CA2205426A1 - Method for measuring the water thickness above a bottom cable - Google Patents
Method for measuring the water thickness above a bottom cableInfo
- Publication number
- CA2205426A1 CA2205426A1 CA 2205426 CA2205426A CA2205426A1 CA 2205426 A1 CA2205426 A1 CA 2205426A1 CA 2205426 CA2205426 CA 2205426 CA 2205426 A CA2205426 A CA 2205426A CA 2205426 A1 CA2205426 A1 CA 2205426A1
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 title claims description 46
- 238000000034 method Methods 0.000 title claims description 30
- 238000005314 correlation function Methods 0.000 claims abstract description 4
- 230000003111 delayed effect Effects 0.000 claims description 4
- 230000003094 perturbing effect Effects 0.000 claims description 3
- 238000012935 Averaging Methods 0.000 claims 1
- 230000003044 adaptive effect Effects 0.000 claims 1
- 230000001934 delay Effects 0.000 claims 1
- 230000001131 transforming effect Effects 0.000 claims 1
- 230000000694 effects Effects 0.000 description 3
- 238000001914 filtration Methods 0.000 description 3
- 238000009472 formulation Methods 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 230000008569 process Effects 0.000 description 3
- 238000012876 topography Methods 0.000 description 3
- 238000013459 approach Methods 0.000 description 2
- 238000003491 array Methods 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 230000001427 coherent effect Effects 0.000 description 2
- 238000007689 inspection Methods 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 238000002310 reflectometry Methods 0.000 description 2
- 230000004044 response Effects 0.000 description 2
- 238000001228 spectrum Methods 0.000 description 2
- 230000002123 temporal effect Effects 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 230000002547 anomalous effect Effects 0.000 description 1
- 238000005311 autocorrelation function Methods 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 238000012937 correction Methods 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 230000033001 locomotion Effects 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 230000010363 phase shift Effects 0.000 description 1
- 238000007781 pre-processing Methods 0.000 description 1
- 150000003839 salts Chemical class 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 230000001052 transient effect Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/38—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/28—Processing seismic data, e.g. for interpretation or for event detection
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- Life Sciences & Earth Sciences (AREA)
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- Remote Sensing (AREA)
- Engineering & Computer Science (AREA)
- Geology (AREA)
- Environmental & Geological Engineering (AREA)
- Acoustics & Sound (AREA)
- General Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- Geophysics (AREA)
- Oceanography (AREA)
- Geophysics And Detection Of Objects (AREA)
- Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)
Abstract
Dual-mode seismic sensors are used to detect the velocity signature and the pressure signature of a reflected acoustic wavefield. The velocity and pressure signatures are resolved into up-going and down-going wavetrains which may be transformed from the time domain to the frequency domain. The transforms are cross-correlated.
The time delay that maximizes the zero-lag cross-correlation function is the two-way water-layer travel time.
The time delay that maximizes the zero-lag cross-correlation function is the two-way water-layer travel time.
Description
CA 0220~426 1997-0~
NETHOD FOR MEASURING THE WATER THICKNESS ABOVE A BOTTON
CABLE
A method for measuring the water column thickness above a seismic bottom cable using seismic reflection data gleaned from routine dual-sensor seismic data-acquisition operations.
In the art of seismic exploration, numbers of spaced-apart seismic sensors are distributed over a designated area of survey. At sea, the sensors may be towed through the water in a streamer cable or, in shallow water on the order of 150 meters depth or less, they may be laid directly on the sea floor. In the latter case, the sensors are mechanically and electrically interconnected by signal communication channels in a bottom cable. The cable is coupled to data-recording and processing equipment mounted aboard a seismic service ship as is well known. An acoustic source generates a wavefield in the water at a succession of designated stations over the survey area.
The station spacing is usually 25 to 50 meters, comparable to the spacing of the seismic sensors. The wavefield propagates radially in all directions to insonify sub-bottom earth layers whence the wavefield is reflected back towards the sea floor where it is detected by the sensors. The sensors convert the mechanical motions or pressure variations due to the seismic wavefield to amplitude-modulated electrical signals which are recorded for archival storage and, perhaps, partially processed in ship-borne data-recording equipment following discretization. The recorded seismic data are processed to provide a CA 0220~426 1997-0~
representation of the topography of selected sub-sea strata.
Sensors used in marine seismic exploration are usually pressure-sensitive hydrophones. For certain projects, geophones, which are responsive to particle velocity may be used in combination with the hydrophones as dual-mode sensors. For purposes of this disclosure, the term dual-mode means that sensors of different genera, which are used jointly to register a common seismic wavefield, may either be mounted together in a single case or they may be separate instruments that are laid next to each other on the water bottom. In some instances, accelerometers may be substituted in place of or in combination with the hydrophones. Accelerometers are responsive to changes in particle velocity.
Reference will be made to seismic signatures. For purposes of this disclosure, a seismic signature, such as the velocity signature, is defined as the variation in phase and amplitude, expressed in the time domain, of a waveform that is representative of a quantity under consideration. The unqualified term velocity means the velocity of propagation of an acoustic wavefield through a medium of interest. The term pressure refers to the pressure variation, usually in a fluid, due to the passage of a compressional wavefront.
Seismic signals are usually contaminated by noise.
Noise is defined as any unwanted signal such as the random noise of a ship's screw, marine-life soundings, wind noise, and the crashing of waves. Random noise of that type may be reduced by temporal or spatial filtering.
