High-resolution Geophysical Surveys for Engineering Purposes

Abstract

In recent years it has become increasingly evident that there is a need to bridge the gap that existed between the ultra-high resolution (but with limited penetration) provided by analogue equipment and the so-called high-resolution shallow seismics. The latter generally provides resolution of about 10 m, with limited data in the top few hundred milliseconds, but gives satisfactory information at greater depths. To some extent, vertical resolution can be improved by paying more attention to the deconvolution techniques at the processing stage, and lateral resolution can be improved by migration of the data. However, these options — particularly that of migration which is very costly and therefore depends on the client’s willingness to pay — are seldom within the control of the survey company. Hence, two years ago, Gardline’s Geophysical Research and Development Department undertook the design of a new acquisition system which would do much towards achieving such objectives. The approach to this project is outlined, and an account of the system is a high-resolution multichannel hydrophone array (the mini-streamer). The technical specifications of this mini-streamer are described, together with the theoretical characteristics and responses of the array design.

The mini-streamer, moreover, is part of an integrated digital package consisting of source (mini-sleeve exploder), array (the mini-streamer), recording system (an upgraded DFS V) and a processing capability developed in close co-operation with a specialist high-resolution processing house.

The new system was tested over three separate locations, and the results of these test lines are discussed. As a result of these tests — all of which were very successful — it has been possible to assess the optimum operational capabilities of the system, as well as its geophysical potential.

Since then, nine site surveys have been successfully completed with the system, and a case study of some of these sites is presented along with illustrations of the quality of data obtained.