METHOD FOR DETERMINING SOURCE AND RECEIVER STATICS IN MARINE SEISMIC EXPLORATION

Abstract: Offshore seismic exploration over anomalous, near surface, low velocity layers (12) whereby reflection (13) and refraction (16) signals of generated seismic waves are used to correct for source and receiver status.

Description: METHOD FOR DETERMINING SOURCE AND RECEIVER STATICS IN MARINE SEISMIC EXPLORATION This invention relates to a method for determining source and receiver statics in marine seismic exploration. A major problem in seismic exploration has been the estimatio of time delays under source and receiver positions due to weathering, elevation, and shallow velocity changes. These time delays;are often simply called "static errors", and the corrections necessary to correc seismic traces for this error are called "static corrections" or "statics". The widepsread use of multiple coverage seismic profiling and the stacking of the multifold common depth point data as described in "Common Reflection Point Horizontal Data Stacking Techniques" by W. Harry Mayne, "Geophysics", Vol. XXVII, No. 6, Part 2 (Dec, 1961), pp. 927-938, requires the elimination of these static errors. Several methods are known for determining these static errors, one of which is described in "The Application and Limitations of Automatic Residual Static Correction Techniques" by B.M. Irving and J.K. Worley, presented at the 39th Annual International Meeting of the Society of Exploration Geophysicists in Calgary, Alberta, Canada. This method, as well as others, provides means for resolving the static estimates into source statics and receiver statics. It is these two components, source statics and receiver statics, which are utilized to correct, the seismic data prior to common depth point stacking. Some marine exploration areas are characterized by severe statics problems caused by the existence of a variety of subaqueous features and other sea floor irregularities. Such anomalies generally lie immediately below the water bottom and contain sediments having abnormally low velocities relative to normal sediments at shallow depths. They characteristically resemble channels ranging in depth up to a few hundred feet and lateral extent from a few hundred feet to thousands of feet. Velocities in these sediments are commonly less than half that of normal sediments. This decrease in velocity is due OMPI to gas bubbles in the sediment, causing a time delay in seismic energy waves traveling through the sediment relative to seismic energy waves traveling the same distance through normal sediments. One such marine exploration area is near the modern Mississippi River delta. The present invention resides in a method for determining source and receiver statics at a marine exploration site having laterally varying low velocity layers below the water bottom, comprising the steps of a) deploying a marine seismic detector cable along a seismic exploration line for which seismic reflection signals traveling to said seismic detector cable pass through a near surface low velocity layer, b) deploying a marine seismic monitor at a distance remote from said seismic detector cable such that the first energy received at said monitor is in the form of seismic refraction signals which have passed through said near surface low velocity layer, and c) generating first seismic energy at an offset distance from said seismic detector cable so as to produce a first seismic signal which travels downward into the earth, is refracted through the earth, and travels upward through said near surface low velocity layer to intersect said seismic detector cable in a generally orthogonal direction, whereby said first seismic signal, as received by said detector cable, includes a receiver statics component attributable to said near surface low velocity layer, and d) generating second seismic energy so as to produce a second seismic signal which travels downward through said near surface low velocity layer into the earth, is refracted through the earth, and is detected by said remote seismic monitor whereby said second seismic signal, as detected by said remote monitor, includes a source statics component attributable to said near surface low velocity layer. In the accompanying drawings, which illustrate a method according to one example of the present invention, Figure 1 is a pictorial representation of the technique for determining source statics over a marine seismic exploration area, and Figure 2 is a pictorial representation of the technique for determining receiver statics over a marine seismic exploration area.. Referring to the drawings, in the example shown exploration is carried out with a seismic detector cable 14 fixed in position on the water bottom and extending along a seismic exploration line. Seismic energy is generated in the water by a source 11 towed by a marine vessel 10 and reflections of the energy from subsurface formations are detected by hydrophones located along the cable 14. The detected signals are then transferred to the marine vessel through the cable wiring for recording and processing. If a hydrophone overlies an anomalous layer of low velocity sediments, an undesirable receiver statics effect will be imposed on the seismic reflection signals. Also, if the seismic energy source is energized over such a low velocity sediment, undesirable source statics will be recorded in the seismic reflection signals. Referring now more particularly to Figure 1 which illustrates determination of source statics, seismic energy waves from the source 11 are shown as travelling downward through the water and passing into a subsurface formation through a low velocity sediment in a buried channel 12. Reflections of the seismic energy from the subsurface formation travel upward by way of path 13 and are received by the on-bottom seismic detector cable 14. To identify the extent to which source statics is introduced into the seismic reflection signals received and recorded by the detector cable 14, there is provided at least one remote seismic monitor 15. After passing through the channel 12 into the subsurface formations, the seismic energy waves are also refracted along a path 16 and received and recorded by the remote monitor 15 simultaneously with the recording of the seismic reflection ' signals by the detector cable 14. With a sourcε-to-detector cable offset of, for example, 0-4000 feet, a suitable source-to-remote monitor offset will be in the order of 15,000-25,000 feet. The recorded refraction data may then be compared with the recorded reflection data to determine the low velocity layer statics for each source location during the exploration operation. OMPI . _ ~ Referring now to Figure 2, there is shown the technique by which receiver statics introduced into the detector cable 14 may be determined. In Figure 2, the detector cable 14 is shown as being deployed on the ocean bottom such that seismic reflection signals will pass upward through the low velocity sediment of the channel 12 and be recorded by the hydrophones in the cable 14. To identify the extent to which receiver statics is introduced into the seismic record along with the seismic reflection signals, the vessel 10 locates the seismic energy source at an offset from the detector cable 14 so that the seismic energy waves from the source travel through refraction paths in the subsurface formations upward through the channel 12 and then intersect the cable 14 from a generally broadside or orthogonal direction. The first refraction arrivals recorded by the hydrophones in the cable 14 are compared to conventionally recorded refraction data from non-low velocity layer areas to determine the receiver statics for the selected exploration site. Corrections to the recorded seismic reflection data are then carried out by removing the distortions caused by such source and receiver statics introduced by the low velocity sediments in the buried channel at the selected exploration site. In the above described preferred embodiment, the seismic exploration system, may be of the type described in U.S. Patent No. 2,590,531 wherein the seismic detector cable is deployed on the ocean bottom from a first marine vessel while the seismic energy source is deployed from a second marine vessel. A suitable seismic energy source produces repetitive pulses of seismic energy from an air gun and is described in U.S. Patent No. 3,687,218. An on-bottom seismic receiver system that is particularly suitable for use as the remote seismic monitor 15 for refraction measurements is disclosed in U.S. Patent application Serial No. 163,757, filed June 27, 1980, to Bowden et al.