The acoustic wavefields are not only reflected from subsurface earth strata but may also be reflected many times between the sea floor and the sea surface much like the multiple reflections as seen in mirrors positioned facing each other. Both the sea floor and the sea surface are efficient reflectors having a CA 0220~426 1997-0~
coefficient of reflection that may approach unity.
Reflections from the water-surface, also referred to as ghost reflections, may have a very high amplitude and often reside in the same part of the seismic spectrum as desired primary reflections. Ghost reflections constitute a severe type of coherent interference which is not necessarily amenable to attenuation by temporal or spatial filtering.
US patent 5,36S,492, issued November 15, 1994, to William H. Dragoset and assigned to the assignee of this invention, teaches a method for canceling ghost reflections. In that method, a geophone and a hydrophone are co-located so as to see both the pressure signature and the velocity signature characteristic of a particular seismic transient. The pressure signature is adaptively filtered and subtracted from the velocity signature to isolate a nearly pure noise signature. The noise signature is added back to the velocity signature with opposite sign to clear away the embedded random noise, leaving a refined velocity signature. The refined velocity signature is scaled and summed with the pressure signature from the hydrophone to cancel the coherent noise of the ghost reflection.
A similar method is taught by W. H. Ruehle in US
patent 4,486,865 who reduces the ghost effect using dual-mode sensors. The output of one of the pair of sensors is gain-adjusted and filtered using a deconvolution operator having a preselected amount of white noise added to the zero lag of the autocorrelation function. The deconvolved gain-adjusted signal is added to the signal output from the other sensor to cancel the ghost. The two above references are concerned with ghost reflections but do not address water depth measurements.
US patent 4,146,871, issued March 27, 1979 also to W. H. Ruehle, teaches a ghost elimination method that employs measurements of the water depth as well as the CA 0220~426 1997-0~
sea floor reflectivity using arrays of hydrophones towed near the water surface. In the case of towed arrays, calculation of the water depth beneath surface-deployed sensors is a trivial task by means of first-arrival times using well-known methods.
K. P. Allen et al in US patent 4,234,938, issued November 18, 1980, discloses a method for the determination of the water depth using a towed array of hydrophones. The autocorrelation coefficients of a window of conventionally-produced seismograms are iteratively generated using n-sample lags where n=0,1,2,...,N. The coefficients are combined for various values of n to determine a minimum energy function, which value for n is a measure of the water depth. This patent teaches use of but a single genus of sensor towed near the surface.
A technique for separating an up-going wavefield from a down-going wavefield is taught by D. W. Bell et al in US Patent 4,794,573, issued December 27, 1988, which primarily applies to vertical seismic profiling in boreholes, but the method may be of interest in marine exploration applications. The process operates on two vertically-separated detectors at a time and is based on the concept that waves traveling in opposite directions have spatial derivatives of opposite sign. The derivative is approximated by the difference between the signals which is time integrated to recover the phase.
The resulting integrated difference signal I is then amplitude-scale corrected and combined by addition or subtraction with a signal S~ representing the sum of the two detector signals to form a succession of filtered signals which, when recorded in alignment in order of detector depths to form a vertical seismic profile, preserves either the up-going or the down-going seismic wave.
As earlier explained, the purpose of seismic data processing is to create a cross section of the earth to CA 0220~426 1997-0~
determine the depth and attitude of sub-sea earth layers. Depths to the respective strata are customarily referred to a sea-level datum. Since the seismic sensors are reposing on the water bottom, reflection times measured at the sensors must be referred back up the water surface. Water has a relatively low velocity relative to the velocity of earth layers. Variations in the water-layer thickness create false structures in the subsurface topography if not properly compensated.
Conventional water-depth measurements from first arrival times of seismic recordings are not possible for bottom-disposed sensors. The depth of the water can be measured using tools such as a fathometer, but that procedure would require that a special survey ship visit each and every seismic sensor in the survey area.
Inasmuch as many thousands of sensors may have been distributed over a large survey region, that practice would be decidedly uneconomical.
There is a need for a practical way to measure the depth of water above water-bottom deployed seismic sensors.
The present invention allows one to measure the water thickness above each of the sensors by measuring the time lag between primary reflections and the corresponding ghost reflection, using ordinary routinely-gathered dual-sensor reflection data. The present invention therefore puts to good use the nuisance data that was heretofore considered to be unusable.
A method is provided for measuring the thickness of a water layer above an array of dual-mode seismic sensors emplaced on the water bottom. A plurality of reflected wavefields are successively generated from each of a plurality of source locations. The reflected wavefields are characterized by a velocity signature and a pressure signature. The velocity and pressure signatures corresponding to the respective reflected CA 0220~426 1997-0~
wavefields are detected jointly by a dual-mode sensor and formatted in the time domain at a selected receiver location as members of a common receiver gather. The pressure and velocity signatures comprising each member of the common receiver gather are resolved into up-going and down-going wavetrains which are transformed from the time domain into the frequency domain. A time delay operator is applied to the transforms of the up-going wavefields. The delayed transforms of the up-going wavefields and the transforms of the downgoing wavefields are iteratively cross-correlated, discretely perturbing the time delay operator after each iteration.
The time lags that maximize the zero-lag cross correlation functions for each member of the common receiver gather are averaged to provide a measure of the thickness of the water layer.
In an aspect of this method, the transforms of the up-going and the down-going wavefields are auto-correlated. The auto-correlated members of the common receiver gather are stacked, followed by application of a time delay operator and subsequent cross-correlation of the auto-correlations.
