CA1149925A - Multiple reflection suppression in marine seismic exploration - Google Patents
Multiple reflection suppression in marine seismic explorationInfo
- Publication number
- CA1149925A CA1149925A CA000336129A CA336129A CA1149925A CA 1149925 A CA1149925 A CA 1149925A CA 000336129 A CA000336129 A CA 000336129A CA 336129 A CA336129 A CA 336129A CA 1149925 A CA1149925 A CA 1149925A
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- CA
- Canada
- Prior art keywords
- seismic
- floats
- elements
- water
- energy
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- Geophysics And Detection Of Objects (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The invention is concerned with the suppression of multiple reflections from the water surface in marine seismic exploration. More specifically, the invention contemplates the use of a plurality of energy reflective elements, for example floats, which may or may not be coupled together, to substantially cover the surface of the body of water above the seismic detectors. The size of each element, and the distance between adjacent elements being commensurable to the dominant wave length of the seismic waves. The elements will dissipate the energy arising from the water surface.
The invention is concerned with the suppression of multiple reflections from the water surface in marine seismic exploration. More specifically, the invention contemplates the use of a plurality of energy reflective elements, for example floats, which may or may not be coupled together, to substantially cover the surface of the body of water above the seismic detectors. The size of each element, and the distance between adjacent elements being commensurable to the dominant wave length of the seismic waves. The elements will dissipate the energy arising from the water surface.
Description
~45~9Z5 The invention relates to marine seismic exploration, and more particularly to a method and apparatus which facili-tates the suppression of multiple reflections from the surface of a body of water.
This invention rela~es to methods and apparatus of marine seismic exploration and particularly to methods and apparatus for suppressing multiple reflections.
The problem of suppression of multiples is one of the most important and difficult problem areas in marine seismic exploration. Many methods have been employed to im-prove the signal-to-noise ratio. Predictive deco~volution and stacking techniques have been successfully used for shallow marine data. However, in case deep water layers these procedures produced generally unsatisfactory results.
In many cases the energy of multiple reflections is so high that they cannot be sufficiently suppressed by the stacking technlques and standard deconvolution. Therefore no post-shooting processing will make the data useful. In a preferred embodiment of the invention it is desirable to perform the exploration with :a group of floats which rejects these multiple reflections.
In accordance with the present invention, elements, for example floats, adapted to reflect seismic energy pulses, while at least partially immersed in a body of water, are used to suppress substantially all multiples reflected from the water surface. This suppression of multiple reflections, being the result of dissipation of the energy of seismic waves reflected from the uneven surface created by the immersed sur-face area of the elements, and the surface of the water between each element.
.
13L~9~5 In accordance with another important aspect of the present in~ention, the size of the elements and the horizontal spacing between adjacent elements are commensurable with the dominant wavelength of the registered seismic waves.
The invention will now be described by way of example only, with reference to the accompanying drawings in which:-Figure 1 is a top diagramatic view of an array offloats according to a first embodiment of the invention;
Figure 2 is a diagramatic side view of the embodi-ment according to Figure 1, showing the disposition of the seismic source, and seismic detectors relative to the array of floats;
Figure 3 shows schematically, the dissipation of seismic energy by one of the floats according to Figures l and 2;
Figure 4 shows graphically, the effect of curvature of interfaces on the spherical divergence; and Figure 5 shows an alternative float configuration designed to reduce the effect of surface wind on the float array.
Referring particularly to the appended drawings, Figures 1 and 2 show a marine seismic exploration system according io the present invention.
A marine vessel (lO), tows cable with detectors, or receivers tll), located beneath an array of partially submerged and interconnected floats (12).
Adjacent floats may be interconnected by any suit-able means, such as lines or cable (13), and additional cables interconnect the array with the towing vessel.
While a specific array of floats (12) is shown in ~4~925 Figure 1, it will be appreciated that such does not limit the scope of the present invention. The number of floats, is dependent upon the sizes of the floats. It being necessary only that the array of floats (12) be sufficiently spread to substantially cover the immediate surface of the body of water above detectors (11), and a sufficiently extended area there-around. As an example only, the surface geometric patterning of floats can be arranged in a chess order.
As to the shape of floats (12) and the material used in their construction, these are not critical features. How-ever, for ease of understanding, floats (12) have been shown as spherical in form, and it is suggested that they be hollow, and formed from a suîtable plastics material. Weights may be utilized to ensure partial submersion.
