CA1219944A - Method for finding the acoustic properties of seabed materials - Google Patents
Method for finding the acoustic properties of seabed materialsInfo
- Publication number
- CA1219944A CA1219944A CA000474528A CA474528A CA1219944A CA 1219944 A CA1219944 A CA 1219944A CA 000474528 A CA000474528 A CA 000474528A CA 474528 A CA474528 A CA 474528A CA 1219944 A CA1219944 A CA 1219944A
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- seabed
- seismogram
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- 238000000034 method Methods 0.000 title claims abstract description 77
- 239000000463 material Substances 0.000 title claims description 15
- 239000013049 sediment Substances 0.000 claims abstract description 20
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 18
- 238000013178 mathematical model Methods 0.000 claims abstract description 8
- 238000000518 rheometry Methods 0.000 claims description 12
- 239000012530 fluid Substances 0.000 claims description 7
- 239000011148 porous material Substances 0.000 claims description 7
- 238000011835 investigation Methods 0.000 claims description 5
- 230000008569 process Effects 0.000 claims description 5
- 238000003491 array Methods 0.000 claims description 4
- 238000010276 construction Methods 0.000 claims description 4
- 238000010304 firing Methods 0.000 claims description 3
- 238000002310 reflectometry Methods 0.000 claims description 3
- 230000001960 triggered effect Effects 0.000 claims description 3
- 239000002360 explosive Substances 0.000 claims description 2
- 230000008571 general function Effects 0.000 claims description 2
- 230000001788 irregular Effects 0.000 claims description 2
- 238000005259 measurement Methods 0.000 claims description 2
- 230000007246 mechanism Effects 0.000 claims description 2
- 230000035699 permeability Effects 0.000 claims description 2
- 238000012545 processing Methods 0.000 claims description 2
- 239000011343 solid material Substances 0.000 claims description 2
- 238000004364 calculation method Methods 0.000 claims 1
- 230000021715 photosynthesis, light harvesting Effects 0.000 claims 1
- 230000000007 visual effect Effects 0.000 description 2
- 238000012935 Averaging Methods 0.000 description 1
- 241000271566 Aves Species 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 238000009795 derivation Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000003631 expected effect Effects 0.000 description 1
- 244000145841 kine Species 0.000 description 1
- 238000001615 p wave Methods 0.000 description 1
- 229920000136 polysorbate Polymers 0.000 description 1
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- 238000012360 testing method Methods 0.000 description 1
- 239000003643 water by type Substances 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
- G01V1/3808—Seismic data acquisition, e.g. survey design
-
- 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
- G01V1/282—Application of seismic models, synthetic seismograms
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V2210/00—Details of seismic processing or analysis
- G01V2210/60—Analysis
- G01V2210/61—Analysis by combining or comparing a seismic data set with other data
- G01V2210/614—Synthetically generated data
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V2210/00—Details of seismic processing or analysis
- G01V2210/60—Analysis
- G01V2210/67—Wave propagation modeling
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- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Remote Sensing (AREA)
- Acoustics & Sound (AREA)
- Environmental & Geological Engineering (AREA)
- Geology (AREA)
- General Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- Geophysics (AREA)
- Oceanography (AREA)
- Geophysics And Detection Of Objects (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method of investigating surficial sea floor sediments comprises towing a plurality of acoustic sensors through the water near the sea floor, generating acoustic signals in the water at locations such that the acoustic signals propagate at least in part through the surficial sediment to the acoustic sensors recording the waveform of the signals received by the acoustic sensors to produce a field seismogram, constructing a synthetic seismogram on the basis of a postulated earth structure for the surficial sedi-ment using mathematical modelling techniques, comparing the observed field seismogram with the synthetic seismogram, and adjusting the mathematical model to best fit the recorded data so as to obtain information relating to the sediment structure. With this method the geologist can obtain considerably more information about the sediment structure than is available from simple time-of-travel techniques.
A method of investigating surficial sea floor sediments comprises towing a plurality of acoustic sensors through the water near the sea floor, generating acoustic signals in the water at locations such that the acoustic signals propagate at least in part through the surficial sediment to the acoustic sensors recording the waveform of the signals received by the acoustic sensors to produce a field seismogram, constructing a synthetic seismogram on the basis of a postulated earth structure for the surficial sedi-ment using mathematical modelling techniques, comparing the observed field seismogram with the synthetic seismogram, and adjusting the mathematical model to best fit the recorded data so as to obtain information relating to the sediment structure. With this method the geologist can obtain considerably more information about the sediment structure than is available from simple time-of-travel techniques.
Description
The invention relates to a method of investigating surEicial seafloor materials.
