WO2001077488A1

WO2001077488A1 - Seismic surveying

Info

Publication number: WO2001077488A1
Application number: PCT/NO2001/000148
Authority: WO
Inventors: Helge Brandsaeter; Tore Kjos; Tor Hilton
Original assignee: Read Asa
Priority date: 2000-04-06
Filing date: 2001-04-06
Publication date: 2001-10-18
Also published as: AU2001250685A1; GB0008545D0

Abstract

A system for surveying a volume of rock or sub-surface below the seabed comprising at least one acoustic generating means for generating intermittent acoustic pulses, and a first signal detection means for detecting the signals generated by the acoustic pulses and/or reflections thereof at a first detection means arranged in a well or hole penetrating beneath the seabed, characterised in that a second signal detecting means is arranged on or above the seabed, and data processing means are provided for combining and processing the results from the first and second detection means.

Description

Seismic surveying

The present invention relates to a method and system for investigating the geological properties below the seabed.

More particularly the invention relates to seismic profiling which is a measurement procedure in which a seismic signal is generated at the earth's surface and is recorded by geophones. Typically the seismic signal is generated at or above the earth's surface and the signal is recorded by geophones or accelerometers which are also located at the surface of the earth.

The geophones are typically arranged over a sufficiently wide area on the earth's surface to survey the target area below and this method is known as horizontal seismic profiling. Recently four component sensing systems have been used in which different devices are arranged at the surface such as two horizontal geophones, a vertical geophone and a hydrophone. These four sensing devices give rise to four components to the data. More recently the geophones have been located vertically and secured at various depths to the wall of a drilled hole or well. This vertical seismic profiling (VSP) has the advantage that, in theory, more information can be obtained because of the various depths at which the signal can be recorded and that the signal can be recorded on the way down as well as on the way back up. However a great deal of data is recorded by this technique and the problem is to interpret this data in a meaningful way to accurately determine the properties of the rock being surveyed.

It is an objective of the invention to provide an improved method of determining the properties of the rock more accurately.

It is also an objective of the invention to reduce the costs of carrying out a seismic survey both in terms of the handling of the equipment required and in providing the required energy sources.

It is also an objective of the invention to reduce the time taken to carry out a seismic survey.

According to the invention there is provided a system for surveying a volume of rock below the seabed comprising at least one acoustic generating means for generating intermittent acoustic pulses, and a first signal detection means for detecting the signals generated by the acoustic pulses and/or reflections thereof at a first detection means arranged in a well or hole penetrating beneath the seabed, and a second signal detecting means arranged on or above the seabed, and data processing means for combining and processing the results from the first and second detection means.

The second signal detecting means may be towed by a vessel as a horizontal array, arranged horizontally on the sea bed, arranged vertically in the sea between the seabed and a vessel.

The at least one acoustic generating means may be towed by a vessel, positioned on the sea bed, or, arranged in a well or hole penetrating beneath the seabed.

Preferably the data from the first dection means is processed by a data processor and combined with the data from the second detection means to arrive at a final representation of the area being surveyed.

Preferably one operation that is made of the data is velocity control for P waves.

Preferably a further operation is velocity control for S waves. Using three separate geophones or three geophones and one hydrophone in the first detection means, four component down hole measurement may be made of the downwards wavefield.

Preferably a further operation is anisotropy estimation.

Preferably a further operation is the tieing together of P and S waves.

Preferably a further operation is wavelet estimation and Q compensation.

According to a further aspect of the invention there is provided a method for surveying a volume of rock below the seabed comprising generating intermittent acoustic pulses from at least one energy source, said at least one energy source, detecting the signals generated by the acoustic pulses and/or reflections thereof at a first detection means arranged in a well or hole penetrating beneath the seabed and at a second detecting means arranged above the seabed and combining and processing the results from the first and second detection means.

Further aspects and objects of the invention is acheived by means of the features in the dependent claims.