In another aspect of this invention, a noise-abatement filter operator is applied to the velocity wavefield prior to the step of resolving.
The novel features which are believed to be characteristic of the invention, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the invention is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the invention:
FIGURE 1 illustrates the geometry of the wavefield trajectories useful in measuring the thickness of a water layer;
CA 0220~426 1997-0~
FIGURE 2 is a suite of four time-scale traces showing the pressure signature, velocity signature, resolved up-going and resolved down-going seismic transients as derived from one member of a common receiver gather;
FIGURE 3 is a graph of the time delay progression during an iterative cross-correlation process.
In Figure 1, a particle-velocity responsive geophone V1 and a pressure responsive hydrophone P1 are laid next to each other on the bottom 10 of a body of water 12, the surface of which is designated as 14. For purposes of this disclosure, units V1 and P1 are sensor components that comprise a dual-mode sensor. To avoid complicating the drawings, only three spaced-apart dual-mode sensors are shown, but it is to be understood thatin practice many hundreds or even thousands of such sensors may be laid out in practice by a cable servicing ship.
An acoustic source, which may be towed at or near the water surface by a shooting ship, generates a wavefield in the water at successive spaced-apart stations such as 16, 18, 20 that are offset from the surface projections of the dual-mode sensor locations.
The offset is a selected multiple of the station spacing which, in turn may be the same as the dual-sensor spacing.
Each wavefield, as it is generated, propagates along appropriate trajectories such as 22, 24, and 26, through water layer 12 to insonify subsurface earth strata such as 28, whence the wavefield is reflected back towards the surface to be jointly detected by both components of the dual-mode sensor such as V1 and P1 on the water bottom 10. Trajectories 22, 24 and 26 are shown converging at V1 to avoid complicating the drawings but it should be understood that P1 is co-located with V1 and receives the same energy over the same travel path. Wavefield trajectories from three CA 0220~426 1997-0~
source stations converging towards V1,P1, form a common receiver gather having three members. Figure 2 is a display of the pressure and velocity signatures for one member of a common receiver gather formatted as a time scale display. Five reflected seismic events are distributed over a reflection-time gate of 1.5 seconds.
In addition to the reflected wavefields, seismic energy arrives at the sensors by way of a direct path such as 30, through the water 12 or refracted along the water/bottom interface such as by path 32. For modest-length offsets up to 600 meters or less, the direct and refracted arrivals appear quite early on the time scale recordings, well ahead of the shallow reflected arrivals. In addition, by employing such a relatively short offset gate the data trajectories approximate normal-incidence ray paths as indicated by dashed line 34, Figure 1, thus avoiding the need for extensive preprocessing. The angularity of the trajectories 22, 24, 26 in Figure 1 has been exaggerated for illustrative purposes. In actual field work, the offset is short compared to the depth to a reflector of interest, so that the ray paths do indeed approach normal incidence.
Figure 2 is a display of four time-scale seismic traces. Trace A represents the synthetically-generated pressure signatures for the primary and first ghost reflection of five reflected events. Trace B represents the corresponding velocity signatures. The remaining two traces will be discussed later.
Referring first to Figure 1, consider a geophone at depth h in a water layer having a velocity c (about 1500 m/s in salt water). Let the normally-incident pulse 34 traveling upward past geophone V1 at time t=0, impinge on the bottom of the geophone, to generate a positive-going output signal such as 36, trace B, Figure 2. For purposes of clarity, the second well-developed cycle of the signal envelope is chosen as the argument.
Continuing upward at 38, Figure 1, the pulse encounters CA 0220~426 1997-0~
the surface at time 0.5r = 2h/c. The air has a lower impedance than the water. The reflection of the velocity wave suffers no phase shift, that is, an upward-moving particle remains upward moving after reflection. After time r = 2h/c, the downward traveling pulse 40, Figure 1, impinges on the top of the geophone V1 which again registers positive as shown by 41, trace B Figure 2. The impulse response G(~)v of the velocity-signature ghost reflection is therefore G(~)V = 1 - RsZ~ O<Rs<l, Z = e~ i~r Values for the surface reflectivity Rs lie in the range of 0.7 - 0.9.
Consider now the hydrophone, Pl also at depth h in the water. A normally-incident seismic pressure pulse 34 traveling upward registers as a positive hydrophone output pulse at time t=O as shown at 43, trace A, Figure 2 and continuing upward along 38, Figure 1, as before.
For an ideal reflector, the pressure at the surface is zero; up- and down-going pulses cancel at the surface.
Therefore, the reflected downgoing ghost pressure reflection 44 is a rarefaction. At time r [r=2h/c] the hydrophone registers a negative event 46, trace A, Figure 2. The impulse response G(~)p for the hydrophone pressure pulse is therefore G(~)p = 1 + RsZ-It is apparent that the velocity (V) and the pressure (P) signatures can be resolved into up-going (U) and down-going (D) energy components from:
U = P + V, (1) D = P - V. (2) The two-way travel time in the water layer can be determined from reflection seismic data by finding the time delay for which the up-going wave most closely corresponds with the down-going wave as will be explained next.
CA 0220~426 1997-0~
In the presently preferred method of operation, a time gate within the wavefield is selected. A time gate is selected having a span of about one second which would include the first four events on traces A and B of Figure 2, for example. The time window should be selected from a shallow portion of the section where relatively clean reflection data are available but not so shallow as to receive interference from direct and refracted arrivals. Common receiver gathers including a desired number, such as 25-50, time scale traces representative of velocity and pressure wavefields in the time domain, are selected, preferably having offsets of less than 600 meters.
