GB2180341A - Method of acquiring and processing seismic energy signals and marine seismic array - Google Patents

Abstract

A two-dimensional array of seismic energy sources 12 and receiver elements 14 is disclosed for use in a marine environment. The two-dimensional array has element spacing d',D', chosen to attenuate horizontally travelling noise of certain wavelengths. Also disclosed is a seismic signal processing scheme for processing seismic signals obtained from the two-dimensional array to further attenuate horizontally travelling seismic noise. The signal amplitudes are scaled and then summed for elements aligned in the cross-line direction. Then the signals are scaled and summed for elements aligned in the in-line direction. The processed signals are then output for analysis. <IMAGE>

Description

SPECIFICATION Method of acquiring and processing seismic energy signals and marine seismic array The present invention relates to marine methods for acquiring seismic energy signals and, more particularly, to such methods specialized to attenuate essentially horizontally traveling seismic noise waves.

In the acquisition of seismic energy signals used to locate subterranean oil and/or gas deposits, it is known to utilize linear arrays of seismic energy receivers, i.e., long in-line distributions of hydrophones (in a marine environment). While these linear arrays of seismic energy receivers are especially good at attenuating, i.e., reducing or eliminating, seismic noise waves that travel parallel or nearly parallel to the linear array, these linear arrays are relatively ineffective at attenuating horizontally traveling seismic noise waves that travel in a nonparailel direction to the linear array.This nonattenuation problem is particularly associated with types of seismic waves travelling parallel to the ground surface, i.e., boundary waves, which sometimes causes the signal-tonoise ratio of the collected seismic traces to be so poor that the seismic traces are of little use.

Various processing methods, as well as various seismic energy receiver array layouts, have been tried to better attenuate these horizontally traveling seismic noise waves. The processing methods include stacking or summing the seismic energy signals; however, these methods have been found to not always greatly improve the quality of the resultant seismic energy traces.

Some of the various array layouts developed include a plurality of linear arrays of seismic energy receivers. One such array of seismic energy receivers is disclosed in U.S. Patent 4,403,312 to Thomason. In Thomason, arrays of seismic energy sources and seismic energy receivers are used to increase the signal-tonoise ratio by increasing the total number of signals (traces) which are reflected from any one common depth point (CDP). Thomason accomplishes this by using a particular sourcereceiver geometry where the receivers are on the perimeter of a box layout and the sources are in the interior of the box layout or visaversa. The recorded seismic signal traces are then summed together after assembling specified traces for a number of specified and different source records.The configuration of the arrays of the seismic energy sources and seismic energy receivers is determined by trial and error to obtain the best signal-to-noise ratio.

It is not disclosed or suggested within Thomason to attenuate horizontally traveling seismic noise waves by combining the seismic traces by sorting and summing (stacking) not during the acquisition of the seismic energy signals, but during processing, which greatly improves the attenuation, as will be described later. It is not disclosed or suggested within Thomason to space the seismic energy receivers or individual groups of seismic energy receivers a certain distance apart to attenuate seismic noise waves of a particular wavelength.

Specifically, in the marine environment, it is known to tow one or two elongated receiver arrays or streamers, containing a plurality of linearly arranged hydrophones, behind one or more seismic energy sources, such as waterguns or airguns. However, the inventors hereof know of no seismic receiver and source arrangement that utilizes one or more seismic energy sources with more than two receiver streamers towed behind the seismic energy sources.

U.S. Patent 4,254,480 to French discloses a method for attenuating seismic noise waves arriving at some angle from the vertical. He achieves this by deploying sources and/or receivers at different depths in a vertical plane parallel to the seismic profile. However, there is no disclosure or suggestion within French to locate the seismic energy receivers in a horizontal plane, as is the case with the present invention.

European Patent Application 0018053 to Lamb discloses a method to reduce horizontally traveling seismic noise by using arrays of seismic energy sources which extend in a lateral direction in addition to the in-line direction with respect to the seismic profile. Nowhere is it disclosed or suggested within Lamb to utilize a linear array of seismic energy receivers with a plurality of seismic energy sources, or to space the receivers a certain distance apart to attenuate horizontally travel ingseismic noise waves of a certain wavelength.

There is a need for a method that can be utilized in a marine environment for attenuating essentially horizontally traveling noise waves which are not traveling in the direction of the seismic profile in a manner that takes advantage of the particular spacing of the seismic energy receivers to attenuate seismic noise waves of a certain wavelength.