Of prime importance however, is the size, and the spacing between adjacent floats and the principal feature of the present invention according to the embodiment disclosed herein, is that the size of the individual floats (14) is com-mensurable to the dominant wavelength (~) of the registered seismic wave.s, i.e. the diameter (A) of the float is preferably in the range:-A* 0.2~ to 5.0~ -----------------(1) Furthermore, the distance (D) between adjacent floats must, according to the present invention, be com}nensurable with the size of the floats, or with the wavelength of the seismic pulse. Therefore distance (D) is also in the range.
i.e. D~ .2 ~to 5.0 ~ --------------(2) To further explain, it is known that the velocity (V) of propagation of the seismic waves through water is ap-proximately equal to 1500 meters/second, and the basic frequency 1~45'~ZS
(F) of the seismic waves registered in marine seismic explora-tion is approximately S~ Hz. Therefore, the dominant wave-length (~) of the basic frequency of the propagating pulse in water is given by:-~ - V ~30 meters therefore, since A~ D falls in the range 0.2 ~ to 5.0~
then A~ D may fall within the range 6 meters to 150 meters As previously indicated, the suppression of multiple reflection according to the present invention arises as a result of dissipation of the energy of seismic waves from the uneven surface of the body of water created by the position-ing of floats (14) thereon.
Figure 3, shows schematically the dissipation of seismic energy by one of floats (12). In this figure, numeral (14) refers to a seismic energy source; numeral (15) refers to a geological reflecting interface; R is the radius of sur-face curature of the float (14) and; H is a known distance, measured between the bottom of float (:L4) and the geological interface (15).
It has been established that the eneryy, per unit area of the wave front, will substantially decrease after reflection from this uneven surface.
For the approximate calculation of this decrease in energy, the standard equation of Spherical Divergance (L) of seismic waves reflected from an uneven interface may be applied and is given by:-L = cos ~ (3) 4 H (1 + 2H~
R ) ~1~9~2S
where is the angle of slope of the reflecting interface:
Obvîously, it will be appreciated that in the case of a water surface without the array of floats, equation (3) above, will transform to the standard equation of Spherical Divergance for first multiples reflected from the water surface and will be given by:-L = 1 Therefore the presence of the floats results in the appearance, in the denominator, of an additional multiplier.
i.e. (1 + 2H) R ) which increases from the value one as the depth (H) of the interface, and the curvature of the floats (represented by 1) increases.
Figure 4 has been introduced to graphically show the effect of curvature of floats on the spherical divergence. From this figure it can be seen the spherical divergence decreases rapidly. As additionally seen from Figure 4, this decrease is even more rapid, as the principal dimension (R) decreases.
The intensity of seismic waves is therefore proportionally dependent upon the spherical divergence, thus, the intensity of all multiple reflections, from the uneven surface of the body of water, will correspondingly decrease at the same rate.
It must be mentioned at this point however that the method of the present invention may be practiced without the requirement of a towing vessel, or any interconnection between the energy reflective elements since as previously indicated, the suppression of multiple reflections arises as a direct result of dissipation of the energy of seismic waves from the uneven surface of the body of water, created by the positioning and size of the energy reflective elements.
Thus, it will be appreciated that, for example, motorized vessels such as small boats could effectively take the place of the floats (14) previously mentioned, it being only a requirement that the size of the vessels, and the relative spacing being maintained between each vessel fall within the range calculated from equation (1) and (2) above.
As also will be appreciated, individual elements whether they be floats, boats, or any suitable submersible object, interconnected or not, need not be all of the same size, nor maintained apart an identical distance, the only criterian being that the chosen sizes and spacing be pre-ferably within the ranges calculated on the basis of equations (1) and (2) above.
Further modifications and alternative embodiments ofl theinvention will be apparent in view of the foregoing descrip-tion, and accordingly, this description is to be construed as illustrative only, and for the purpose of teaching those skilled in the art, the manner of carrying out the invention.
It is further understood that the form of the inven-tion herewith shown and described is to be taken as the pre-sently preferred embodiment, various changes may be made in the shape, size and general arrangement of components dis-cussed herein, for example equivalent elements may be sub-stituted for those illustrated and described, as will be apparent to one skilled in the art to which this invention relates, after having the benefit of the description of the subject invention.
This invention rela~es to methods and apparatus of marine seismic exploration and particularly to methods and apparatus for suppressing multiple reflections.
The problem of suppression of multiples is one of the most important and difficult problem areas in marine seismic exploration. Many methods have been employed to im-prove the signal-to-noise ratio. Predictive deco~volution and stacking techniques have been successfully used for shallow marine data. However, in case deep water layers these procedures produced generally unsatisfactory results.