The seismic signals represented by the pressure and velocity signatures corresponding to each trace are resolved into up-going and down-going wavefields from formulations (1) and (2). Traces C and D represent the primary up-going and reflected down-going or ghost wavefields respectively. Because the traces shown in Figure 2 were synthetically constructed for tutorial purposes, the time delay, 48, that is, the difference in arrival times, indicated by 52 and 54 between primary and ghost due to the thickness of the water layer, readily can be determined by inspection to be about 40 milliseconds which, at a water velocity of 1500 m/s, would be 30 meters.
In real life, however, noise, instrumental artifacts, the filtering effect of the earth itself, as well as mutual reflection interference in the presence of complex geology, all conspire to so complicate field data as to require substantially more sophisticated analyses than simple inspection as suggested by Figure 2. A preferred method of operation employs well-known cross-correlation methods.
The wavefields resolved into up-going and down-going wavetrains are transformed from the time domain to the frequency domain such as by the well-known fast CA 0220~426 1997-0~
Fourier transform algorithm. A time delay operator is applied to the transform of the up-going wavefield. The delayed up-going and the down-going transforms are iteratively cross-correlated, discretely perturbing the time delay operator after each iteration, such as by a time shift corresponding to a preferred increment of water depth, such as 10 centimeters, divided by the water velocity. Cross-correlation can most easily be done in the frequency domain because the correlation process reduces to simple multiplication of the Fourier spectra of the delayed up-going and the down-going signal. The time lag that maximizes the zero-lag cross-correlation function is a measure of the thickness of the water layer as shown by the dotted function 50 shown on the graph of Figure 3. Figure 3 shows the results of iteratively cross-correlating traces C and D of Figure 2 with a zero-lag correlation peak at about 30 meters.
Preferably, because of the sheer volume of calculations needed for a large survey area, the method of this invention is computer implemented.
Preferably, the results of the cross-correlation of each of the members of a common receiver gather are averaged to provide an average water layer thickness.
In an alternate method, the transforms of the velocity and pressure signatures of each member of a common receiver gather are auto-correlated. The auto-correlations are then stacked and the stacked auto-correlations are then cross-correlated.
In areas of severe noise, it is contemplated that the noise abatement filter, such as is taught by the '492 reference, will be applied.
It is to be understood that the seismic data are not gathered to provide a solution for a naked formulation. The method and formulations disclosed here are provided to more clearly depict sub-bottom earth-layer topography by providing a continuous profile of the water-layer thickness. Using that information, CA 0220~426 1997-0~
static corrections can be generated to compensate for the anomalous effect of an irregular low-velocity water layer.
This invention has been described with a certain degree of specificity by way of example but not by way of limitation. Those skilled in the art will devise obvious variations to the examples given herein. For example, ghost reflection operators can be computed for the velocity and pressure signatures to create a second pair of pseudo-pressure and pseudo-velocity traces from the up-going wavefield. Minimization of the measured and pseudo pressure and velocity traces is a measure of the correct two-way travel time in the ghost operator.
Another method would use the effects of the ghost filters to equalize the reflection sequence for the pressure and the velocity signals. Minimization of the difference between the equalized traces indicates the correct parameters in the ghost filter. By way of example, but not by way of limitation, the acoustic source is shown at or near the water surface. The source may be deployed anywhere within the water layer or it may be immersed in the mud at the bottom of the water. All such alternate but equivalent methods will fall within the scope of this invention which is limited only by the appended claims.
NETHOD FOR MEASURING THE WATER THICKNESS ABOVE A BOTTON
CABLE
A method for measuring the water column thickness above a seismic bottom cable using seismic reflection data gleaned from routine dual-sensor seismic data-acquisition operations.
In the art of seismic exploration, numbers of spaced-apart seismic sensors are distributed over a designated area of survey. At sea, the sensors may be towed through the water in a streamer cable or, in shallow water on the order of 150 meters depth or less, they may be laid directly on the sea floor. In the latter case, the sensors are mechanically and electrically interconnected by signal communication channels in a bottom cable. The cable is coupled to data-recording and processing equipment mounted aboard a seismic service ship as is well known. An acoustic source generates a wavefield in the water at a succession of designated stations over the survey area.
The station spacing is usually 25 to 50 meters, comparable to the spacing of the seismic sensors. The wavefield propagates radially in all directions to insonify sub-bottom earth layers whence the wavefield is reflected back towards the sea floor where it is detected by the sensors. The sensors convert the mechanical motions or pressure variations due to the seismic wavefield to amplitude-modulated electrical signals which are recorded for archival storage and, perhaps, partially processed in ship-borne data-recording equipment following discretization. The recorded seismic data are processed to provide a CA 0220~426 1997-0~
representation of the topography of selected sub-sea strata.
Sensors used in marine seismic exploration are usually pressure-sensitive hydrophones. For certain projects, geophones, which are responsive to particle velocity may be used in combination with the hydrophones as dual-mode sensors. For purposes of this disclosure, the term dual-mode means that sensors of different genera, which are used jointly to register a common seismic wavefield, may either be mounted together in a single case or they may be separate instruments that are laid next to each other on the water bottom. In some instances, accelerometers may be substituted in place of or in combination with the hydrophones. Accelerometers are responsive to changes in particle velocity.