The present invention provides the novel method and related arrangement of equipment for overcoming the foregoing disadvantage and meeting the above described needs. Disclosed herein is a two-dimensional array of seismic energy receiver elements, such as streamers, spaced a predetermined distance apart in the in-line direction and in the crossline direction specifically for attenuating chosen wavelengths of seismic noise.

A feature of one form of this invention is a method for processing seismic energy signals specifically for attenuating undesired seismic noise. This method comprises acquiring the seismic energy signals from an areal array of seismic energy receive elements, the receiver elements being spaced a predetermined distance in the in-line direction and the cross-line direction, the spacing chosen to permit attenuation of certain wavelengths of seismic noise.

The amplitudes of the seismic energy signals are scaled and then the seismic energy signals from receiver elements aligned in a first direction are mixed. The seismic energy signals from receiver elements aligned in a second direction are then mixed, and thereafter, the processed seismic energy signals are outputted for analysis.

To aid in the understanding of the present invention the following non-limiting discussion is provided. First of all, the present invention is concerned with the spacing of the seismic energy receiver elements and, if desired, the seismic energy sources. The seismic energy receiver elements are arranged in a grid or an array pattern where the spacing is chosen to attenuate unwanted wavelengths of horizontally traveling seismic noise. Each seismic energy receiver element is a separately recorded, i.e., the seismic signals from each separate receiver element is not added, stacked or summed during recording.

Another feature of a form of this invention is a processing method that can be used with the present invention. After the seismic energy signals or traces are recorded, the traces are amplitude balanced. This means that the trace amplitudes are adjusted so that in any given time window, t, the signal amplitudes are approximately the same from trace to trace. This is accomplished by adjusting the trace valuers with a running sum or a single factor applied to all values of the trace, as is known in the art. Thereafter, each of the traces in the cross-line direction are multiplied by a factor and then summed together. The multiplication factor is determined by the method of weighting desired, again as is well known.The seismic traces are amplitude balanced as before, then each of the traces in the in-line direction are multiplied by a factor and summed in the same manner as before.

Thereafter, the processed traces are outputted for analysis.

The important point to emphasize about the deployment of the equipment used in the present invention is that there is a specific spacing of the seismic energy receivers elements with each individual receiver element being individually recorded. For the processing of the data which follows acquisition, the important points are trace balancing, trace weighting, and trace summing. The result of this novei processing method is to attenuate seismic noise waves traveling from any direction in the horizontal plane and to emphasize waves traveling in the vertical direction, but not necessarily in any one vertical plane.

Figure 1 is a semi-diagrammatic plan view of two prior art methods of towing seismic receiver streamers and seismic energy sources behind a vessel.

Figure 2 is a plan view of a vessel towing a plurality of seismic energy receiver elements and seismic energy sources in accordance with one embodiment of the present invention.

Figure 3 is a semi-diagrammatic plan view in variations of hydrophone patterns, as well as variations in seismic energy source patterns that can be utilized in the present invention.

Figure 4 is a processing flow sequence for one method of the present invention.

Figure 5 is a graphical representation of the effect of wave length vs frequency attenuation vs minimum seismic receiver element spacing.

Figure 6 is a left side elevational view of a vessel towing a plurality of seismic energy sources and seismic energy receiver elements in accordance with one embodiment of the present invention.

Figure 7 is a plan view of a vessel towing a plurality of seismic energy sources and seismic energy receiver elements in accordance with one embodiment of the present invention.

Disclosed herein is a two-dimensional array of seismic energy receiver elements spaced specifically for attenuating horizontally traveling seismic noise. The seismic energy receiver elements are spaced a predetermined distance apart in the in-line direction and the cross-line direction to attenuate certain wavelengths of seismic noise. A method for processing seismic energy signals in a manner to attenuate undesired seismic noise is also disclosed herein. In the method, the seismic energy signals are acquired from the two-dimensional array of seismic receiver elements. The amplitude of the seismic energy signals are scaled and the seismic energy signals are mixed from receiver elements aligned in a first direction.

The seismic energy signals are then sealed and mixed from receiver elements aligned in a second direction and thereafter the processed seismic energy signals are outputted for analysis.