In many cases the energy of multiple reflections is so high that they cannot be sufficiently suppressed by the stacking technlques and standard deconvolution. Therefore no post-shooting processing will make the data useful. In a preferred embodiment of the invention it is desirable to perform the exploration with :a group of floats which rejects these multiple reflections.
In accordance with the present invention, elements, for example floats, adapted to reflect seismic energy pulses, while at least partially immersed in a body of water, are used to suppress substantially all multiples reflected from the water surface. This suppression of multiple reflections, being the result of dissipation of the energy of seismic waves reflected from the uneven surface created by the immersed sur-face area of the elements, and the surface of the water between each element.
.
13L~9~5 In accordance with another important aspect of the present in~ention, the size of the elements and the horizontal spacing between adjacent elements are commensurable with the dominant wavelength of the registered seismic waves.
The invention will now be described by way of example only, with reference to the accompanying drawings in which:-Figure 1 is a top diagramatic view of an array offloats according to a first embodiment of the invention;
Figure 2 is a diagramatic side view of the embodi-ment according to Figure 1, showing the disposition of the seismic source, and seismic detectors relative to the array of floats;
Figure 3 shows schematically, the dissipation of seismic energy by one of the floats according to Figures l and 2;
Figure 4 shows graphically, the effect of curvature of interfaces on the spherical divergence; and Figure 5 shows an alternative float configuration designed to reduce the effect of surface wind on the float array.
Referring particularly to the appended drawings, Figures 1 and 2 show a marine seismic exploration system according io the present invention.
A marine vessel (lO), tows cable with detectors, or receivers tll), located beneath an array of partially submerged and interconnected floats (12).
Adjacent floats may be interconnected by any suit-able means, such as lines or cable (13), and additional cables interconnect the array with the towing vessel.
While a specific array of floats (12) is shown in ~4~925 Figure 1, it will be appreciated that such does not limit the scope of the present invention. The number of floats, is dependent upon the sizes of the floats. It being necessary only that the array of floats (12) be sufficiently spread to substantially cover the immediate surface of the body of water above detectors (11), and a sufficiently extended area there-around. As an example only, the surface geometric patterning of floats can be arranged in a chess order.
As to the shape of floats (12) and the material used in their construction, these are not critical features. How-ever, for ease of understanding, floats (12) have been shown as spherical in form, and it is suggested that they be hollow, and formed from a suîtable plastics material. Weights may be utilized to ensure partial submersion.
Of prime importance however, is the size, and the spacing between adjacent floats and the principal feature of the present invention according to the embodiment disclosed herein, is that the size of the individual floats (14) is com-mensurable to the dominant wavelength (~) of the registered seismic wave.s, i.e. the diameter (A) of the float is preferably in the range:-A* 0.2~ to 5.0~ -----------------(1) Furthermore, the distance (D) between adjacent floats must, according to the present invention, be com}nensurable with the size of the floats, or with the wavelength of the seismic pulse. Therefore distance (D) is also in the range.
i.e. D~ .2 ~to 5.0 ~ --------------(2) To further explain, it is known that the velocity (V) of propagation of the seismic waves through water is ap-proximately equal to 1500 meters/second, and the basic frequency 1~45'~ZS
(F) of the seismic waves registered in marine seismic explora-tion is approximately S~ Hz. Therefore, the dominant wave-length (~) of the basic frequency of the propagating pulse in water is given by:-~ - V ~30 meters therefore, since A~ D falls in the range 0.2 ~ to 5.0~
then A~ D may fall within the range 6 meters to 150 meters As previously indicated, the suppression of multiple reflection according to the present invention arises as a result of dissipation of the energy of seismic waves from the uneven surface of the body of water created by the position-ing of floats (14) thereon.
Figure 3, shows schematically the dissipation of seismic energy by one of floats (12). In this figure, numeral (14) refers to a seismic energy source; numeral (15) refers to a geological reflecting interface; R is the radius of sur-face curature of the float (14) and; H is a known distance, measured between the bottom of float (:L4) and the geological interface (15).
It has been established that the eneryy, per unit area of the wave front, will substantially decrease after reflection from this uneven surface.
