ABSTRACT

Digitally processed geophysical data has been compared to ground truth from geotechnical investigations. The processing method used was a calculation of layer acoustic impedance; a technique which has been referred to as the acoustic core technique. Correlations of the locations of geological units identified with geophysical and geotechnical techniques have been sought. The units identified by the acoustic core technique are shown to compare well to those identified with the piezometric cone.

The acoustic core technique supplemented by geotechnica1 ground truthing shows promise as a powerful three-dimensional site-imaging tool. The technique extracts additional, more subtle information from high resolution survey results than does traditional analog interpretations.

INTRODUCTION

Severa1 of Canada I s major oil companies are actively exploring the oil resource potential of the continental shelf of the Canadian Beaufort Sea. Gulf Canada Resources Inc. (GCRI) is an exploration operator in this region (Figure 1).

At potential exploration drilling sites, complete and accurate geophysical and geotechnical evaluations of the upper 100 m of bottom sediments are required to:

  • assess the stability of gravity base structures and artifical islands under large horizontal ice loads during year round operations.

  • estimate the available holding power of the mooring system of floating drilling units.

  • identify any shallow dri1ling hazards such as shallow gas and permafrost.

  • identify potential borrow material for foundations for gravity base structures and for artificial island construction.

Present site testing techniques include:

  • high resolution shallow seismic mapping over a 5 sq. km area on an approximate 250 m to 500 m grid pattern to give a three dimensional image of site stratigraphy, complemented by

  • geotechnical sampled boreholes and in situ tests to obtain detailed engineering parameters and stratigraphy of a specific foundation site.

The analog signal recorded from shallow high resolution seismic data is often presented on a graphic recorder without prior processing. Figure 2 presents an example of an EPC graphic record of data collected from a boomer sound source. The maximum dynamic range of this graphic recorder is 20dB, which limits detection of subtle changes in signal level outside of the dynamic range. In order to extract additional information and improve the signal-to-noise ratio, the reflected signal is digitized and then processed. The final numerical results can then be interpreted.

The digital processing involves calculation of the reflection coefficients through interpretation of the signa1 energy content and then calcu1ation of the acoustic impedance of the soil layers present relative to sea water.

The results from this digital processing technique will be compared to geotechnica1 results from a sample site shown in Figure 3.

Keywords:
epc record, interpretation, impedance, geology, high-resolution seismic data, calculation, reservoir characterization, reflection coefficient, reflector, acoustic core technique
Subjects:
Reservoir Characterization, Seismic processing and interpretation
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