The investigation of seabed materials is of interest to geologists, engineers and underwater acousticians. Geoloyists and engineers are interested in the acoustic properties oE these materials in that such proper-ties can be related to the rnechanical properties of the materials, whereas underwater acousticians generally require to know the acoustic properties in order to make more accurate predictions about sound propagation in the overlying water column. Methods used at present for determining the acoustic properties of such ma-terials require either that equipment be lowered to the seabed from a stationary vessel, or else that samples be taken at sea and analysed in the laboratory. soth oE these methods are costly in -terms of ship time, and the lat-ter method suffers from difficulties encountered in taking undisturbed samples.
It is desirable to be able to conduc-t areal surveys to determine as many acoustic parameters as possible by the use ~0 of equipment which can be towed -through the water for making kh~ necessary observations without requiring the -towing vessel to stop.
Conventional surveying techniques based on travel time methods have been employed, but these are crude and little more than modiEied depth sounding technic~ues. In such a method -the time of travel of an acoustic signal reflected in the sediment from a source to the hydrophones is measured, and from the travel -times informa-tion about the underlying sediment structure can be derived. These techniques have severe limitations, particularly as no information is given c~bout -the variation with depth of the seabed properties such as the acoustic velocity and 1 ~ qC
~2~ ~g~
attenuation. Gross changes can be expec-ted in these properties if the sediments are gasified or ice-bonded.
The above considerations apply equally to lacustrine, estuarine, deltaic, fluvial and other aquatic environments. Hereinafter, -the word "seabed" is to be understood as referring to -the bed oE any body of water and the term "sea" includes inland bodies of water.
Accordingly, -the present invention provides a method of investigating surficial sea floor sediments, comprising towing a plurality of acoustic sensors through the water near the sea floor, generating acoustic signals in the water at locations such that -the acoustic signals propagate at least in par-t through the surficial sediment to the acoustic sensors recording the waveform of the signals received by the acoustic sensors to produce a field seismogram, constructing a synthetic seismogram on the basis of a postulated earth structure for the surficial sediment using mathematical modelling techniques, comparing the observed field seismogram with the synthetic seismogram, and O adjusting the mathematical model to bes-t fit the recorded data so as to obtain information relating -to the sediment structure.
The plurality of acoustic sensors may be used in conjunction with one or a plurality of acoustic sources such that the assemblage of the said source or sources and sensors can be towed at varying heights above the seabed.
Such heights can be optimised for -the specific seabed under investiga-tion by the mathematical or physical cons-truction of a corresponding model seismogram which is compared with the observed seismogram. In the present specification, the plurality of observed time series which are recorded either simultaneously or sequentially from the output of the L9~
plurality of acoustic sensors are collectively referred -to hereinaEter as a field seismogram, whereas a corresponding model, whether construc-ted by physical or mathematical processes is referred to hereinafter as a synthet:ic seismogram.
The said sensors may be arranged spat:ially in different ways, bu-t it is a charac-teristic of the invention that the spatial location of these sensors be known wi-th reference to the acous-tic source, either by constraining the said sensors to move through the water in such a way that their location rela-tive to the said acoustic source be invariant or else by providing a means of locating continuously ~he sensors relative to the source. Like provision is made -to constrain or determine con-tinuously the location of the assemblage of acoustic source and sensors relative -to the seabed. Ambiguities of interpretation which can arise in the deriva-tion of seabed acoustic properties from field seismograms are minimised by either or both of the following procedures:
a)Where some pre-existing knowledge of the seabed is available, synthetic seismograms are constructed prior to fieldwork in order to optimise the spatial characteristics of the equipment deployment such as to obtain the bes-t possible discrimination of -the seabed properties which remain unknown.
b)Where no pre-existing knowledge of the field area exis-ts, equipment is deployed and field seismograms are collected. Modelling methods are used to construct synthetic seismograms on the basis bf postulated seabed characteristics which are varied until a fi-t is ob-tained between the field seismogram and the synthetic seismogram. If any ambiguity exists amony sets of characteristics which yive a fit between the field seismogram and the synthetic seismogram, then are calculated the ad~ustments to the deployment of the field equipment which are required to remove such ambigui-ties, and more field seismograrns are collected.
The above procedures for gathering the field seismogram are referred to herein after as -the direct (a -above) and the recursive (b - above) methods. In implementing the direct or the recursive method, use may be made of common kinematic methods for a prelirninary determination of acous-tic veloci-ties so that the number of unknown parameters characterising the seabed may be reduced and the modelling process be simplified. Such a constrain-t on values for the seabed acoustic velocities may be relaxed at a later stage in the dynamic modelling procedure, because kine~matic methods all suffer from the disadvantage of averaging acoustic velocities over the thickness of a postulated layer.
~ he invention will now be described in more detail, by way of example only, with reference -to -the accompanying drawings, in which:
Figure 1 is a diagram of the field equipment;
Figure 2 shows -the parameters characterising the lossy elastic rheology for a three layer model of the seabed;
Figure 3 sh~ws a field seismogram;
Figures 4a, 4b and 5a, 5b are -two synthetic seismograms toge-ther with -the seabed characteristics on ~L2~
which they are based (in the form of graphs showing variation with depth below the seafloor of the parameters);
and Figures 6a, 6b are charts of p-velocity against depth for different seabed structures.