It should be understood that each seismic energy receiver element can comprise a single hydrophone or a plurality, usually about 10, of hydrophones. It should also be understood that the spacing of the seismic energy receiver elements is such that they are close enough to each other to prevent spatial aliasing for a given frequency of noise waves, as is well known. The seismic receiver elements, as stated above, are arranged in a two-dimensional array having a total in-line extent D and a total cross-line extend d. D is usually greater than, but can be equal to, d. The spacing of the receiver elements is chosen on the basis of attenuating a predetermined range of wavelengths of seismic noise waves. The determination of this spacing is made as follows.Us ing the well-known equation: frequency (f) =velocity (v)/wavelength (A) and to prevent spatial aliasing, the relation: A=2*spacing distance (d); the correct spacing of receiver elements is: d = v/2f.

For example, if the frequency of seismic noise is 50 Hz and the velocity of the noise wave is v= 1500 m/sec, then d= 1500 m/sec/2*50Hz thus d= 15 meters.

As shown in Fig. 1, it is known to tow behind a vessel 10 at least one seismic energy source or array of sources 12, such as an airgun, watergun, or the like, and then one streamer 14, containing a plurality of spaced hydrophones. Also known is to tow only one seismic energy source 12 and then one or no more than two streamers. However, the inventors know of no arrangement of a plurality of sources 12 and/or more than two streamers 14 spaced to form a two-dimensional array and having a predetermined spacing chosen specifically to attenuate certain wavelengths of seismic noise.

In one embodiment of the present invention, as shown in Fig. 2, a vessel 10 can tow one or a plurality of seismic energy sources 12 from booms 15 and/or with paravanes 16, as is well known. The seismic energy sources 12 are located between the vessel 10 and the streamers 14, possibly at the head of each streamer 14 or not attached to any particular streamer 14. The seismic energy sources can be initiated simultaneously or in sequence, as desired.

Variations in the arrangement of seismic energy sources 12 along with arrangements of the streamers 14 are shown in Fig. 3. The total distance between the outermost streamers 14 (d in Fig. 2) is preferably wider than that of the vessel 10 and the cross-line (d') and in-line spacings (D') of seismic energy receiver elements are chosen so that horizontally traveling energy waves are sampled often enough to prevent aliasing, a distance which depends upon the frequency of the signals and the seismic velocity of the noise wave. In one embodiment of the method, the seismic energy source 12 is placed as near as possible (typically 50 meters) to the leading hydrophone within a streamer but not so close as to induce excessive noise from the source.

The spacing of the individual seismic energy receiver elements (D') within the streamers 14 and the streamers 14 (d') themselves can be equal or approximately equal, but again, close enough to prevent spatial aliasing. The in-line extent of the seismic energy receiver elements, i.e, the total length of the streamers 14 (D) can be about 2 to about 10 times as large as d.

Once the seismic energy signals from each of the receiver elements have been individually recorded and filtered to remove undesired frequency signals, i.e., the frequencies to be processed are filtered to the desired band width, the seismic signals are processed in any known manner. However, it is preferred that they are processed in the manner shown in Fig. 4. The signals are first scaled by multiplying each trace by a trace-constant or a timevariant scaler calculated from the values of the traced samples, as is known in the art, to ensure that the trace-to-trace amplitudes within any given time window (t) are approximately equal. Each element trace can then be multiplied by factors, as is well known, to provide greater attenuation than unit weighted arrays.Each seismic trace from the seismic receiver elements in the cross-line direction (perpendicular to the direction of the seismic line) are summed after any appropriate corrections are made for statics, or even for time shifting to highlight subsurface features. This summing is a process whereby a sample at a particular time, t, from one trace is multiplied by a scaler or a weight, and then is added to scaled samples from other traces, and then divided by the total number of weights added together. These scaler numbers or weights can be identical (unit weights) or unequal in some fashion corresponding to various computational methods, such as chebyshev weights. The amplitudes of the mixed traces can be equalized or scaled as before.

The cross-line traces which have been combined into a single trace, free from offending cross-line noise, can then be processed using known two-dimensional geophysical processing techniques, such as velocity filtering, common midpoint stacking, partial migration before stack, or plane wave domain methods.

Further, it should be understood that the individual seismic receiver elements (one or more hydrophones) can be weighted and summed in the cross-line direction any time during the processing. Also, the elements can be processed as individual receiver lines and then summed in the cross-line directions.

The increased attenuation of seismic noise afforded by the above described method is thought to be due to the trace scaling before mixing and the choice of the weights utilized during the mixing. The amount of noise wave attenuation can be adjusted by varying the weights within certain limits of the number and spacing of the arrays, as well as the bandwidth desired. The use of chebyshev weighting, in particular, results in greatly increased attenuation over that attained by single large areal arrays.

