Detailed Site Survey Using VHR 3D Seismic
Authors: Marsset B. , Meunier J. , Marsset T. , Noble M. 
Published: October 2008

We present a preliminary attempt to achieve 3D seismic survey for Very High Resolution purposes. The term Very High Resolution (VHR) seismic refers to mean source frequencies over 500 Hz, i.e. to signal wavelengths of metric scale. The 3D field system includes 4 streamers, 35 meters long, with an active section of 10 meters, each streamer containing 6 traces (2 meters spacing).

The processing of 3D seismic data requires accurate positioning of the acquisition device, whereas real time navigation is carried out using standard D-GPS, off-line processing is performed using natural GPS corrected for trajectographic errors. Seismic processing includes 3D prestack Kirchohff depth migration which allows easy handling of the irregular position of both source and receivers. Comparison between depth slices extracted from the 3D volume and conventional 2D geological interpretation, shows a profusion of details, underlying the advantage of this method for small scale geological studies.

Introduction

Very High Resolution single channel seismic survey gives access to an image of the different sedimentary layers encountered, but does not allow a quantitative characterization of the subsurface. Furthermore the image is distorted due to traveltime representation. The use of multichannel arrays resolves most of these problems as it gives access to velocity information. Nevertheless the resulting subsurface image remains inaccurate due to the 2D approximation, i.e. the data gathered by the receivers are interpreted in a vertical plane along the direction of the ship's track. Lateral effects, e.g. related to dipping layers or diffracting points, decrease the resolution of the image and the accuracy of the calculated seismic attributes. With the design of marine structure foundations needing more and more accurate surveys, IFREMER has decided to conduct a project on the adaptation of the deep exploration 3D method for shallow geotechnical and geological site investigations of small to intermediate size.

The Very High Resolution 3D seismic acquisition method was applied in the Mediterranean sea, near the present Rhone delta. The goal was to image coastal sediments deposited since the last ice age and thought to be linked to the river activity of the Rhone. The area of interest was one square kilometer large and was surveyed with 100 seismic profiles, 20 meters apart. Each 3D track line consisted of four streamers with a 10 m spacing between the different streamers therefore corresponding to a 30 m wide survey. The acquisition geometry of the four streamer array was handled using divergent panels, these panels are linked together by a 5mm section wire on which the four streamers are hooked. The lines were shot at 3 knots, at a 1 second shooting interval of a sparker at 300 Joules. The 3D acquisition survey required about 4 days.

Fig. 1-Flexible divergent panels. (Available in full paper)

Data acquisition

The solution to handle the 3D seismic array involved the use of flexible "parachute-like" panels, which were recently developed at Ifremer in the framework of fishery research. The panels (Fig. 1) are basically parachute wings made of plastic canvas with a flat intrados and a concave extrados.