Reflection seismology is a well-known technique for prospecting for sub-surface oil and gas reservoirs, both on land and in marine environments. As is fundamental in this technique, acoustic waves are imparted into the earth, generally by activation of a seismic source. Acoustic receivers detect the acoustic waves after their reflection from sub-surface strata and interfaces. Analysis of the source-receiver travel time of the acoustic waves, together with the known position of the source and receiver and the known acoustic wave velocity, provides an indication of the depth of the point of reflection at a location below the surface in the region between the source and receiver. As is well known in the art, sub-surface acoustically reflecting interfaces often correspond to the location of an oil or gas reservoir.
As is well known in this field, the depths of a reflecting sub-surface strata, taken at many points in a survey area, are necessary in order to generate a sufficiently accurate survey upon which drilling can be done. In addition, conventional seismic analysis techniques "stack" multiple traces of acoustic amplitude versus time for midpoints within a certain vicinity of the same location (such vicinity generally referred to as the "bin"), reinforcing the "signal" portion of the traces while the random acoustic "noise" tends to cancel out. As is well known, it is preferable that the multiple stacked traces for a given bin correspond to varying source-receiver offset distances, with normal move-out ("NMO") or dip move-out ("DMO") operations adjusting for the difference in travel time versus offset for a particular midpoint. Such stacking, or gathering, of trace data is conventionally referred to as common depth point ("CDP") or common midpoint ("CMP") stacking, with the number of traces for a given bin generally referred to as the "fold" of the survey for that bin.
Many conventional surveys are performed in the so-called two-dimensional (2-D) manner. 2-D land surveys are generally performed by deploying the acoustic receivers in a line and by activating the seismic source at locations collinear with, or offset from but parallel to, the line of receivers. In marine environments, 2-D surveys are generally performed by a vessel which is towing a source, such as an air gun, followed by a streamer of hydrophones. Other types of 2-D marine surveys use bottom-fixed receivers in combination with a towed source. As is apparent to those in the art, each 2-D survey is assumed to provide survey information relative only to a vertical plane into the earth. Surveying an area of the earth using these prior techniques requires multiple parallel 2-D surveys; an example of this technique is described in Canadian Patent No. 1 232 349, issued Feb. 2, 1988.
The surveying of an area by way of multiple parallel 2-D surveys has been referred to in the art as "3-D" seismic surveys, as a map of sub-surface geology over a surface region of the earth is generated. However, this type of survey is not truly "3-D", as information is acquired only at two source-receiver angles, or azimuths, with these two azimuths at a 180.degree. angle relative to each other. In particular, certain sub-surface formations of importance from the hydrocarbon prospecting standpoint may not be appropriately imaged in a survey made up of parallel 2-D surveys, depending on the azimuths of the survey, but would appear in a survey using reflections at varying azimuths. True 3-D seismic prospecting, utilizing acoustic reflections at varying azimuths as well as at varying offsets to provide surface-versus-time data from which a volumetric image can be generated, is quite beneficial in locating such formations.
In order to provide an accurate 3-D volumetric image from reflection results at many azimuths and offsets, the spatial sampling interval must be sufficiently fine to avoid spatial aliasing (i.e., the inability to resolve spatial structures, analogous to time-domain aliasing in the field of signal processing). In addition, in order to adequately suppress noise and focus the image, each bin must not only have statistically sufficient fold, but also should have traces at varying offset distances and azimuths, and therefore coupling, to allow the use of conventional processes such as 3-D velocity determination, spatial filtering, correction for statics, dip moveout (DMO) corrections and migration.
Of course, the acquisition of such data can be quite cumbersome and expensive. An example of a prior 3-D survey technique, particularly directed to land surveying, is described in U.S. Pat. No. 4,933,912. In this technique, an areal array of sources and receivers is used to obtain a large amount of seismic data at varying azimuths and offset distances. As is apparent from this reference (especially considering its disclosed technique for selecting only some of the source-receiver combinations for analysis), 3-D surveys taken in such a manner can be quite cumbersome, slow, and expensive.
Other prior techniques are directed to facilitating the acquisition of sufficient seismic data to generate a 3-D survey. A common one of such prior techniques is the so-called "swath" survey, where the receiver array consists of a number of relatively closely spaced parallel lines of receivers, for example spaced by a distance on the order of one-eighth of a mile apart, each line of receivers being several miles long. In the marine context, the swath of receivers may be deployed as parallel lines of bottom-fixed receivers. According to this method, the seismic source location moves in a direction parallel to the lines of the swath, to provide parallel subsurface lines of midpoints.
