waves at a set of vertical positions respectively in the other and in the one borehole, the sets extending from substantially above said interface to substantially beneath said interface.

Thus it is enabled to acquire advantageously an accurate map of said structure of subsurface formation layers.

The invention will now be described by way of example in more detail with reference to the accompanying drawings, in which: fig. 1 shows schematically a vertical section through a structure of subsurface formation layers to which the method in accordance with the invention is applied; fig. 2 is a time-record of seismic waves propagating through the structure as shown in fig. 1 and subsequently detected in accordance with the method of the invention; fig. 3 is a graph of arrival times vs. arrival angles of the seismic waves of fig. 2; fig. 4 shows schematically the structure of subsurface formation layers as shown in fig. 1 now passed by seismic waves which are first arriving waves when detected in the other borehole; and fig. 5 presents schematically a graph of angles of first arriving seismic waves vs. corresponding receiver depths for the set up as shown in fig. 4.

In the figs. 1 an'd 4 similar reference numerals are used for corresponding elements.

Referring now to fig. l-a vertical section through a structure 1 of subsurface formation layers comprising a layer 2 and a layer 3 having therebetween a layer interface 10 is shown. In this exemplary structure the layers are assumed homogeneous, i.e. each layer has a constant wave propagation velocity, whereas for layer 2 the wave propagation velocity v2 is greater than the wave propagation v3 for layer 3 since layer 2 has a greater acoustical density than layer 3. Two boreholes 4 and 5 extend through said structure. Furthermore the layer interface 10 is extending substantially in lateral directions relative to the interborehole distance, indicated with D in the fig. For reason of simplicity both boreholes 4 and 5 are assumed to be substantially normal to the earth's surface and to the layer interface 10.

In the one borehole 4 a generator G for generating seismic waves is arranged at a depth dc from the earth's surface and at an height h from the layer interface 10. Generally pressure (or P-, or longitudinal) waves (as such known for those skilled in the art and not explained in detail) will be generated for surveying purposes.

In the other borehole 5 a receiver R for directionally detecting seismic waves is positioned at a depth dR from the earth's surface. For reason of simplicity receiver R is positioned also at said height h from the layer interface 10.

As can be seen in fig. 1 generator G is generating seismic waves in different directions. Three exemplary wave propagation modes arrowed differently have been shown. The following wave modes are represented: firstly a so-called direct wave from G to R, propagating through layer 3 with propagation velocity v3 along a singularly arrowed line, secondly a so-called reflecting wave, originating at G with an angle a1, with respect to the direct wave line, and reflecting against layer 2 at layer interface 10, which functions as a seismic boundary because of the different acoustical densities as mentioned above, in the direction of receiver R along doubly arrowed lines, the wave propagating also with wave propagation velocity v3, and thirdly a so-called critically refracting wave, originating at G with an angle a2 with respect to the direct wave line and propagating in layer 3 with velocity v3, critically refracting a first time with a corresponding critical angle, i.e. (E 2) and travelling along the layer interface 10 with wave propagation velocity v2, and finally critically refracting a second time at the layer interface with said critical angle in the direction of R and propagating with velocity v3, the whole critically refracting wave propagation line being arrowed triply.

The phenomena of wave reflection and critical wave refraction as such are well known to those skilled in the art and will not be explained. Furthermore it will be appreciated by those skilled in the art that the exemplifying arrangement for generator G and receiver R as chosen for fig. 1 results in a symmetrical propagation line pattern.

In fig. 2 a time-record of seismic waves propagating through the structure as shown in fig. 1 and subsequently detected in accordance with the method of the present invention is shown. As can be seen the seismic waves appear as wiggles on such a record, for example being voltage variations generated in an accelerometer used as a seismic detector.

When analyzing such a record two main problems have to be solved. First, which propagating mode the wave (or waves) did follow, and second, from which direction(s) said wave (or waves) did reach the detector.

In the case of the structure of fig. 1 the three above-mentioned wave modes are to be expected. Since the last wiggle has a significantly reduced amplitude it can be identified as the arriving reflecting wave which phenomenon as such is known to those skilled in the art.

The further identification will be carried out now by the reasoning that in said exemplary structure critically refracting waves are propagating faster than direct waves, which is known to those skilled in the art.

In order to enable a complete determination of the structure arrival directions of the above said waves have to be determined.

Thus the present invention provides a method wherein seismic waves are detected at least at one position and in two directions, the first being the axial direction and the second the radial direction within the other borehole, thereby obtaining layer interface related structure parameters.

Then by using basic geometry the arrival direction a for such a wave is defined as the arctan of the ratio of the axial and radial components of said waves detected as explained above.

Moreover it has to be notified that further checks on the above determinations can be carried out.

For example a third component of said wave being the component in the tangential direction normal to the axial and radial directions can be detected.