Reference will be made to seismic signatures. For purposes of this disclosure, a seismic signature, such as the velocity signature, is defined as the variation in phase and amplitude, expressed in the time domain, of a waveform that is representative of a quantity under consideration. The unqualified term velocity means the velocity of propagation of an acoustic wavefield through a medium of interest. The term pressure refers to the pressure variation, usually in a fluid, due to the passage of a compressional wavefront.
Seismic signals are usually contaminated by noise.
Noise is defined as any unwanted signal such as the random noise of a ship's screw, marine-life soundings, wind noise, and the crashing of waves. Random noise of that type may be reduced by temporal or spatial filtering.
The acoustic wavefields are not only reflected from subsurface earth strata but may also be reflected many times between the sea floor and the sea surface much like the multiple reflections as seen in mirrors positioned facing each other. Both the sea floor and the sea surface are efficient reflectors having a CA 0220~426 1997-0~
coefficient of reflection that may approach unity.
Reflections from the water-surface, also referred to as ghost reflections, may have a very high amplitude and often reside in the same part of the seismic spectrum as desired primary reflections. Ghost reflections constitute a severe type of coherent interference which is not necessarily amenable to attenuation by temporal or spatial filtering.
US patent 5,36S,492, issued November 15, 1994, to William H. Dragoset and assigned to the assignee of this invention, teaches a method for canceling ghost reflections. In that method, a geophone and a hydrophone are co-located so as to see both the pressure signature and the velocity signature characteristic of a particular seismic transient. The pressure signature is adaptively filtered and subtracted from the velocity signature to isolate a nearly pure noise signature. The noise signature is added back to the velocity signature with opposite sign to clear away the embedded random noise, leaving a refined velocity signature. The refined velocity signature is scaled and summed with the pressure signature from the hydrophone to cancel the coherent noise of the ghost reflection.
A similar method is taught by W. H. Ruehle in US
patent 4,486,865 who reduces the ghost effect using dual-mode sensors. The output of one of the pair of sensors is gain-adjusted and filtered using a deconvolution operator having a preselected amount of white noise added to the zero lag of the autocorrelation function. The deconvolved gain-adjusted signal is added to the signal output from the other sensor to cancel the ghost. The two above references are concerned with ghost reflections but do not address water depth measurements.
US patent 4,146,871, issued March 27, 1979 also to W. H. Ruehle, teaches a ghost elimination method that employs measurements of the water depth as well as the CA 0220~426 1997-0~
sea floor reflectivity using arrays of hydrophones towed near the water surface. In the case of towed arrays, calculation of the water depth beneath surface-deployed sensors is a trivial task by means of first-arrival times using well-known methods.
K. P. Allen et al in US patent 4,234,938, issued November 18, 1980, discloses a method for the determination of the water depth using a towed array of hydrophones. The autocorrelation coefficients of a window of conventionally-produced seismograms are iteratively generated using n-sample lags where n=0,1,2,...,N. The coefficients are combined for various values of n to determine a minimum energy function, which value for n is a measure of the water depth. This patent teaches use of but a single genus of sensor towed near the surface.
A technique for separating an up-going wavefield from a down-going wavefield is taught by D. W. Bell et al in US Patent 4,794,573, issued December 27, 1988, which primarily applies to vertical seismic profiling in boreholes, but the method may be of interest in marine exploration applications. The process operates on two vertically-separated detectors at a time and is based on the concept that waves traveling in opposite directions have spatial derivatives of opposite sign. The derivative is approximated by the difference between the signals which is time integrated to recover the phase.
The resulting integrated difference signal I is then amplitude-scale corrected and combined by addition or subtraction with a signal S~ representing the sum of the two detector signals to form a succession of filtered signals which, when recorded in alignment in order of detector depths to form a vertical seismic profile, preserves either the up-going or the down-going seismic wave.
As earlier explained, the purpose of seismic data processing is to create a cross section of the earth to CA 0220~426 1997-0~
determine the depth and attitude of sub-sea earth layers. Depths to the respective strata are customarily referred to a sea-level datum. Since the seismic sensors are reposing on the water bottom, reflection times measured at the sensors must be referred back up the water surface. Water has a relatively low velocity relative to the velocity of earth layers. Variations in the water-layer thickness create false structures in the subsurface topography if not properly compensated.
Conventional water-depth measurements from first arrival times of seismic recordings are not possible for bottom-disposed sensors. The depth of the water can be measured using tools such as a fathometer, but that procedure would require that a special survey ship visit each and every seismic sensor in the survey area.
Inasmuch as many thousands of sensors may have been distributed over a large survey region, that practice would be decidedly uneconomical.
There is a need for a practical way to measure the depth of water above water-bottom deployed seismic sensors.
The present invention allows one to measure the water thickness above each of the sensors by measuring the time lag between primary reflections and the corresponding ghost reflection, using ordinary routinely-gathered dual-sensor reflection data. The present invention therefore puts to good use the nuisance data that was heretofore considered to be unusable.