In -the described embodiment use was made of the mathematical reflectivity method of synthetic seismogram cons-truction on the basis of a lossy elastic rheology together with the manual me-thod of comparison with field seismograms which had been obtained with a 12-channel linear hydrophone array operated in conjunction with a 0.15 litre capacity pneuma-tic acoustic source in shelf waters of the Canadian Beaufort Sea.
Figure 1 shows a vessel 1 towing an acoustic source 2 Eollowed by a linear array of acoustic sensors 3 at a height h above the seabed 4. For convenience the body of ~ater 5 in which the sensors are towed is referred to as the "sea", although i-t will be realised that the invention is of course applicable to any sui-table body of wa-ter, whether it ~ be the sea or an inland lake.
The acoustic source 2 can be a single source or a plurality of sources, each comprising individual source elements. The source or sources can comprise one or more of thc Eollowing types of device: spark discharge, electromagnetic, piezo-electric, magnetos-trictive parame-tric, explosive, hydraulic, pneumatic and percussive.
The source can be a source of either spherical, cylindrical, or plane wave fronts. In the case of a plurality of sources, the resulting wave can be formed by the interference of waves from the aorementioned sources.
The sources can emit w~aves at different frequencies and be triggered either simul-taneously or with a -time delay be-tween successive firings.
The acoustic sensors 3 or hydrophones, are shown in Flgure 1 in the form of a linear array. The height h of the array above the seabed 4 can be varied frorn the towiny vessel 1. The acoustic sensors 3 can also be arranged in the form of a series of arrays that are ei-ther co-planar or deployed such tha-t the resulting plurality of sensors creates a three-dimensional pattern in the water. The individual sensors can have regular or irregular spacing.
The height and geometrical disposi-tion of -the sensors can be op-timised for the seabed under investigation.
Where pre-existing knowledge of the seabed struc-ture is available, a synthetic seismogram is constructed and the parameters set to optimise the -test results. Where no knowledge of ~he seabed structure is available, a field survey is first conduc-ted and a synthetic seismogram constructed to give the best fit with the observed seismogram. A further field survey can -then be conducted wi-th the parameters set to optimise the results based on the
The investigation of seabed materials is of interest to geologists, engineers and underwater acousticians. Geoloyists and engineers are interested in the acoustic properties oE these materials in that such proper-ties can be related to the rnechanical properties of the materials, whereas underwater acousticians generally require to know the acoustic properties in order to make more accurate predictions about sound propagation in the overlying water column. Methods used at present for determining the acoustic properties of such ma-terials require either that equipment be lowered to the seabed from a stationary vessel, or else that samples be taken at sea and analysed in the laboratory. soth oE these methods are costly in -terms of ship time, and the lat-ter method suffers from difficulties encountered in taking undisturbed samples.
It is desirable to be able to conduc-t areal surveys to determine as many acoustic parameters as possible by the use ~0 of equipment which can be towed -through the water for making kh~ necessary observations without requiring the -towing vessel to stop.
Conventional surveying techniques based on travel time methods have been employed, but these are crude and little more than modiEied depth sounding technic~ues. In such a method -the time of travel of an acoustic signal reflected in the sediment from a source to the hydrophones is measured, and from the travel -times informa-tion about the underlying sediment structure can be derived. These techniques have severe limitations, particularly as no information is given c~bout -the variation with depth of the seabed properties such as the acoustic velocity and 1 ~ qC
~2~ ~g~
attenuation. Gross changes can be expec-ted in these properties if the sediments are gasified or ice-bonded.
The above considerations apply equally to lacustrine, estuarine, deltaic, fluvial and other aquatic environments. Hereinafter, -the word "seabed" is to be understood as referring to -the bed oE any body of water and the term "sea" includes inland bodies of water.
Accordingly, -the present invention provides a method of investigating surficial sea floor sediments, comprising towing a plurality of acoustic sensors through the water near the sea floor, generating acoustic signals in the water at locations such that -the acoustic signals propagate at least in par-t through the surficial sediment to the acoustic sensors recording the waveform of the signals received by the acoustic sensors to produce a field seismogram, constructing a synthetic seismogram on the basis of a postulated earth structure for the surficial sediment using mathematical modelling techniques, comparing the observed field seismogram with the synthetic seismogram, and O adjusting the mathematical model to bes-t fit the recorded data so as to obtain information relating -to the sediment structure.
The plurality of acoustic sensors may be used in conjunction with one or a plurality of acoustic sources such that the assemblage of the said source or sources and sensors can be towed at varying heights above the seabed.
