A New Method for Improving the Resolution of Shallow Sub-bottom Seismic Reflections in Deep Water

A new method of achieving very high resolution of seismic reflections near the sea floor in deep water has been developed and tested during several sea trials. The method employs a conventional energy source deployed on the sea surface and a single channel receiver deployed at few hundred meters depth. As much as possible, the source is towed directly above the receiver. The intention of this geometry is to record a far-field source signature that is essentially a vertically traveling plane wave. The principal restriction is that the receiver be located far enough above the sea floor that the direct arrival is not contaminated by the sea-floor reflection. This means that, if pneumatic sources are to be used, the water should be at least a hundred meters deeper than the receiver tow depth. During data acquisition, the procedure known as "over sampling" is practiced by digitizing the received signals at a rate on the order of 100,000 samples per second. This is sufficient to achieve good waveform fidelity in the 5-1 kHz range. Post processing then uses the source signature to collapse the waveforms of the reflected wavelets on a trace-by-trace basis. The result is that the reflected wavelets are transformed into waveforms that are generally symmetric about a central peak or trough, depending on the sign of the reflection coefficient. Self-calibration permits the magnitude of the central lobe to be used to estimate the value of reflection coefficients. Sea trials using 15 and 80 cu.in. waterguns were carried out in water depths ranging from 300m to 1600m. Results indicated that the thicknesses of layers up to 75ms below the sea floor were resolved to within a few tenths of a millisecond. In one trial across a well where a potential shallow flowing sand 400 ms below the sea floor had been encountered but controlled, the upper boundary of the sand body was resolved to less than a millisecond.