It has become common practice to undertake detailed engineering geophysical investigations of areas around proposed offshore drilling locations with the intention of assessing potential construction, anchoring or drilling hazards In this type of survey, sub-bottom profiling is regularly carried out, utilizing both single channel analogue and digitally recorded multichannel, reflection seismic acquisition techniques. The resolution and penetration of the typical systems used can be tabulated as in Table I.

It has become apparent that the quality of data acquired with surface towed sparkers is inadequate to provide information between the good quality data obtained from deep-tow sparkers and from digital seismic recordings. Typically, surface towed sparker data is totally unusable below the first multiple, where it is degraded by noise from other multiple events which follow The data are further degraded by the variable nature of the sparker signature and by the poor attenuation of swell noise with the single fold technique Racal Survey Limited has, over the past few years, attempted to improve the data quality by using alternative sources, for example small air-guns and waterguns This has gone some way to improving the data quality but has still not overcome the noise and multiple problems.

Digitally recorded seismic data is usually band limited by the sampling rate used in recording and by the source spectrum Data in the first hundred milliseconds sub sea bed are often degraded by source-generated noise and surface reverberation.

Racal Survey Limited have set out to develop an acquisition technique which will provide the highest possible resolution and signal to noise ratio in the first few hundred metres below the sea bed. The first attempts at solving the problem involved the use of a single 15 cubic inch watergun as the source with a 16 trace, 6 25 m group streamer The data was recorded at the very high sampling rate of 0 25 ms using TT Survey's recently developed TTS 96 system.

This chapter presents the results and compares them with conventional multi-channel site survey data and with conventionally recorded, single fold watergun data.

GEOLOGICAL HAZARDS

The geological features that may cause problems or hazards during structure emplacement or drilling operations may be subdivided as follows.

Gas hazards Pockets of shallow gas which are usually detected by the presence of amplitude, phase and velocity anomalies in the seismic records.

Structural hazards Faults, buried channels, slumps etc which may cause differential settlement under a structure or deviate a well from its planned route.

Lithological hazards Gravel sheets, boulder beds etc. which may prevent penetration of legs or piles and which may also cause well deviation or loss of circulation of the drilling fluid.

EQUIPMENT REQUIREMENTS

In choosing sensors to resolve these features, the site survey contractor should take into account the work limitations.

  1. Features which are resolvable at frequencies lower than around 80 Hz will have already been noted on conventional seismic data, and so the system need not be too rich in low frequencies

TABLE I Resolution and penetration of typical systems(available in full paper)

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