Such heights can be optimised for -the specific seabed under investiga-tion by the mathematical or physical cons-truction of a corresponding model seismogram which is compared with the observed seismogram. In the present specification, the plurality of observed time series which are recorded either simultaneously or sequentially from the output of the L9~
plurality of acoustic sensors are collectively referred -to hereinaEter as a field seismogram, whereas a corresponding model, whether construc-ted by physical or mathematical processes is referred to hereinafter as a synthet:ic seismogram.
The said sensors may be arranged spat:ially in different ways, bu-t it is a charac-teristic of the invention that the spatial location of these sensors be known wi-th reference to the acous-tic source, either by constraining the said sensors to move through the water in such a way that their location rela-tive to the said acoustic source be invariant or else by providing a means of locating continuously ~he sensors relative to the source. Like provision is made -to constrain or determine con-tinuously the location of the assemblage of acoustic source and sensors relative -to the seabed. Ambiguities of interpretation which can arise in the deriva-tion of seabed acoustic properties from field seismograms are minimised by either or both of the following procedures:
a)Where some pre-existing knowledge of the seabed is available, synthetic seismograms are constructed prior to fieldwork in order to optimise the spatial characteristics of the equipment deployment such as to obtain the bes-t possible discrimination of -the seabed properties which remain unknown.
b)Where no pre-existing knowledge of the field area exis-ts, equipment is deployed and field seismograms are collected. Modelling methods are used to construct synthetic seismograms on the basis bf postulated seabed characteristics which are varied until a fi-t is ob-tained between the field seismogram and the synthetic seismogram. If any ambiguity exists amony sets of characteristics which yive a fit between the field seismogram and the synthetic seismogram, then are calculated the ad~ustments to the deployment of the field equipment which are required to remove such ambigui-ties, and more field seismograrns are collected.
The above procedures for gathering the field seismogram are referred to herein after as -the direct (a -above) and the recursive (b - above) methods. In implementing the direct or the recursive method, use may be made of common kinematic methods for a prelirninary determination of acous-tic veloci-ties so that the number of unknown parameters characterising the seabed may be reduced and the modelling process be simplified. Such a constrain-t on values for the seabed acoustic velocities may be relaxed at a later stage in the dynamic modelling procedure, because kine~matic methods all suffer from the disadvantage of averaging acoustic velocities over the thickness of a postulated layer.
~ he invention will now be described in more detail, by way of example only, with reference -to -the accompanying drawings, in which:
Figure 1 is a diagram of the field equipment;
Figure 2 shows -the parameters characterising the lossy elastic rheology for a three layer model of the seabed;
Figure 3 sh~ws a field seismogram;
Figures 4a, 4b and 5a, 5b are -two synthetic seismograms toge-ther with -the seabed characteristics on ~L2~
which they are based (in the form of graphs showing variation with depth below the seafloor of the parameters);
and Figures 6a, 6b are charts of p-velocity against depth for different seabed structures.
In -the described embodiment use was made of the mathematical reflectivity method of synthetic seismogram cons-truction on the basis of a lossy elastic rheology together with the manual me-thod of comparison with field seismograms which had been obtained with a 12-channel linear hydrophone array operated in conjunction with a 0.15 litre capacity pneuma-tic acoustic source in shelf waters of the Canadian Beaufort Sea.
Figure 1 shows a vessel 1 towing an acoustic source 2 Eollowed by a linear array of acoustic sensors 3 at a height h above the seabed 4. For convenience the body of ~ater 5 in which the sensors are towed is referred to as the "sea", although i-t will be realised that the invention is of course applicable to any sui-table body of wa-ter, whether it ~ be the sea or an inland lake.
The acoustic source 2 can be a single source or a plurality of sources, each comprising individual source elements. The source or sources can comprise one or more of thc Eollowing types of device: spark discharge, electromagnetic, piezo-electric, magnetos-trictive parame-tric, explosive, hydraulic, pneumatic and percussive.
The source can be a source of either spherical, cylindrical, or plane wave fronts. In the case of a plurality of sources, the resulting wave can be formed by the interference of waves from the aorementioned sources.
The sources can emit w~aves at different frequencies and be triggered either simul-taneously or with a -time delay be-tween successive firings.
The acoustic sensors 3 or hydrophones, are shown in Flgure 1 in the form of a linear array. The height h of the array above the seabed 4 can be varied frorn the towiny vessel 1. The acoustic sensors 3 can also be arranged in the form of a series of arrays that are ei-ther co-planar or deployed such tha-t the resulting plurality of sensors creates a three-dimensional pattern in the water. The individual sensors can have regular or irregular spacing.
The height and geometrical disposi-tion of -the sensors can be op-timised for the seabed under investigation.
Where pre-existing knowledge of the seabed struc-ture is available, a synthetic seismogram is constructed and the parameters set to optimise the -test results. Where no knowledge of ~he seabed structure is available, a field survey is first conduc-ted and a synthetic seismogram constructed to give the best fit with the observed seismogram. A further field survey can -then be conducted wi-th the parameters set to optimise the results based on the
2~ best fit model to resolve any ambiguities in data.
