RIASSUNTO
ABSTRACT
Sea trials were conducted on deep tow Sparker profiling equipment. Preliminary results indicate that data can be obtained of higher quality and in significantly worse sea conditions compared with equivalent surface towed systems. The addition of a tape recording facility should partly eliminate the problem of combining high resolution with good penetration. Cost will be comparable to existing surface towed systems, with mobility and operational flexibility only slightly reduced.
1. INTRODUCTION
For many years, engineering seismic profiling surveys have been conducted under a number of limitations. Foremost amongst these is the problem of obtaining consistently good records in sea conditions resulting from wind forces greater than about force 3. In the North Sea this problem has become progressively more important as oil activity has flowed northwards into rougher, more exposed waters. At the latitude of the Shetlands in winter, successful profiling work is normally restricted to a very few short lulls of seldom more than a few hours duration and, as often as not, at times when accurate navigational systems cannot be used. Ship's propeller noise can be an allied problem. This varies markedly from ship to ship, is difficult to predict and requires skill and experience to eliminate. Some vessels are virtually unusable. A third problem is that increasing resolution tends towards decreasing penetration. Inevitably same compromise is necessary between the power and the frequency of the seismic source.
With such limitations in mind targets were set to increase the usefulness of the profiling system. These targets are:-
SEA NOISE
To obtain good quality records in sea states corresponding to winds up to at least force 6.
ENGINE NOISE
To be able to use any vessel without interference from engine noise.
PENETRATION AND RESOLUTION
To be able to obtain records combining good penetration with high resolution from a single system. (Specifically 300 feet of penetration through sediment, with an ability to resolve down to 1 foot in the uppermost 30 feet).
COST
Costs should not differ markedly from conventional surface towed equipment.
PORTABILITY
The system should be fully portable and capable of operating from boats down to about 30 feet in length.
Targets 1 and 2 (sea and engine noise) can be overcome by the use of deep-tow equipment. However to satisfy 4 and 5 (cost and portability) it was necessary to use existing equipment wherever possible. Target 3 (penetration and resolution) is more difficult. A Multielectrode Sparker has the necessary frequency content and power handling capability, but like all other sources cannot normally achieve both aspects of the target on a single-pass. A partial way round this is to tape input. To keep within the cost target an analogue or very simple digital system is required and, it must be confessed here, this target has not yet been satisfactorily achieved.