Specifically, for a single array of equally sensitive, equally spaced seismic receiver eie- ments, the maximum attenuation possible at any one wavelength is a function of the number of seismic receiver elements used. Theoretically, the response of an array of seismic energy receivers can be greatly improved by unequal detector gain or unequal detector spacing according to several methods for calculating array responses, i.e., chebyshev and Savit. Experience with the weighted arrays on land indicates that the actual results often fail to equal the expected theoretical attenuation improvement. This is often caused by the difficulty of ensuring equal coupling on the individual seismic energy receivers to the ground and of spacing the seismic energy receivers with the required accuracy in the field situations.Areal receiver arrays have not been utilized at all in the marine environment. The present disclosed method minimizes these field problems because the coupling of the array is an average of individual couplings used. This average should not vary greatly from one array to the next. Even if variations do result, the trace scaling prior to mixing minimizes these trace-to-trace variations. Also, any errors in receiver spacing is a very small percentage of the total area.

It is believed that the method and apparatus of the present invention has a wide variety of applications and can be particularly useful wherever scattered noise affects the data in the cross-line (horizontal) direction. Situations where the present invention can be utilized inciude the following: where high velocity limestone is on the sea floor, areas where a high velocity inhomogeneous layer forms the sea floor; where pseudo-Rayleigh or boundary waves travel in a cross-line direction; whenever the target depth is so deep that it lies within the noise cone, even if the noise velocity is not high; and in certain marine areas where a hard, rough bottom creates scattered waves traveling in the cross-line direction in the water column or sea floor.

The combination of the widely distributed seismic energy sources and seismic energy receiver elements can result in attenuation of horizontally traveling noise. Since the receiver elements are distributed in a horizontal plane, the arrival time patterns at the receiver element array of noise waves traveling mainly horizontally is much different than reflection signals traveling primarily vertically. The difference in arrival patterns can be used to attenuate these noise waves traveling horizontally by using the method, previously discussed, of the present invention. The energy waves traveling mainly verticaliy can on the other hand be ac centuated by shifting and summing by methods known as "beam steering," as is known in the art, that can be used with the present method.Another advantage of the present method is the arrangement of the hy drophones, i.e., the nearest hydrophones to the sources receive the most benefit from the directivity of the sources, which for a wide source array, helps to reduce side scatter or noises. Also, because the distance between the sources and the hydrophones is relatively short, the seismic signals are reflected from subsurface layers at a nearly perpendicular angle. This permits known processing methods of signal deconvolution to perform better since they are formulated for normally incident waves.

Fig. 5 is a graphical plot of seismic receiver element spacing (in seawater) vs alias frequency, which illustrates that to apply unaliased digital filtering requires sampling appropriate to the frequency desired (two spatial samples per wavelength for horizontally traveling waves). As an example, to achieve spatial filtering with areal arrays of seismic energy receiver elements in the marine environment, a minimum receiver element spacing of 15 meters is required to attenuate a frequency of 50 Hz. The longest wavelength that can be rejected by spatial filtering is approximately the number of elements multiplied by the elements spacing.For an example, using d'= 15 and D'=12.5, the longest rejected wave length in the cross-line direction is 6 stream ers X 15 (30 meters/2)=90 meters (16.7 Hz) and the in-line direction may be 16 hydrophone elementsX12.5=(25 meters/2) 200 meters (7.5 Hz).

The design of the source and receiver arrays are chosen so that the desired spatial filtering can be accomplished during processing. The receiver element spacing controls the highest frequencies (shortest wavelength) that can be rejected while the number of receiver elements controls the lowest frequency (longest wavelength) that can be rejected.

Figs. 6 and 7 show one embodiment of the present invention. A vessel 10 is shown towing a float 16 which has suspended there underneath a plurality of seismic energy sources 12, such as airguns. Extending from each float 17 is a stretch and several elastic vibration filtering cords 18, a connector section 20 with a depth controller 22, with the streamer 14 extending there behind.

The method of deployment of the arrays is by spooling a series of stretch sections 20 and the streamers 14 in series on a streamer reei. The deployment procedure for each is as follows: deploy the active sections, attach a depth controller 22 to the section 20 at about 20 meters from the active section. Thereafter, the forward end of the stretch section 20 is detached from the streamer real and the con nector 20 is brought around the deck support and connected to the signal lead-in cable 19 in parallel with elastic cords 18. Next, the sources 12 are deployed and attached to the lead-in cable and to the air supply tow cable, if airguns are used. This whole procedure is repeated for all subarrays, deploying the out board arrays first, then the inboard arrays.