An additiona:L array (no-t shown) of electrical sources and senors can also be deployed to take additional measurements of electrical resistance as a further aid in investigating the seabed characteris-tics, particularly -the porosity.
The precise loca-tions of the acoustic sensors must be known at all times. These can be determined from the travel times of the acoustic waves propaga-ting between the sources and sensors, which have travelled between the sources and sensors either directly, by reflection at the water surface, or refl~ction a-t the seabed. ~l-ternatively, the locations of the acoustic sources and sensors can be .
determined by means of one or more sensing devices of the following types: iner-tial, pressure, magnetic, acoustic. In an alternative embodiment, the acoustic sources and sensors can be constrained to move wi-th invariant internal re:Lative posi-tions.
To conduct a survey, the source 2 and sensors 3 are deployed in the water at height h as shown in Figure 1.
If some pre-existing knowledge of the seabed floor is available, the height h and relative positions of the sensors are set to op-timise -the results. If not, a best guess is made and a synthetic seismogram created from -the data as explained above, following which -the parameters are varied and reset -to resolve any ambigui-ties.
Figure 3 shows a field seismogram from the Ukalerk area oE the Canadian Beaufor-t Sea taken with an arrangement similar to that shown in Figure 1. The travel time is shown on the X axis and the offset distance in me-ters on the Y
axis. The first pulse d in the seismogram represents -the time of arrival of -the direct water-wave and is not shown in ~he synthetic seismograms.
The synthetic seismogram is construc-ted on -the basis of a lossy elastic rheology or Biot-Stoll rheology.
:Ln the former case, the seabed material is characterised by the following parameters: compressional velocity, shear velocity, density, compressional attenuation, and shear attenuation. In the latter case, the seabed material is characterised by the following parame-ters: density of sediment grains, bulk modulus of sediment grains, densi-ty of pore fluid, bulk modulus of pore fluid, viscosity of pore fluid, porosity, permeability, pore size parame-ter, structure factor, she,~r modulus of frame, and bulk modulus of Erame. In the case of lossy elastic rheology, the loss ~L2~4~L
mechanism for acoustic energy can be some general function oE the velocity oE the dis-turbance of the seabed material by the radiated acous-tic field. The synthetic seisrnogram can also be construc-ted by a physical modelling process, which uses combinations of fluid and solid materials to represent the water column and the seabed materials, wi-th such scaling as may be necessary for realisation on a laboratory scale.
The synthetic seismograrns are then constructed mathematically by one or more of the following methods:
finite difference method, finite element method, reflectivity method, perturbation method, asymp-totic me-thod, and Gaussian beam method.
In order to obtain information rela-ting to -the structure of the seabed, the syn-thetic seismogram is compared by one of -the following methods:
a)Manual comparison of field and syn-thetic seismograms b)Computational multivariate optimisation wherein a leas-t squares difference or equivalent criterion is used to judge when a fit is obtained, ~the main computational procedure must be of a non-linear optimising type, although linear methods can be used for first approximations to decrease computational time), c)Compu-tational comparison, wherein the use of image processing techniques compare the field and synthetic seismograms.
The mathematical models and controlling parameters for the syn-the-tic seismograms are varied until the best fit wi-th the observed data is found.
Figure 2 sh~ws -the parameters characterising the lossy elastic rheology for a three-layer model of the seabed. The significant parameters are the p-wave velocity (compression wave), s-wave velocity (shear wave), -the density, and the compressional and shear attenuation (, 1 over q sub p, 1 over q sub s).
Figures 4a and 4b show an unsuccessEul at-ternpt -to create a synthe-tic seismogram based on a sirnple layer model to fit the field seismogram shown in Figure 3. The postulated parameters are shown in Figure 4b. It is clear from visual comparison that the synthetic seismogram of Figure ~ does not fi-t -the observed field seismogram of Figure 3.
Figure 5a shows a synthetic seismogram constructed by removing the constraint of simple layering and allowing a more continuous variation in p and s velocity, as shown in E`igure 5b. A visual comparison with the field seismogram of Figure 3 shows that some approach to the observed seismogram has been obtained. sy manlpulating the mathematical model and -the parame-ters, and comparing the synthe-tic seismogram constructed with the observed seismogram, much can be cleduced about the structure of the underlying seabed.
Figure 6a shows the variation of p-velocity wi-th depth arising from a dynamic analysis of the field data for near uniform layers, and Figure 6b shows the variation of p-velocity for a much greater variation in velocity gradience. Figures 6a, 6b illustrate -the result of attempting to model a different field seismogram~ which turned out to be impossible to interpret unambiyuously.
