Digtal CDP Procedures Provide New Capability for High Resolution Seismic Data
Authors: Dee Mark 
Published: October 2008

With proper regard for frequency content, conventional data processing procedures, including common depth point (CDP) horizontal stacking, have been applied to digitally recorded high resolution sparker data. The resulting processed section displays accurately-determined details of structure, stratigraphy, and reflectivity from the seafloor to depths reaching 8000 feet. Detailed velocity analyses prove to be a useful by-product of data processing, enabling lithologic and overpressure determinations. The CDP SUPERSPARKER * data have been used for both engineering and exploration applications.

INTRODUCTION

Through utilization of common depth point (CDP) stacking and other digital data processing techniques, significant improvement in depth penetration and record quality has been obtained for digitally recorded, high resolution, seismic data. In addition use of relative amplitude preservation procedures enables detection of amplitude and phase anomalies with a sensitivity far exceeding that of conventional seismic data. Conventional CDP processing procedures have been specifically adapted for the frequency content of seismic signals generated by a highly efficient electric discharge sparker system (SUPERSPARKER). The energy source has a broad spectrum with a central peak frequency of approximately 100 Hz.

DATA ACQUISITION

A one-inch diameter hydrophone streamer consisting of twelve independent, active sections, was used to record seismic signals. Each active section consists of thirty-two acceleration cancelling hydrophones arranged in a binomial tapered array in order to afford cancellation of horizontally traveling waves. The hydrophone response, filtered against aliasing, was digitized at a one millisecond sample rate, summed in a field minicomputer, and recorded on magnetic tape in a demultiplexed format. Concurrently, an analog display of the near trace was recorded on an EPC graphic recorder. This record is used primarily to monitor system operation and to maintain quality control. In addition to assisting the data processor in orienting himself with respect to the surveyed line, it also serves as a final comparison of processed results with original data.

RECORDING GEOMETRY

The standard 12-trace recording geometry is illustrated in Figure 1. The group interval of the streamer employed is 150 feet. The traces are numbered in decreasing order from near (12) to far (1), corresponding to industry convention. Total cable length is 1650 feet. The near and far offset distances are 350 feet and 2000 feet, respectively.

The NMO times for the far offset trace were calculated using both a constant water velocity of 4850 feet per second and a linear increasing velocity function with a gradient of .6 ft/sec/ft and are tabulated in Table 1. From the magnitude (42.1 ms) of NMO at 2.0 seconds at water velocity, one can appreciate that in deep water recent depositional environments the amount of normal move out correction to high resolution data would be substantial. In the situation where a complete geologic section exists (linear increasing velocity) and the average velocity at a time of two seconds is 12,000 feet per second, there would still be approximately 7 milliseconds move out on the far trace.