A number of the advantages can be realized with the present invention including the fact that the width of the seismic source and receiver element arrays provides a spatial filter for side scatter noise, and the presence of two wide arrays (the sources and receivers) multiplies the effect of the responses in the cross-line direction and results in an increased cross-line noise attenuation even over the use of only a widesource array. In-line seismic noise attenuation occurs as with a conventional streamer. Thus, with the wide seismic receiver element arrays, side scattered noise is attenuated in all directions, not just in one as with a single conventional streamer.

The data received from the present method has been found to be relatively insensitive to changes in sub-bottom seismic velocities, that is, because the offsets are very short and the amount of normal move out is small. A consequence of this is that both flat and dipping events can be preserved and the data provides good input for two-dimensional migration. Also, it has been found that the data is insensitive to sideswipe, and structural noise from reflection points offline, which is due to the arrays having the most sensitivity to vertically traveling signals.

Further, the array configuration has several advantages when performing 3-D surveys in that the arrays form a spatial antialias filter in all directions and reduce 3-D migration noise.

Also, the time for conducting the survey is reduced because the distance required for the ship to turn around is reduced by a factor of 5 for a typical survey because of the relative shortness of the length of the streamer arrays.

Also, 3-D survey data obtained from the present method encourages very simple processing by reducing the problems of streamer tracking. Also, the data is from primarily vertically traveling waves, thus migration before stack is unnecessary.

Wherein the present invention has been described in particular relation to the drawings attached hereto, it should be understand that other and further modification, apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.

Claims (18)

1. A method of acquiring and processing seismic energy signals to attenuate chosen wavelengths of seismic noise, comprising: (a) acquiring seismic energy signals from a two-dimensional array of seismic receiver elements, the seismic receiver elements being spaced a predetermined distance apart in the cross-line and in-line direction to attenuate chosen wavelengths of seismic noise, (b) balancing the amplitude of the seismic energy signals acquired in step (a), (c) summing seismic energy signals from seismic receiver elements that are aligned in a first direction, and (d) outputting the processed seismic energy signals.

2. The method of Claim 1 wherein between steps (b) and (c) the seismic energy signals from each seismic receiver element are weighted.

3. The method of Claim 1 wherein the seismic energy signals from each seismic receiver element is individually recorded.

4. The method of Claim 1 wherein step (b) comprises multiplying each seismic energy signal by a trace-constant scalar.

5. The method of Claim 1 wherein step (b) comprises multiplying each seismic energy signal by a time-varying scalar.

6. The method of Claim 1 wherein the first direction in step (c) is perpendicular to the inline direction of the seismic receiver elements.

7. The method of Claim 1 wherein the seismic energy signals are summed in a second direction parallel to the in-iine direction of seismic receiver elements.

8. The method of Claim 7 wherein the summing comprises multiplying samples from the seismic energy signals at a time t, by a scalar, summing all of the samples, and then dividing the samples by the total number of samples summed.

9. The method of Claim 1 wherein between steps (b) and (c) the seismic energy signals are time shifted.

10. The method of Claim 1 wherein the seismic energy signals are combined by areal beam forming.

11. A marine seismic array having elements spaced for attenuating horizontally traveling seismic noise, comprising: (a) a seismic energy source array having lateral extent, and (b) a seismic receiver element array having lateral extent, the seismic receiver elements spaced a predetermined distance apart in the in-line and the cross-line direction to attenuate certain wavelengths of seismic noise.

12. The marine seismic array of Claim 11 wherein each seismic receiver element comprises one hydrophone.

13. The marine seismic array of Claim 11 wherein each seismic receiver element comprises more than one hydrophone.

14. The marine seismic array of Claim 11 wherein the seismic receiver element array has an in-line extent D and a cross-line extent d, where D2d.

15. The marine seismic array of Claim 14 wherein D is equal to from 2 to about 10 times d.

16. The marine seismic array of Claim 14 wherein the seismic receiver elements have an in-line spacing D' and a cross-line spacing d', where D' is approximately equal to d'.

17. A method of acquiring and processing seismic energy signals to attenuate chosen wavelengths of seismic noise substantially as hereinbefore described with reference to Figs.

2 to 7 of the accompanying drawings.

18. A marine seismic array substantially as hereinbefore described with reference to and as shown in Figs. 2, 3, 6 or 7.