The modelling process showed how -to change the geome-trical disposition of the field equipment so as to obtain an unambiguous field seismogram. This method of obtaining a field seismogram, constructing the best fit synthe-tic seismogram, and then obtaining a fur-ther field seismogram with -the deployment of the sensors determined by the synthetic seismogram, is -the recursive rnethod referred to above.
As can be seen from -the above description, rnuch more data can be obtained about the underlying seabed structure than could be obtained in the prior art frorn simple time of travel techniques with the aid of a synthetic seismogram based on a mathematical model constructed from the parameters discussed above.
2n
An additiona:L array (no-t shown) of electrical sources and senors can also be deployed to take additional measurements of electrical resistance as a further aid in investigating the seabed characteris-tics, particularly -the porosity.
The precise loca-tions of the acoustic sensors must be known at all times. These can be determined from the travel times of the acoustic waves propaga-ting between the sources and sensors, which have travelled between the sources and sensors either directly, by reflection at the water surface, or refl~ction a-t the seabed. ~l-ternatively, the locations of the acoustic sources and sensors can be .
determined by means of one or more sensing devices of the following types: iner-tial, pressure, magnetic, acoustic. In an alternative embodiment, the acoustic sources and sensors can be constrained to move wi-th invariant internal re:Lative posi-tions.
To conduct a survey, the source 2 and sensors 3 are deployed in the water at height h as shown in Figure 1.
If some pre-existing knowledge of the seabed floor is available, the height h and relative positions of the sensors are set to op-timise -the results. If not, a best guess is made and a synthetic seismogram created from -the data as explained above, following which -the parameters are varied and reset -to resolve any ambigui-ties.
Figure 3 shows a field seismogram from the Ukalerk area oE the Canadian Beaufor-t Sea taken with an arrangement similar to that shown in Figure 1. The travel time is shown on the X axis and the offset distance in me-ters on the Y
axis. The first pulse d in the seismogram represents -the time of arrival of -the direct water-wave and is not shown in ~he synthetic seismograms.
The synthetic seismogram is construc-ted on -the basis of a lossy elastic rheology or Biot-Stoll rheology.
:Ln the former case, the seabed material is characterised by the following parameters: compressional velocity, shear velocity, density, compressional attenuation, and shear attenuation. In the latter case, the seabed material is characterised by the following parame-ters: density of sediment grains, bulk modulus of sediment grains, densi-ty of pore fluid, bulk modulus of pore fluid, viscosity of pore fluid, porosity, permeability, pore size parame-ter, structure factor, she,~r modulus of frame, and bulk modulus of Erame. In the case of lossy elastic rheology, the loss ~L2~4~L
mechanism for acoustic energy can be some general function oE the velocity oE the dis-turbance of the seabed material by the radiated acous-tic field. The synthetic seisrnogram can also be construc-ted by a physical modelling process, which uses combinations of fluid and solid materials to represent the water column and the seabed materials, wi-th such scaling as may be necessary for realisation on a laboratory scale.
The synthetic seismograrns are then constructed mathematically by one or more of the following methods:
finite difference method, finite element method, reflectivity method, perturbation method, asymp-totic me-thod, and Gaussian beam method.
In order to obtain information rela-ting to -the structure of the seabed, the syn-thetic seismogram is compared by one of -the following methods:
a)Manual comparison of field and syn-thetic seismograms b)Computational multivariate optimisation wherein a leas-t squares difference or equivalent criterion is used to judge when a fit is obtained, ~the main computational procedure must be of a non-linear optimising type, although linear methods can be used for first approximations to decrease computational time), c)Compu-tational comparison, wherein the use of image processing techniques compare the field and synthetic seismograms.
The mathematical models and controlling parameters for the syn-the-tic seismograms are varied until the best fit wi-th the observed data is found.
Figure 2 sh~ws -the parameters characterising the lossy elastic rheology for a three-layer model of the seabed. The significant parameters are the p-wave velocity (compression wave), s-wave velocity (shear wave), -the density, and the compressional and shear attenuation (, 1 over q sub p, 1 over q sub s).
Figures 4a and 4b show an unsuccessEul at-ternpt -to create a synthe-tic seismogram based on a sirnple layer model to fit the field seismogram shown in Figure 3. The postulated parameters are shown in Figure 4b. It is clear from visual comparison that the synthetic seismogram of Figure ~ does not fi-t -the observed field seismogram of Figure 3.
Figure 5a shows a synthetic seismogram constructed by removing the constraint of simple layering and allowing a more continuous variation in p and s velocity, as shown in E`igure 5b. A visual comparison with the field seismogram of Figure 3 shows that some approach to the observed seismogram has been obtained. sy manlpulating the mathematical model and -the parame-ters, and comparing the synthe-tic seismogram constructed with the observed seismogram, much can be cleduced about the structure of the underlying seabed.
