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Abstract The Mini 3D project was undertaken by TOTAL and ELF with the support of CEPM (Comite d'Etudes Petrolieres et Marines) to define an economical method of obtaining 3D seismic HR data for shallow gas assessment. An experimental 3D survey was carried out with classical site survey techniques in the North Sea. From these data 19 simulations, were produced to compare different acquisition geometries ranging from dual, 600 m long cables to a single receiver. Results show that short offset, low fold and very simple streamer positioning are sufficient to give a reliable 3D image of gas charged bodies, The 3D data allow a much more accurate risk delineation than 2D HR data. Moreover on financial grounds Mini-3D is comparable in cost to a classical HR 2D survey. In view of these results, such HR 3D should now be the standard for shallow gas surveying. Introduction Advantages of the 3D seismic method compared to classical 2D for oil and gas exploration are obvious, particularly in applications requiring high spatial resolution. However, exploration 3D puts heavy constraints on data acquisition and processing. Costs are consequently high. High resolution 3D seismic investigation is not commonly used so for such purposes mainly because cost reasons in spite of the technical advantages. The objective of the Mini 3D project was to investigate economic HR 3D seismic for shallow gas risk assessment. The strategy was to try to define the simplest data acquisition configuration capable of 3D visualization of hazardous shallow gas that is consistent with the equipment currently used in the site survey industry. The work progressed as follows : The first step was to acquire a full, state of the art HR 3D seismic and to process the data with full processing sequence. The subsequent 3D volume was by definition the reference. Further steps were taken to simulate reduced acquisition geometries by means of successive decimations. The results were then compared to the reference volume Data acquisition Site selection To be representative, to possess the following attributes:proven shallow gas occurrencesprevious sumying for shallow gaswell data in the immediate vicinity the experimental site hadlocation in an active oil and gas producing region Such a site was found in the British sector of the North sea, in block 22/7 operated by TOTAL. Acquisition equipment and recording parameters The objective was to acquire a HR 3D volume corresponding to a drilling site making use, to the greatest extent possible, of standard 2D HR equipment including : vessel, seismic source, and streamer. The desired highest HR geometry was a 6,25 × 12.5 m bin size to be acquired along lines 12.5 m apart.

ABSTRACT It is recognised by most offshore foundation engineers that a poor level of correlation still exists between standard seismic records and geotechnical investigation data. The GEOSIS project aimed to define and develop new investigation procedures enabling a better integration of seismic and geotechnical data. Research has focused on the two following techniques : multi-channel very high resolution seismic surveying and seismic measurements in geotechnical boreholes. Toward the end of 1992, tests were carried out in 50 m of water depth offshore Monaco. the tests included multi-channel seismic surveys, continuous coring and sampling down to 80 m, sonic and nuclear logging and for the first time, P and S- wave vertical seismic profiling. The paper describes the offshore field tests and the Data processing procedures. Results demonstrate that both seismic cone and standard VSP techniques are able to provide data usable for the calibration and the interpretation of very high resolution 24-channel seismic surveying carried out on the same site. INTRODUCTION The installation of offshore facilities requires a precise spatial understanding of seabed stratigraphy and horizontal variations in soil characteristics. A typical site survey prior to the installation of offshore production facilities is done in two phases :an initial geophysical survey (including bathymetry, sonar, shallow seismic down to about 100 m below seabed) over an area of one to several sq. km.a dedicated geotechnical investigation at the location of the intended installation. These two surveys are generally carried out at different periods of the project, by different contractors. Consequently there is quite often a lack of cross-correlation between the two data sets, Sometimes resulting in critical uncertainties for foundations engineers. Moreover, when a potential change of platform location k likely to happen, it should be particularly interesting to optimise the number of boreholes needed to obtain sufficient knowledge on soil layer distribution and characteristics over the entire production facilities layout1. The objective of the GEOSIS project is to define and develop new investigation procedures enabling a better integration of geophysical and geotechnical data and a significant improvement of methods used to extrapolate soil data around geotechnical boreholes. Current practice in offshore soil Investigation In current practice, seismic reflection surveys for offshore civil engineering projects combine two main techniques :single-channel seismic surveying, analogue for the most part, with sources and receivers of very high frequency (0.5 to 3 kHz).multi-channel high resolution seismic surveying with sources and receivers of lower frequency( Single channel seismic numerical acquisition has been developed recently and is being increasingly used. It allows improvement of the output quality and presents the possibility of post-processing.