A method is provided for measuring the thickness of a water layer above an array of dual-mode seismic sensors emplaced on the water bottom. A plurality of reflected wavefields are successively generated from each of a plurality of source locations. The reflected wavefields are characterized by a velocity signature and a pressure signature. The velocity and pressure signatures corresponding to the respective reflected CA 0220~426 1997-0~
wavefields are detected jointly by a dual-mode sensor and formatted in the time domain at a selected receiver location as members of a common receiver gather. The pressure and velocity signatures comprising each member of the common receiver gather are resolved into up-going and down-going wavetrains which are transformed from the time domain into the frequency domain. A time delay operator is applied to the transforms of the up-going wavefields. The delayed transforms of the up-going wavefields and the transforms of the downgoing wavefields are iteratively cross-correlated, discretely perturbing the time delay operator after each iteration.
The time lags that maximize the zero-lag cross correlation functions for each member of the common receiver gather are averaged to provide a measure of the thickness of the water layer.
In an aspect of this method, the transforms of the up-going and the down-going wavefields are auto-correlated. The auto-correlated members of the common receiver gather are stacked, followed by application of a time delay operator and subsequent cross-correlation of the auto-correlations.
In another aspect of this invention, a noise-abatement filter operator is applied to the velocity wavefield prior to the step of resolving.
The novel features which are believed to be characteristic of the invention, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the invention is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the invention:
FIGURE 1 illustrates the geometry of the wavefield trajectories useful in measuring the thickness of a water layer;
CA 0220~426 1997-0~
FIGURE 2 is a suite of four time-scale traces showing the pressure signature, velocity signature, resolved up-going and resolved down-going seismic transients as derived from one member of a common receiver gather;
FIGURE 3 is a graph of the time delay progression during an iterative cross-correlation process.
In Figure 1, a particle-velocity responsive geophone V1 and a pressure responsive hydrophone P1 are laid next to each other on the bottom 10 of a body of water 12, the surface of which is designated as 14. For purposes of this disclosure, units V1 and P1 are sensor components that comprise a dual-mode sensor. To avoid complicating the drawings, only three spaced-apart dual-mode sensors are shown, but it is to be understood thatin practice many hundreds or even thousands of such sensors may be laid out in practice by a cable servicing ship.
An acoustic source, which may be towed at or near the water surface by a shooting ship, generates a wavefield in the water at successive spaced-apart stations such as 16, 18, 20 that are offset from the surface projections of the dual-mode sensor locations.
The offset is a selected multiple of the station spacing which, in turn may be the same as the dual-sensor spacing.
Each wavefield, as it is generated, propagates along appropriate trajectories such as 22, 24, and 26, through water layer 12 to insonify subsurface earth strata such as 28, whence the wavefield is reflected back towards the surface to be jointly detected by both components of the dual-mode sensor such as V1 and P1 on the water bottom 10. Trajectories 22, 24 and 26 are shown converging at V1 to avoid complicating the drawings but it should be understood that P1 is co-located with V1 and receives the same energy over the same travel path. Wavefield trajectories from three CA 0220~426 1997-0~
source stations converging towards V1,P1, form a common receiver gather having three members. Figure 2 is a display of the pressure and velocity signatures for one member of a common receiver gather formatted as a time scale display. Five reflected seismic events are distributed over a reflection-time gate of 1.5 seconds.
In addition to the reflected wavefields, seismic energy arrives at the sensors by way of a direct path such as 30, through the water 12 or refracted along the water/bottom interface such as by path 32. For modest-length offsets up to 600 meters or less, the direct and refracted arrivals appear quite early on the time scale recordings, well ahead of the shallow reflected arrivals. In addition, by employing such a relatively short offset gate the data trajectories approximate normal-incidence ray paths as indicated by dashed line 34, Figure 1, thus avoiding the need for extensive preprocessing. The angularity of the trajectories 22, 24, 26 in Figure 1 has been exaggerated for illustrative purposes. In actual field work, the offset is short compared to the depth to a reflector of interest, so that the ray paths do indeed approach normal incidence.
Figure 2 is a display of four time-scale seismic traces. Trace A represents the synthetically-generated pressure signatures for the primary and first ghost reflection of five reflected events. Trace B represents the corresponding velocity signatures. The remaining two traces will be discussed later.
Referring first to Figure 1, consider a geophone at depth h in a water layer having a velocity c (about 1500 m/s in salt water). Let the normally-incident pulse 34 traveling upward past geophone V1 at time t=0, impinge on the bottom of the geophone, to generate a positive-going output signal such as 36, trace B, Figure 2. For purposes of clarity, the second well-developed cycle of the signal envelope is chosen as the argument.
Continuing upward at 38, Figure 1, the pulse encounters CA 0220~426 1997-0~
the surface at time 0.5r = 2h/c. The air has a lower impedance than the water. The reflection of the velocity wave suffers no phase shift, that is, an upward-moving particle remains upward moving after reflection. After time r = 2h/c, the downward traveling pulse 40, Figure 1, impinges on the top of the geophone V1 which again registers positive as shown by 41, trace B Figure 2. The impulse response G(~)v of the velocity-signature ghost reflection is therefore G(~)V = 1 - RsZ~ O<Rs<l, Z = e~ i~r Values for the surface reflectivity Rs lie in the range of 0.7 - 0.9.
Consider now the hydrophone, Pl also at depth h in the water. A normally-incident seismic pressure pulse 34 traveling upward registers as a positive hydrophone output pulse at time t=O as shown at 43, trace A, Figure 2 and continuing upward along 38, Figure 1, as before.
For an ideal reflector, the pressure at the surface is zero; up- and down-going pulses cancel at the surface.