For the approximate calculation of this decrease in energy, the standard equation of Spherical Divergance (L) of seismic waves reflected from an uneven interface may be applied and is given by:-L = cos ~ (3) 4 H (1 + 2H~
R ) ~1~9~2S
where is the angle of slope of the reflecting interface:
Obvîously, it will be appreciated that in the case of a water surface without the array of floats, equation (3) above, will transform to the standard equation of Spherical Divergance for first multiples reflected from the water surface and will be given by:-L = 1 Therefore the presence of the floats results in the appearance, in the denominator, of an additional multiplier.
i.e. (1 + 2H) R ) which increases from the value one as the depth (H) of the interface, and the curvature of the floats (represented by 1) increases.
Figure 4 has been introduced to graphically show the effect of curvature of floats on the spherical divergence. From this figure it can be seen the spherical divergence decreases rapidly. As additionally seen from Figure 4, this decrease is even more rapid, as the principal dimension (R) decreases.
The intensity of seismic waves is therefore proportionally dependent upon the spherical divergence, thus, the intensity of all multiple reflections, from the uneven surface of the body of water, will correspondingly decrease at the same rate.
It must be mentioned at this point however that the method of the present invention may be practiced without the requirement of a towing vessel, or any interconnection between the energy reflective elements since as previously indicated, the suppression of multiple reflections arises as a direct result of dissipation of the energy of seismic waves from the uneven surface of the body of water, created by the positioning and size of the energy reflective elements.
Thus, it will be appreciated that, for example, motorized vessels such as small boats could effectively take the place of the floats (14) previously mentioned, it being only a requirement that the size of the vessels, and the relative spacing being maintained between each vessel fall within the range calculated from equation (1) and (2) above.
As also will be appreciated, individual elements whether they be floats, boats, or any suitable submersible object, interconnected or not, need not be all of the same size, nor maintained apart an identical distance, the only criterian being that the chosen sizes and spacing be pre-ferably within the ranges calculated on the basis of equations (1) and (2) above.
Further modifications and alternative embodiments ofl theinvention will be apparent in view of the foregoing descrip-tion, and accordingly, this description is to be construed as illustrative only, and for the purpose of teaching those skilled in the art, the manner of carrying out the invention.
It is further understood that the form of the inven-tion herewith shown and described is to be taken as the pre-sently preferred embodiment, various changes may be made in the shape, size and general arrangement of components dis-cussed herein, for example equivalent elements may be sub-stituted for those illustrated and described, as will be apparent to one skilled in the art to which this invention relates, after having the benefit of the description of the subject invention.
Claims (2)
PROPERTY OR PRIVILEGE IS CLAIMED, ARE DEFINED AS FOLLOWS:
1. A method of marine seismic exploration which facili-tates the suppression of multiples reflected from the surface of a body of water comprising:-(a) traversing a seismic exploration path with a marine vessel;
(b) towing behind said vessel:-(i) a plurality of spaced-apart seismic energy sources; and (ii) means for detecting reflective seismic waves;
(c) arranging in a horizontal geometric pattern above said sources and detectors, a plurality of spaced apart energy reflective elements, each of which elements being supported, at least in part, on the surface of said body of water, and (d) maintaining each of said elements apart from an adjacent element, a distance, substantially equal to the dominant wavelength of the said seismic wave.
(b) towing behind said vessel:-(i) a plurality of spaced-apart seismic energy sources; and (ii) means for detecting reflective seismic waves;
(c) arranging in a horizontal geometric pattern above said sources and detectors, a plurality of spaced apart energy reflective elements, each of which elements being supported, at least in part, on the surface of said body of water, and (d) maintaining each of said elements apart from an adjacent element, a distance, substantially equal to the dominant wavelength of the said seismic wave.
2. The method according to Claim 1 wherein said energy reflective elements are provided in the form of floats, said floats being submerged, at least in part, having an effective size equal to that of the dominatnt wavelength on the said seismic wave.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000336129A CA1149925A (en) | 1979-09-21 | 1979-09-21 | Multiple reflection suppression in marine seismic exploration |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000336129A CA1149925A (en) | 1979-09-21 | 1979-09-21 | Multiple reflection suppression in marine seismic exploration |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1149925A true CA1149925A (en) | 1983-07-12 |
Family
ID=4115184
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000336129A Expired CA1149925A (en) | 1979-09-21 | 1979-09-21 | Multiple reflection suppression in marine seismic exploration |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1149925A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4625302A (en) * | 1983-10-24 | 1986-11-25 | Exxon Production Research Co. | Acoustic lens for marine seismic data multiple reflection noise reduction |
-
1979
- 1979-09-21 CA CA000336129A patent/CA1149925A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4625302A (en) * | 1983-10-24 | 1986-11-25 | Exxon Production Research Co. | Acoustic lens for marine seismic data multiple reflection noise reduction |
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