Broad-Band Recording Through Ghost Elimination

Abstract

ABTRACT Recent acquisition and processing of marine seismic data using the variable depth hydrophone array or detuned array produces much better data. This technique has been utilized in many areas and water depths in the Gulf of Mexico, North Sea, Angola Cabinda, Congo, and Ethiopia. comparisons are made from data acquired simultaneously with twin hydrophone streamers; one streamer was towed slanted and one towed flat. Acquisition and processing techniques are demonstrated and benefits discussed. Comparisons include examples, diagnostics, and final processed data sets. Results indicate alternate solutions to the tuning effect of narrow band flat arrays and the broad band effect of slant-arrays, This broad band approach, possible with the s1ant array, yields much better stratigraphic data at the reservoir level. INTRODUCTION Normally, marine seismic data is acquired using multiple hydrophone groups arranged in continuous towed streamer arrays, deployed at relatively shallow uniform water depths. Flat streamer cables are typically towed at nominal speeds of 4 to 6 knots to enable CRP data to be gathered as efficiently and as noise free as possible. In the late 1970's, new Cable Levelers were developed allowing hydrophone depths to be controlled remotely from the recording vessel. This, coupled with the development of digital streamers, set the stage for hydrophone deployment at independently selected depths, In 1979, the first Slant Cable operations were conducted using a high resolution seismic system. The purposeful benefits were to remove the surface reflection distortion, or "Ghost", so that high resolution could be maintained and surface noise reduced as the cable was deepened. The procedure worked well and several programs were conducted with the system, after which u.s. Patent #4,353,121 was issued, It remained until 1986, before the larger exploration systems were deployed in the Slant Cable mode. BY this time, computer capacity and software capability had both been developed sufficiently to make processing cost-effective and competitive with typical Flat Cable technology, New software was developed, automating the procedures, allowing 2-D data to provide new insight into better structural and stratigraphic views of reservoir geology. This undertaking involved analyses of the Slant Cable seismic system response in both time and frequency domains, using actual Air gun array signatures for wavelet processing, DISCUSSING Air gun array data, when recovered from variable depth hydrophone groups, demonstrates some startling changes, The far field Air gun signature, as depicted in (Figure 1, Trace 1 on the left of the expanded view) as a down going wavelet, ghosted from deployment 25 feet below the water surface, The detected upward returning signature now characterized as a reflection, is then further ghosted by a second down going return from the water surface on trade 2 for a two foot hydrophone depth; on trace 3 for a 3 foot depth, and continuing at one foot deeper increments to a hydrophone depth of 200 feet on the right, The vertical sofa: lines are 20 milliseconds apart. surface ghost returns, in both cases, is reversed in polarity and nearly equal in amplitude to the primary.