Source specifications for high resolution seismic surveys have only recently been introduced into contract documents and this development is significant in terms of the data quality which can be achieved if the specifications are properly applied. Over the past ten years it has been traditional that site surveys, and other similar projects involving the use of digitally-recorded high resolution seismic data, have not been regarded very highly by the managements and exploration and production departments of the Client companies. Accordingly, it was often left to the survey department, if one existed, or an operations group, to acquire the high resolution data as quickly and as cheaply as possible, with minimal disruption to the "more important" operations in progress. Processing and interpretation of the data were viewed in the same way. As long as a basic "bright spot" detection exercise could be said to have been carried out, all the requirements and Interests existing at that time were satisfied. Given these circumstances, it is not surprising that seismic source specifications were not even considered when planning high resolution surveys.
Finally, in 1990, it appears that a small proportion of the offshore operators now recognize that there is a requirement for acquisition of optimum quality high resolution seismic data, for the efficient detection of shallow gas hazards. This recognition has developed as offshore drilling activity has continued at a high level all over the world and the safety considerations have escalated in response to a number of well-publicized accidents and attendant public awareness. The importance of optimizing high resolution seismic data, in order to provide information vital for safety during offshore drilling, has been clearly demonstrated in numerous instances in the past. Seismic source specifications are fundamental to the improvement of high resolution data, and to the detection of gas hazards, and this paper demonstrates the effects which can be created by properly-applied and improperly-applied specifications. It is hoped that an awareness of these effects may encourage other operators, who currently have little regard for high resolution seismic data, to recognize the potential for effective hazard detection offered by the method and the risks involved if inadequate site surveys are carried out.
In the late 1970's and early 1980's the source used by most high resolution seismic contractors was the sparker, or sparkarray. This was because it was the cheapest and easiest source to operate at the time. The output pulse characteristics of the source, which determine the limits of resolution and penetration attainable using that source, were of no concern and were not usually specified. This was just as well, because a typical sparker pulse, shown in Figure 1, is rather difficult to provide specifications for, in the normally accepted terms.