Figure 6a shows the variation of p-velocity wi-th depth arising from a dynamic analysis of the field data for near uniform layers, and Figure 6b shows the variation of p-velocity for a much greater variation in velocity gradience. Figures 6a, 6b illustrate -the result of attempting to model a different field seismogram~ which turned out to be impossible to interpret unambiyuously.
The modelling process showed how -to change the geome-trical disposition of the field equipment so as to obtain an unambiguous field seismogram. This method of obtaining a field seismogram, constructing the best fit synthe-tic seismogram, and then obtaining a fur-ther field seismogram with -the deployment of the sensors determined by the synthetic seismogram, is -the recursive rnethod referred to above.
As can be seen from -the above description, rnuch more data can be obtained about the underlying seabed structure than could be obtained in the prior art frorn simple time of travel techniques with the aid of a synthetic seismogram based on a mathematical model constructed from the parameters discussed above.
2n
Claims (22)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of investigating surficial sea floor sediments, comprising towing a plurality of acoustic sensors through the water near the sea floor, generating acoustic signals in the water at locations such that the acoustic signals propagate at least in part through the surficial sediment to the acoustic sensors recording the waveform of the signals received by the acoustic sensors to produce a field seismogram, constructing a synthetic seismogram on the basis of a postulated earth structure for the surficial sedi-ment using mathematical modelling techniques, comparing the observed field seismogram with the synthetic seismogram, and adjusting the mathematical model to best fit the recorded data so as to obtain information relating to the sediment structure.
2. A method as claimed in claim 1 wherein the assembly of said source and sensors can be towed at varying heights above the seabed, such heights being optimised for the specific seabed under investigation by a) where some pre-existing knowledge of the seabed is available, constructing the synthetic seismogram prior to conducting the field survey, and determining such heights from the synthetic seismogram, or b) where no pre-existing knowledge of the seabed is available, conducting the field survey to produce a field seismogram, varying the mathematical model forming the basis of the synthetic seismogram until the best fit is obtained, and conducting a further field survey with such heights determined by the best fit model to resolve any ambiguities.
3. A method as claimed in claim 1 wherein the plurality of sensors is in the form of a series of arrays having individual sensors within each array at either regular or irregular spacing towed such that the arrays are either coplanar with one another or are deployed such that the resulting plurality of sensors creates a 3-dimensional pattern in the water column, the geometrical disposition of said source and sensors can be varied with respect to one another, and the assembly of the said source and the said sensors can be towed at varying heights above the seabed, the said heights and the said geometrical disposition being optimised for the specific seabed under investigation by a) where some pre-existing knowledge of the seabed is available, constructing the synthetic seismogram prior to conducting the field survey, and determining such heights and geometrical disposition for the synthetic seismogram, or b) where no pre-existing knowledge of the seabed is available, conducting the field survey to produce a field seismogram, varying the mathematical model forming the basis of the synthetic seismogram until the best fit is obtained, and conducting a further field survey with such heights and geometrical disposition determined from the best fit model to resolve any ambiguities.
4. A method as claimed in claims 1, 2 or 3 wherein the acoustic source is a source of spherical wavefronts.
5. A method as claimed in claims 1, 2, or 3 wherein the acoustic source is a source of cylindrical wavefronts.
6. A method as claimed in claims 1, 2 or 3 wherein the acoustic source is a source of plane wavefronts.
7. A method as claimed in claims 1, 2 or 3 wherein the acoustic source is a source of waves formed by the interference of waves arising from sources of spherical, cylindrical, or plane wavefronts.
8. A method as claimed in claim 1 wherein the acoustic source comprises a plurality of acoustic sources which can emit waves at different frequencies.
9. A method as claimed in claim 8 above wherein the plurality of acoustic sources is triggered simultaneously.
10. A method as claimed in claim 8 wherein the plurality of acoustic sources is triggered with time delay between the firing of each source.
11. A method as claimed in claims 1, 2 or 3 wherein the acoustic source comprises a plurality of acoustic sources, and each individual acoustic source comprises spherical, cylindrical, plane wavefronts, there being a time delay between the firing of each source element.
12. A method as claimed in claims 1, 2 or 3 wherein for the synthetic seismogram construction, the rheology used to characterise the seabed materials is that commonly called a lossy elastic rheology wherein each unit of seabed material is characterised by the parameters:
compressional velocity shear velocity density compressional attenuation shear attenuation
compressional velocity shear velocity density compressional attenuation shear attenuation
13. A method as claimed in claims 1, 2 or 3 whrein the rheology used to characterise the seabed materials is that commonly called the Biot-Stoll rheology, wherein each unit of seabed material is characterised by the parameters:
density of sediment grains bulk modulus of sediment grains density of pore fluid bulk modulus of pore fluid porosity permeability pore size parameter structure factor shear modulus of frame bulk modulus of frame
density of sediment grains bulk modulus of sediment grains density of pore fluid bulk modulus of pore fluid porosity permeability pore size parameter structure factor shear modulus of frame bulk modulus of frame
14. A method as claimed in claims 1, 2 or 3 wherein the rheology is one derived from the lossy elastic rheology wherein the loss mechanism for acoustic energy dissipation is some general function of the velocity of disturbance of the seabed material by the radiated acoustic field.