In a further embodiment of the invention a position near the borehole wall can be chosen in order to assure a better transfer of acoustical energy. Additionally it has appeared advantageous to detect seismic waves at least at one subsurface level on a plurality of positions alongside the wall of the said other borehole thereby obtaining polar diagrams for said level.

As shown in fig. 3 for the waves of fig. 2 the corresponding arrival directions a have been determined.

It will be clear for those skilled in the art that the data of the figs. 2 and 3 enable to compute layer interface related structure parameters and to determine the structure as shown in fig. 1 When using basic geometry in the structure of fig. 1 the following formulas for the three different travel times of said waves can be derived: t D (1) direct V3 t reflection

and t critical refraction

2h Jt D-2h (F) (3) 2h j (tan a2} + D - 2h (tan1a ) (3) v3 V2 the parameters h, D, v2, V3 and a2 having been explained with respect to fig. 1. Consequently the unknown structure parameters h, V2 and v3 can be calculated.

Those skilled in the art will appreciate that for similar type layer interfaces, for example a layer structure having a low wave propagation velocity layer above a high wave propagation velocity layer, a dipping layer interface, or a thin intermediate layer, corresponding calculation models can be derived which can be solved similarly.

Referring to fig. 4 the same structure of subsurface formation layers as shown in fig. 1 is now passed by a plurality of seismic waves, generated by generator G positioned substantially at the layer interface 10 and propagating both in the high wave propagation velocity layer 2 and in the low wave propagation velocity layer 3. It is notified that only those wave representing propagation lines have been drawn which represent waves to be labelled as first arriving waves when reaching the other borehole 5 used for arranging the above-mentioned receiver R at a plurality of vertical positions dR.

As can be seen in fig. 4 in layer 2 so-called direct waves are the first arriving waves. From the layer interface 10 at dR = to dR - D tan a2 ((n-a2) being the critical angle as explained above) refracting waves having said critical angle are the first arriving waves. Then for depth values beneath dR - D tan a2 again direct waves, now having wave propagation velocity v3, are the first arriving waves.

Now referring to fig. 5 a graph of angles a of first arriving seismic waves vs. corresponding receiver depths dR for the measuring set up of fig. 4 is shown schematically.

As can be read off from fig. 4 in the upper layer 2 the angles a decrease when lowering the receiver R to dR O.

Then at the layer interface 10 at dR I 0 the first arrival angle a jumps from a - O to a - a2 because of the refraction phenomenon as explained above.

Finally, for the exemplifying structure of fig. 4, for depth values dR > D tan a2 the a-values actually form a continuation of the a-values as found for the direct waves in layer 2.

Thus by determining first arrival directions of seismic waves in accordance with the method of the invention arrival angle discontinuities can be detected thereby layer interfaces being traced immediately.

It will be clear for those skilled in the art that modifications on the procedure can be made. For example the generator G can be lowered when generating seismic waves whereas the receiver R has a fixed position. Furthermore for subsurface structures differing from the structure as shown similar graphs can be obtained resulting in corresponding layer interface detections.

Various modifications of the present invention will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Claims

C L A I M S

1. A method for determining the location of a layer interface, comprised within a structure of subsurface formation layers and functioning as a seismic boundary, said interface having at least a substantial lateral extent between at least two boreholes extending through said structure, the one borehole being used for arranging a seismic generator generating seismic waves, and the other for a seismic receiver directionally detecting arrival times and amplitudes of said waves, the method further comprising, generating seismic waves at least at one position in the one borehole, and detecting seismic waves at least at one position and in two directions, the first'being the axial direction and the second the radial direction within the other borehole, thereby obtaining layer interface related structure parameters.

2. The method as claimed in claim 1, wherein said waves are detected in a third direction being the tangential direction normal to the axial and radial directions.

3. The method as claimed in claim 1 or 2, wherein the seismic receiver is arranged near the wall of the other borehole.

4. The method as claimed in claim 3, wherein the seismic receiver is arranged at least at one subsurface level on a plurality of positions along side the wall of the other borehole.

5. The method as claimed in any one of the foregoing claims wherein said seismic waves are detected at a set of vertical positions within the other borehole, the set extending from substantially above said interface to substantially beneath said interface.

6. The method as claimed in any one of the foregoing claims wherein said seismic waves are generated at a set of vertical positions within the one borehole, the set extending from substantially above said interface to substantially beneath said interface.

7. The method as claimed in claim 5 or 6, wherein a set of first wave arrival directions is obtained thereby acquiring a first wave arrival direction vs. depth profile for said structure.

8. The method as claimed in any one of the foregoing claims, wherein the layer interface related structure parameters comprise interface depth, interface velocity change and relative velocity change corresponding thereto.

9. Method for determining the location of a layer interface, comprised within a structure of subsurface formation layers and functioning as a seismic boundary, substantially as described in the description with reference to the appended drawings of fig. 2, 3, 4 and 5.