Claims: CLAIMS; 1. A method for seismic exploration at a marine exploration site having an anomalous near surface low velocity layer below the water bottom, comprising the steps of: a) deploying a marine seismic detector cable along a seismic exploration line for which seismic reflection signals traveling to said seismic detector cable pass through the near surface low velocity layer b) deploying a marine seismic monitor at a distance remote from said seismic detector cable such that the first energy received at said monitor is in the form of seismic refraction signals which have passed through said near surface low velocity layer, and c) generating first seismic energy at an offset distance from said seismic detector cable so as to produce a first seismic signal which travels downward into the earth, is refracted through the earth, and travels upward through said near surface low velocity layer to intersect said seismic detector cable in a generally orthogonal direction, whereby said first seismic signal, as received by said detector cable, includes a receiver statics component attributable to said near surface low velocity layer, and d) generating second seismic energy so as to produce a second seismic signal which travels downward through said near surface low velocity layer into the earth, is refracted through the earth, and is detected by said remote seismic monitor whereby said second seismic signal, as detected by said remote monitor, includes a source statics component attributable to said near surface low velocity layer. 2. A method as claimed in claim 1 further including the step of detecting said second seismic signal by said seismic detector cable as said signal is reflected from the earth. 3. A method as claimed in claim 2 wherein said reflected second seismic signal detected by said seismic detector cable is compared to said refracted second seismic signal detected by said remote seismic monitor to determine the source statics for the selected exploration site. 4. A method as claimed in claim 2 or claim 3 wherein said second seismic energy is generated at a location such that the reflected second seismic signal is detected by said seismic detector cable at the same time said refracted second seismic signal is detected by said remote seismic monitor. 5. A method as claimed in any preceding claim wherein said detected first seismic signal is compared to recorded refraction data from non-low velocity layer areas to determine the receiver statics for the selected exploration site. 6. A method as claimed in any preceding claim wherein said seismic detector cable is deployed on the ocean bottom. 7. A method as claimed in any preceding claim wherein said remote seismic monitor is deployed on the ocean bottom.

Inventors: WILSON JACK LYNN, WILSON, JACK, LYNN
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1982
Main classification: G01V1
IPC codes: G01V1, G01V13