15. A method as claimed in claims 1, 2 or 3 wherein the synthetic seismogram construction, the physical modelling process uses combinations of fluid and solid materials to represent the water column and the seabed materials, with scaling such as may be necessary for realisation on a laboratory scale.
16. A method as claimed in claims 1, 2 or 3 wherein the acoustic source or sources comprise one or more of the following methods for realisation:
spark discharge explosive electromagnetic hydraulic piezo-electric pneumatic magnetostrictive percussive parametric
spark discharge explosive electromagnetic hydraulic piezo-electric pneumatic magnetostrictive percussive parametric
17. A method as claimed in claims 1, 2 or 3 wherein there is added to the assembly of acoustic equipment, arrays of electrical sources and sensors which are used to make measurements of electrical resistance as a help in constraining the values of seabed characteristics, particularly in respect of the characteristic porosity.
18. A method as claimed in claims 1, 2 or 3 wherein the locations of acoustic sources and sensors are determined by trigonometric calculations based on the travel times of acoustic waves between sources and sensors each to each, such waves having travelled between each source and receiver (a) directly, (b) by reflection at the water surface, or (c) by reflection at the seabed.
19. A method as claimed in claims 1, 2 or 3 wherein the locations of the acoustic sources and sensors are determined by means of one or more sensing devices of the following types:
a. inertial b. pressure c. magnetic d. acoustic
a. inertial b. pressure c. magnetic d. acoustic
20. A method as claimed in claims 1, 2 or 3 wherein the acoustic sources and sensors are constructed to move the invariant internal relative position.
21. A method as claimed in claims 1, 2 or 3 wherein the method of constructing mathematically the required synthetic seismograms is one which comprises or combines one, two or more of the following methods:
a. finite difference method b. finite element method c. reflectivity method d. perturbation method e. asymptotic method f. Gaussian beam method
a. finite difference method b. finite element method c. reflectivity method d. perturbation method e. asymptotic method f. Gaussian beam method
22. A method as claimed in claims 1, 2 or 3 wherein the method of obtaining the fit between the field seismogram and the synthetic seismogram is one which comprises or combines one, two or more of the following methods:
a) Manual comparison of field and synthetic seismograms b) Computational multivariate optimisation wherein a least squares difference or equivalent criterion is used to judge when a fit is obtained, the main computational procedure being of a non-linear optimising type; and c) Computational comparison, wherein the use of image processing techniques replaces the manual comparison of a) above.
a) Manual comparison of field and synthetic seismograms b) Computational multivariate optimisation wherein a least squares difference or equivalent criterion is used to judge when a fit is obtained, the main computational procedure being of a non-linear optimising type; and c) Computational comparison, wherein the use of image processing techniques replaces the manual comparison of a) above.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000474528A CA1219944A (en) | 1985-02-18 | 1985-02-18 | Method for finding the acoustic properties of seabed materials |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000474528A CA1219944A (en) | 1985-02-18 | 1985-02-18 | Method for finding the acoustic properties of seabed materials |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1219944A true CA1219944A (en) | 1987-03-31 |
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ID=4129851
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000474528A Expired CA1219944A (en) | 1985-02-18 | 1985-02-18 | Method for finding the acoustic properties of seabed materials |
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| Country | Link |
|---|---|
| CA (1) | CA1219944A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1996020415A1 (en) * | 1994-12-28 | 1996-07-04 | Thomson-Csf | Acoustic emission method for sonars |
| FR2744224A1 (en) * | 1996-01-26 | 1997-08-01 | Inst Francais Du Petrole | METHOD FOR SIMULATING THE FILLING OF A SEDIMENT BASIN |
-
1985
- 1985-02-18 CA CA000474528A patent/CA1219944A/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1996020415A1 (en) * | 1994-12-28 | 1996-07-04 | Thomson-Csf | Acoustic emission method for sonars |
| FR2729041A1 (en) * | 1994-12-28 | 1996-07-05 | Thomson Csf | ACOUSTIC TRANSMISSION PROCESS FOR SONAR |
| US5856954A (en) * | 1994-12-28 | 1999-01-05 | Thomson-Csf | Process of acoustic emission for sonar |
| FR2744224A1 (en) * | 1996-01-26 | 1997-08-01 | Inst Francais Du Petrole | METHOD FOR SIMULATING THE FILLING OF A SEDIMENT BASIN |
| US5844799A (en) * | 1996-01-26 | 1998-12-01 | Institut Francais Du Petrole | Method for simulating the filling of a sedimentary basin |
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