Various embodiments of the invention will now be described in more detail in which: Fig. 1 is a schematic general arrangement of a first embodiment of the invention;

Fig. 2 is a schematic general arrangement of a second embodiment of the invention; and Fig. 3 is a schematic general arrangement of a third embodiment of the invention.

Referring to fig. 1 an embodiment is shown in which an energy source 2 is located on the sea bed and generates intermittent acoustic pulses 3 downwards into the earth. A vertical well hole 4 is formed within range of the energy source and a downhole VSP (vertical seismic profiling) geophone package 5 is lowered into the hole 4. There may be a number of separate geophone packages arranged along and spaced apart in the well hole. The or each geophone package 5 comprises a number of geophone sensors along its length and which are encased in a container capable of withstanding the fluid pressure at the deployed depth. The geophone package 5 also includes a locking device (not shown) for physically securing the package 5 to the well hole 4 in the desired position.

Accelerometers may also be used instead of geophones, but for the purposes of this embodiment we will refer only to geophones. For convenience the term geophone package will be used throughout the description of the embodiments, although it will be understood that the geophone package may include accelerometers and/or hydrophones, as well as or instead of geophones.

The geophone package 5 senses the acoustic pulses as they pass in the downwards direction from the energy source 2 and also senses the reflected signals on the way back up having been reflected from a certain rock strata below the energy source. Additionally, located adjacent to the energy source on the seabed, is a further detection means 10 such as a geophone which detects the reflected signals only. The further detection means 10 may be any desirable surface sensing system such as a simple geophone arranged at the surface or a four component system.

The data from the package 5 is processed by a data processor 9, shown here located on the vessel 8, and combined with the data from the second detection means 10 to arrive at a final picture of the area being surveyed. Various data processing steps can be undertaken on the data from the package and/or the second detection means.

The data from the package 5 and the data from the second detection means 10, may be processed in two separate processing sequences, a first and a second, where results from various steps in the processing sequence of the package 5 data are used as input values for the processing sequence of the data from the second detection means 10.

A processing sequence for the data from the package 5 can be exemplified by the the following steps (first processing sequence): a) Binning (regularization of the source grid spacing) b) Rotation of the three-component wavefield into a inline/crossline vector wavefield c) Filling data into empty cells (dip adaptive interpolation) d) Forward tau-p transform (not for expanding circle data) e) Wavefield separation (extraction of separate upgoing and downgoing wave modes) f) VSP deconvolution g) Elastic Wavefield Decomposition (P-S separation) h) Inverse tau-p transform i) Transform of the source grid coordinates back to the coordinates prior to the binning (optional) j) New binning (re-gridding) of the upgoing scalar P- and S-wave-fields. k) Velocity estimation/ Anisotropy estimation

1) Imaging

A processing sequence for the data from the second detection means 10 can be exemplified by the the following steps (second processing sequence):

1. Lowpass filter 85(72) Hz(dB/Oct) and resampling to 4ms

2. Sorting into CRGs (Common Reflection Groups) 3. Wiener filter fit of H to Vz (zero offset data filter design)

4. P-P nmo (normal moveout), source grid binning and P-P rnmo (residual normal moveout)

5. Forward tau-p transform

6. 4C eastic wavefied decomposition into downgoing P, upgoing P-P and upgoing P-S: alphal = 1465 m/s, alpha2 = 180 m/s, beta2 = 600 m/s and density = 1.2 g/cm³.

7. Transform of each separate wave mode from vector wavefield to scalar wavefield (time-invariant tau-p domain polarization) 8. Deterministic deconvolution: Filters designed from downgoing P. Desired output = 6[8]-60[48] Hz[dB/Oct] zero phase pulse for P-P and 5[12]-40[32] Hz[dB/Oct] zero phase pulse for P-S.

9. P-P wavefield migration velocity analysis (curvature velocity analysis applied to P-P migrated CIPs (Common Image Points))

10. Final P-P wavefield anisotropic depth migration, removal of residual multiples with radon filtering applied to the CIPs, and finally CIP stack.