While the swath survey is commonly referred to as "3-D" due to the areal distribution of the receivers, true three-dimensional surveys do not result from this method. This is because the geometry of the swath necessarily provides a non-uniform azimuthal distribution of data because most of the source-receiver paths are at nearly the same azimuth; to the extent that there is azimuthal variation in the swath survey, the azimuth is highly dependent on the offset distance. In addition, significantly more data are obtained for each bin from within a narrow azimuthal range (e.g., on the order of 5.degree. or less) than at other angles, with the small amount of data from crossline azimuths limited to relatively short source/receiver offset distances. A directional bias is therefore necessarily present in this type of survey. Furthermore, there is generally insufficient interrelationship between passes or swaths to allow for proper correction of statics.
As a result of each of these limitations of the swath survey, the data processing techniques used therewith are conventionally limited to strictly 2-D analysis, treating the data from varying azimuths as though it is at a common azimuth with the majority of the data. This analysis limits the resulting survey to providing multiple 2-D surveys in parallel vertical planes. Furthermore, many traces in such a swath survey are redundant, i.e., having essentially common azimuth and offset with another trace. As a result, the swath survey is also quite inefficient.
Other examples of full 3-D land and marine surveys are described in U.S. Pat. No. 4,970,696, issued Nov. 13, 1990, assigned to Atlantic Richfield Company, and incorporated herein by this reference. In the land survey case described therein, seismic data of varying azimuths are acquired by arranging the receivers in multiple patterns, and moving the source location around the patch of multiple patterns. A similar survey is described in Crews, et al., "Applications of New Recording Systems to 3-D Survey Designs," Expanded Abstracts with Biographies 1989 Technical Program, 59th Annual International SEG Meeting, Paper SA 1.6, (Society of Exploration Geophysicists, 1989), pp. 624-27, also incorporated herein by this reference. As described at column 3, line 66 through column 4, line 3 of said U.S. Pat. No. 4,970,696, this technique is applicable to marine surveys with the receiver patterns placed on the seafloor or suspended thereabove. According to another embodiment described therein, a marine seismic survey is obtained by the towing of an array of receivers (corresponding to a pattern in the land case) through the off-shore region of interest, where a separate source vessel travels around the towed array to provide source seismic energy at the appropriate locations.
In each of the full 3-D surveys described in said U.S. Pat. No. 4,970,696, seismic data are acquired at many azimuths (i.e., relative angles between source and receiver locations). These data allow a true three-dimensional survey to be obtained, detecting sub-surface geological discontinuities which are at varying angles. In addition, other effects, such as near-surface effects, velocity changes, and the like may be characterized in the three-dimensional sense using these data. It should also be noted that the amount of data obtained (i.e., the fold) by such a true 3-D survey may be reduced, typically by a factor of from three to five, from that acquired according to prior 2-D surveys while maintaining the same degree of random noise attenuation. The theory explaining such fold reduction is described in Krey, "Attenuation of Random Noise by 2-D and 3-D CDP Stacking and Kirchhoff Migration", Geophysical Prospecting 35 (1987), pp. 135-147, also incorporated herein by this reference.
The methods described in U.S. Pat. No. 4,970,696 provide accurate and thorough surveys which are fully three-dimensional, by acquiring data at varying azimuths. It has been observed, however, that such surveys also provide significant redundancy in the data acquired, and accordingly are inefficient in some ways.
By way of further background, U.S. Pat. No. 3,753,222, issued Aug. 14, 1973, describes a three-dimensional land exploration technique including a plurality of parallel lines of geophones. As noted at column 2, line 65 through column 3, line 21 of this reference, seismic wave generating stations are deployed in lines offset from the longitudinal axis of the geophone groups, generally perpendicular to the geophone lines, and preferably aligned with a line of the geophones, with the spacing between stations in a line preferably equal to the spacing between lines of geophones. As disclosed therein, this arrangement provides a three-dimensional survey and is advantageous for static correction. The azimuthal bias of this land survey design should be noted, however.
By way of still further background, U.S. Pat. No. 4,677,598, issued Jun. 30, 1987, discloses a seismic data acquisition method in which corrections for statics is made. The survey includes multiple pairs of parallel receiver lines; source locations are arranged in line segments perpendicular to the receiver lines.
It is an object of the present invention to provide a marine seismic 3-D survey which provides reflection data having statistically significant azimuth and offset distributions for each bin.
It is a further object of this invention to provide such a survey which is particularly efficient in the deployment of marine receivers.
It is a further object of this invention to provide such a survey which allows for time-multiplexing of source and receiver combinations.
It is a further object of this invention to provide such a survey which efficiently provides fine spatial sampling so that aliasing is avoided.
It is a further object of this invention to provide such a survey which allows for flexibility in the fold of the survey without limiting the distribution of azimuth and offset for each bin.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.