Evalutation of sea bottom features from seismic calibration data: A case history

Abstract

Introduction During 1992 a seismic survey was conducted in the West Delta area of the gulf of Mexico. The water depth in the area of the survey varied from approximately 20 feet to over 200 feet, and for this reason, seismic acquisition using a bottom cable technique was selected. Additionally the use of hydrophones and geophone was chosen, because of the potential to minimize ghost and reverberation effects associated with the variable water layer. It was known prior to the commencement of the survey, that in some areas, the “hard” water bottom was overlain by soft mud flows with remarkable physical properties including velocity in some cases of less than 1000 feet/sec. ( ref. Meeder et.al. (1988) ).It was anticipated that the mud flows may present significant problems in both acquisition and processing of the seismic data, and an attempt was made prior to the seismic survey, to map the sea floor features as part of a hazard survey, and some aspects of the seismic acquisition and processing were specifically aimed at trying to resolve the sea floor features. Calibration Data. Vertical incidence “calibration” data was collected as a separate part of the acquisition effort. Calibration data offers one approach to the processing of hydrophone and geophone data for bottom referenced systems, and can be used to determine a scalar, appropriate for application to the geophone component of the seismic data, prior to summation with the hydrophone data, in order to optimally attenuate ghost energy and water bottom reverberations. (ref. Barr and Sanders 1989 ). Other methods of optimally combining bottom referenced hydrophone and geophone data have been proposed, but in this case the calibration data offered an early opportunity to resolve some of the sea bottom features. Note that while the conventional seismic data was in general noisy for a variety of reasons, the calibration data was of very high quality, since only the direct arrivals are examined, and in this seismic data set the source is positioned as close as possible to the receivers ( vertically over each receiver on the bottom ). An example of the calibration data for a set of receivers is shown in Figure 1. Seismic data The quality of seismic data acquired in the survey area was extremely variable. In some areas of the survey, where there were large accumulations of soft mud on the bottom, the seismic records were so weak that only direct arrivals on the near traces were visible when displayed at the same gain as other records where the data amplitudes appeared normal, the mud layers led to large absorption of the seismic energy. Additionally there were at least eight different types of noise regularly observed in the seismic data, and the shallow mud layers resulted in to trace to trace statics which on occasions approached 100 msec.