1 l .P-S wavefield migration velocity analysis using the tie constraint: using mistie between migrated P-S and P-P to update the S-veocity model (the focussing constraint is included later).

12. P-S wavefield receiver static correction: Using those time shifts required to shift a horizon (identified both on P-S and P-P) from where it appears on the P-S migrated section after CRG stack to where it appears on the P-P migrated section after CRG stack to where it appears on the P-P migrated section after CIP stack and strech to vertical P-S time.

13. P-S wavefield migration velocity (anisotropy) analysis using the focussing constraint: Update is done of the anisotropy (epsilon) instead of the vertical S- velocity model.

14. Final P-S wavefield anisotropic depth migration & CIP stack. 15. Compensation for absorbtion (T** l scaling) (in P-S time for P-S) and transmission loss compensation (only for P-P).

Operations may be performed on the two processing sequences that use outputs from the first processing sequence as inputs in the second processing sequence.

An example of an operation, is the deconvolution step, f) in the first processing sequence, where one output value is an estimate of the signature of the acoustic source. An ideal source should have a single peak, a "spike", as its signature, but in the real case multiple small oscillations follow the spike. The estimation of the signature is most efficiently performed by means of the package 5, as the direct wave is registered in the well, and the reflected waves might be separated out during the processing. This estimate is used as an input value in step no. 8 in the second processing sequence, for design of an appropriate filter to filter the noise directly related to the source and thus obtaining better resolution. Further operations uses outputs from the other steps in the processing sequences as inputs for the calculations in the various steps in the other of the processing sequences.

A further operation is velocity control for P waves. By considering the downward acoustic wavefield detected by the package 5, the direct arrivals from the energy source 2 can be selected. The reflection time achieved by the seismic measurements can be related to the depth of each geophone in the geophone package or packages 5. The velocity of P waves can be determined and used as an input for processing of the data from the second detection means. A further operation is velocity control for S waves. Using three or four separate geophones in the geophone package 5, component down hole measurement can be made of the downwards wavefield. The velocity of the direct S waves can be determined and used as input for processing of the date of the second dectection means. The velocity of the converted PS waves can be determined by examining the upward, reflected wavefield and also be used for processing the data from the second detection means.

A further operation is anisotropy estimation. This is the differences in propagation velocities for acoustic waves with changing propagation directions. These measurement are made by the vertical geophone package 5 and used as an input to the second detection means 10.

A further operation is the tieing together of P and S waves. The 3d four component data may result in at least two seismic sections, that is a P section and an S section. Using the data from the vertical geophone package 5 these sections can be tied to match the same depths. The package 5 measures the direct wave from the energy source 2, both S- and P-waves. The following data processing steps will include steps to separate S- and P-waves. This is essential information for structural interpretation purposes as well as for extraction of petrophysical data by combining information from both sections.

A further operation is wavelet estimation and Q compensation. Using the geophone package data, the earth filter can be estimated and used to compensate the data from the second detection means. That is, The frequency band of the data can be increased by the application of inverse filters.

In this embodiment the geophone package is shown extending vertically only. However it is also possible that the well hole extends horizontal or at an curve or angle between the vertical and horizontal. With multiple geophone detention means arranged anywhere along its length to obtain the best results. Fig. 2 shows an further embodiment which is similar in arrangement and the same numerical identifiers are used for common components. In this embodiment the energy source 12 and the second detection means 20 are provided on a wire-line arranged vertically in the sea between the seabed and the vessel 8. Fig. 3 shows an further embodiment which is similar in arrangement and the same numerical identifiers are used for common components. In this embodiment the energy source 22 and the second detection means 30 are provided above the seabed.

In the above embodiments the energy source and further detection means 10 may either remain stationary at the same position on the seabed or they may be towed by the vessel in a desired path.

In all the above embodiments the processing means 9 are shown on the vessel but they may also be located at least in part at either or both or the geophone package or the second detection means or located at a remote location such as a central control centre.