Therefore, the reflected downgoing ghost pressure reflection 44 is a rarefaction. At time r [r=2h/c] the hydrophone registers a negative event 46, trace A, Figure 2. The impulse response G(~)p for the hydrophone pressure pulse is therefore G(~)p = 1 + RsZ-It is apparent that the velocity (V) and the pressure (P) signatures can be resolved into up-going (U) and down-going (D) energy components from:
U = P + V, (1) D = P - V. (2) The two-way travel time in the water layer can be determined from reflection seismic data by finding the time delay for which the up-going wave most closely corresponds with the down-going wave as will be explained next.
CA 0220~426 1997-0~
In the presently preferred method of operation, a time gate within the wavefield is selected. A time gate is selected having a span of about one second which would include the first four events on traces A and B of Figure 2, for example. The time window should be selected from a shallow portion of the section where relatively clean reflection data are available but not so shallow as to receive interference from direct and refracted arrivals. Common receiver gathers including a desired number, such as 25-50, time scale traces representative of velocity and pressure wavefields in the time domain, are selected, preferably having offsets of less than 600 meters.
The seismic signals represented by the pressure and velocity signatures corresponding to each trace are resolved into up-going and down-going wavefields from formulations (1) and (2). Traces C and D represent the primary up-going and reflected down-going or ghost wavefields respectively. Because the traces shown in Figure 2 were synthetically constructed for tutorial purposes, the time delay, 48, that is, the difference in arrival times, indicated by 52 and 54 between primary and ghost due to the thickness of the water layer, readily can be determined by inspection to be about 40 milliseconds which, at a water velocity of 1500 m/s, would be 30 meters.
In real life, however, noise, instrumental artifacts, the filtering effect of the earth itself, as well as mutual reflection interference in the presence of complex geology, all conspire to so complicate field data as to require substantially more sophisticated analyses than simple inspection as suggested by Figure 2. A preferred method of operation employs well-known cross-correlation methods.
The wavefields resolved into up-going and down-going wavetrains are transformed from the time domain to the frequency domain such as by the well-known fast CA 0220~426 1997-0~
Fourier transform algorithm. A time delay operator is applied to the transform of the up-going wavefield. The delayed up-going and the down-going transforms are iteratively cross-correlated, discretely perturbing the time delay operator after each iteration, such as by a time shift corresponding to a preferred increment of water depth, such as 10 centimeters, divided by the water velocity. Cross-correlation can most easily be done in the frequency domain because the correlation process reduces to simple multiplication of the Fourier spectra of the delayed up-going and the down-going signal. The time lag that maximizes the zero-lag cross-correlation function is a measure of the thickness of the water layer as shown by the dotted function 50 shown on the graph of Figure 3. Figure 3 shows the results of iteratively cross-correlating traces C and D of Figure 2 with a zero-lag correlation peak at about 30 meters.
Preferably, because of the sheer volume of calculations needed for a large survey area, the method of this invention is computer implemented.
Preferably, the results of the cross-correlation of each of the members of a common receiver gather are averaged to provide an average water layer thickness.
In an alternate method, the transforms of the velocity and pressure signatures of each member of a common receiver gather are auto-correlated. The auto-correlations are then stacked and the stacked auto-correlations are then cross-correlated.
In areas of severe noise, it is contemplated that the noise abatement filter, such as is taught by the '492 reference, will be applied.
It is to be understood that the seismic data are not gathered to provide a solution for a naked formulation. The method and formulations disclosed here are provided to more clearly depict sub-bottom earth-layer topography by providing a continuous profile of the water-layer thickness. Using that information, CA 0220~426 1997-0~
static corrections can be generated to compensate for the anomalous effect of an irregular low-velocity water layer.
This invention has been described with a certain degree of specificity by way of example but not by way of limitation. Those skilled in the art will devise obvious variations to the examples given herein. For example, ghost reflection operators can be computed for the velocity and pressure signatures to create a second pair of pseudo-pressure and pseudo-velocity traces from the up-going wavefield. Minimization of the measured and pseudo pressure and velocity traces is a measure of the correct two-way travel time in the ghost operator.
Another method would use the effects of the ghost filters to equalize the reflection sequence for the pressure and the velocity signals. Minimization of the difference between the equalized traces indicates the correct parameters in the ghost filter. By way of example, but not by way of limitation, the acoustic source is shown at or near the water surface. The source may be deployed anywhere within the water layer or it may be immersed in the mud at the bottom of the water. All such alternate but equivalent methods will fall within the scope of this invention which is limited only by the appended claims.
Claims (5)
1. A computer-implemented method for measuring the thickness of a water layer above an array of dual-mode seismic sensors, comprising:
a) generating a reflected seismic wavefield from a first acoustic source location, said wavefield being characterized by a pressure signature and a velocity signature;
b) detecting the pressure and velocity signatures of the seismic wavefield by at least one dual-mode seismic sensor at a second location offset by a preselected distance from the source location and formatting the so-detected pressure and velocity signatures in the time domain;
c) resolving the pressure and velocity signatures into up-going and down-going wavetrains;
d) transforming the up-going and down-going wavetrains from the time domain to the frequency domain;
e) applying a time delay operator to the transform of the up-going wavetrain;
f) iteratively cross-correlating the delayed up-going wavetrain transform with the down-going wavetrain transform, discretely perturbing the time delay operator after each iteration;
g) accepting the time delay that maximizes the zero-lag cross-correlation function as the two-way water-layer travel time.
a) generating a reflected seismic wavefield from a first acoustic source location, said wavefield being characterized by a pressure signature and a velocity signature;
b) detecting the pressure and velocity signatures of the seismic wavefield by at least one dual-mode seismic sensor at a second location offset by a preselected distance from the source location and formatting the so-detected pressure and velocity signatures in the time domain;
c) resolving the pressure and velocity signatures into up-going and down-going wavetrains;
d) transforming the up-going and down-going wavetrains from the time domain to the frequency domain;
e) applying a time delay operator to the transform of the up-going wavetrain;
f) iteratively cross-correlating the delayed up-going wavetrain transform with the down-going wavetrain transform, discretely perturbing the time delay operator after each iteration;
g) accepting the time delay that maximizes the zero-lag cross-correlation function as the two-way water-layer travel time.
