1. Field of the Invention
This invention relates to seismic exploration in general. More specifically, it concerns an improved method of reflection-type seismic exploration that is particularly applicable to marine operations.
2. Description of the Prior Art
It has been known for quite some time that in marine seismic operations, there are particular problems that are not encountered in land operations. One aspect stems from the fact that in marine operations the seismic detectors placed below the surface of the water are sensitive to seismic waves in the water regardless of their direction of travel. Furthermore, pressure-type detectors are usually used, whereas in land operations the detectors are ordinarily a displacements or inertia type.
Heretofore, in marine operations, seismic waves were often generated by detonation of an explosive charge which was usually placed at a depth of ten feet or less below the surface. This avoided interference produced by the phenomenon commonly called "bubble bounce" which interference is generated by charges fired at greater depth. The shallow depth of charge also would avoid the problem of ghost reflections which are encountered in land-type shooting where the charge is detonated in competent earth material some distance below a good reflector.
However, the additional problem remained in offshore seismic exploration which was created by the vertically travelling reflection signals that will reflect back down from the surface of the body of water and, consequently, will create an interference pattern.
Consequently, it is an object of this invention to provide an improved method of marine-type seismic surveying. Furthermore, another object is to provide a method of marine-type seismic exploration which greatly simplifies the procedure for elimination of undesirable seismic wave energies from a recording.
Another object of the invention is to provide a marine reflection-type seismic method for creating a record which is independent of depth of the detectors in the water and free of down-going signals.