As well as being extremely useful in carrying out seismic surveys the system and method of the present invention may also be used to assist in the accurate directional control of long horizontal wells. This would greatly improve the existing measurement techniques provided essentially by expensive and vulnerable gyroscopic sensors located downhole. With a geophone package provided as part of the drilling system and coupled with a second detection means it is possible to make distance measurements to the surface and triangulation calculations which will enable much more accurate positioning.

Claims

1. A system for surveying a volume of rock or sub-surface below the seabed comprising at least one acoustic generating means for generating intermittent acoustic pulses, and a first signal detection means for detecting the signals generated by the acoustic pulses and/or reflections thereof at a first detection means arranged in a well or hole penetrating beneath the seabed, characterised in that a second signal detecting means is arranged on or above the seabed, and data processing means are provided for combining and processing the results from the first and second detection means.

2. A system according to claim 1, characterised in that the second signal detecting means is arranged horizontally on the sea bed.

3. A system according to claim 1 , characterised in that the second signal detecting means is arranged vertically in the sea between the seabed and a vessel.

4. A system according to claim 1 , characterised in that the second signal detecting means is located either on a vessel or towed by a vessel.

5. A system according to one of the claims 2-4, characterised in that the data from the first signal detection means is processed by a data processor and combined with the data from the second detection means to produce a final representation of the area being surveyed.

6. A system according to claim 5, characterised in that the data processing means processes the data from the first detection means in a first processing sequence and the data from the second detection means in a second processing sequence and use outputs from the first processing sequence as input in the second processing sequence.

7. A system according to claim 5, characterised in that the data processing operation made on the data is velocity control for P and/or S waves.

8. A system according to claim 5, characterised in that the data processing includes anisotropy estimation.

9. A system according to claim 5, characterised in that the data processing includes the tieing together of P and S waves.

10. A system according to claim 5, characterised in that the data processing includes wavelet estimation and Q compensation.

1 1. A system according to one of the claims 2-4, characterised in that the first detection means comprises two or more separate packages of sensors spaced apart along the well hole.

12. A system according to claim 1 1, characterised in that each package comprises 3 geophones or 3 accelerometers.

1 3. A system according to claim 12, characterised in that each package further comprises a hydrophone.

14. A method for surveying a volume of rock or sub surface below the seabed comprising generating intermittent acoustic pulses from at least one energy source, detecting the signals generated by the acoustic pulses and/or reflections thereof at a first detection means arranged in a well or hole penetrating beneath the seabed and at a second detecting means arranged on or above the seabed and combining and processing the results from the first and second detection means.

15. A method according to claim 14, characterised in that the second detecting means is arranged horizontally on the sea bed.

16. A method according to claim 14, characterised in that the second detecting means is arranged vertically in the sea between the seabed and a vessel.

17. A method according to claim 14, characterised in that the second detecting means is located either on a vessel or towed by a vessel.

18. A method according to one of the claims 15-17, characterised in that the data from the first signal detection means is processed by a data processor and combined with the data from the second detection means to produce a final representation of the area being surveyed.

19. A method according to claim 18, characterised in the data from the first detection means is processed in a first processing sequence and the data from the second detection means is processed in a second processing sequence and outputs from the first processing sequence is used as input in the second processing sequence.

20. A method according to claim 18, characterised in that the data processing operation made on the data is velocity control for P and/or S waves.

21. A method according to claim 18, characterised in that the data processing includes anisotropy estimation.

22. A method according to claim 18, characterised in that the data processing includes the tieing together of P and S waves.

23. A method according to claim 18, characterised in that the data processing includes wavelet estimation and Q compensation.

24. A method according to one of the claims 15-17, characterised in that the first detection means comprises two or more separate packages of sensors spaced apart along the well hole.

25. A system according to claim 24, characterised in that each package comprises 3 geophones or 3 accelerometers.

26. A system according to claim 25, characterised in that each package further comprises a hydrophone.