2. The method as defined by claim 1, comprising:
successively generating a plurality of acoustic wavefields at a like plurality of spaced-apart source locations;
detecting the pressure and velocity signatures of the respective ones of said plurality of wavefields by a dual-mode seismic sensor at a second location offset from the successive source locations and formatted as members of a common receiver gather;
executing steps c) through f) for each of the respective wavefields; and h) averaging the zero-lag time delays resulting from each cross-correlation operation on each member of the common receiver gather.
successively generating a plurality of acoustic wavefields at a like plurality of spaced-apart source locations;
detecting the pressure and velocity signatures of the respective ones of said plurality of wavefields by a dual-mode seismic sensor at a second location offset from the successive source locations and formatted as members of a common receiver gather;
executing steps c) through f) for each of the respective wavefields; and h) averaging the zero-lag time delays resulting from each cross-correlation operation on each member of the common receiver gather.
3. The method as defined by claim 2; comprising:
auto-correlating the transforms of the resolved up-going and down-going wavetrains for each of the respective wavefields in the common receiver gather; and executing steps d) through g)
auto-correlating the transforms of the resolved up-going and down-going wavetrains for each of the respective wavefields in the common receiver gather; and executing steps d) through g)
4. The method as defined by claim 3, comprising:
applying an adaptive noise abatement filter to the velocity signatures of said plurality of acoustic wavefields.
applying an adaptive noise abatement filter to the velocity signatures of said plurality of acoustic wavefields.
5. The method as defined by claim 1, comprising:
performing steps c) through g) over a preselected seismic reflection travel-time gate within the so-generated wavefield.
performing steps c) through g) over a preselected seismic reflection travel-time gate within the so-generated wavefield.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US65199496A | 1996-05-21 | 1996-05-21 | |
| US08/651,994 | 1996-05-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2205426A1 true CA2205426A1 (en) | 1997-11-21 |
Family
ID=24615086
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2205426 Abandoned CA2205426A1 (en) | 1996-05-21 | 1997-05-15 | Method for measuring the water thickness above a bottom cable |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA2205426A1 (en) |
| ID (1) | ID16925A (en) |
| NO (1) | NO971151D0 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2344889A (en) * | 1998-12-18 | 2000-06-21 | Geco As | Marine seismic signal analysis method |
| WO2001020363A2 (en) * | 1999-09-15 | 2001-03-22 | Pgs Data Processing, Inc. | Amplitude spectra estimation |
| WO2002001254A1 (en) * | 2000-06-29 | 2002-01-03 | Geco A.S. | A method of processing seismic data |
-
1997
- 1997-03-12 NO NO971151A patent/NO971151D0/en unknown
- 1997-05-15 CA CA 2205426 patent/CA2205426A1/en not_active Abandoned
- 1997-05-19 ID IDP971647A patent/ID16925A/en unknown
Cited By (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2344889B (en) * | 1998-12-18 | 2001-04-04 | Geco As | Seismic signal analysis method |
| GB2344889A (en) * | 1998-12-18 | 2000-06-21 | Geco As | Marine seismic signal analysis method |
| GB2370666B (en) * | 1999-09-15 | 2004-01-21 | Pgs Data Proc Inc | Amplitude spectra estimation |
| WO2001020363A2 (en) * | 1999-09-15 | 2001-03-22 | Pgs Data Processing, Inc. | Amplitude spectra estimation |
| AU772320B2 (en) * | 1999-09-15 | 2004-04-22 | Pgs Data Processing, Inc | Amplitude spectra estimation |
| WO2001020363A3 (en) * | 1999-09-15 | 2002-03-14 | Pgs Data Proc Inc | Amplitude spectra estimation |
| GB2370666A (en) * | 1999-09-15 | 2002-07-03 | Pgs Data Proc Inc | Amplitude spectra estimation |
| GB2379981A (en) * | 2000-06-29 | 2003-03-26 | Westerngeco As | A method of processing seismic data |
| WO2002001254A1 (en) * | 2000-06-29 | 2002-01-03 | Geco A.S. | A method of processing seismic data |
| GB2379981B (en) * | 2000-06-29 | 2004-05-19 | Westerngeco As | A method of processing seismic data |
| AU2001274411B2 (en) * | 2000-06-29 | 2005-03-24 | Westerngeco As | A method of processing seismic data |
| US7068568B2 (en) | 2000-06-29 | 2006-06-27 | Westerngeco, L.L.C. | Method of processing seismic data |
| NO337327B1 (en) * | 2000-06-29 | 2016-03-14 | Westerngeco As | Procedure for calibrating a seismic receiver |
Also Published As
| Publication number | Publication date |
|---|---|
| NO971151D0 (en) | 1997-03-12 |
| ID16925A (en) | 1997-11-20 |
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