A New, High-Frequency, Broadband Seismic Source

Abstract

ABSTRACT A new seismic source has been developed which emits a reproducible, high frequency, broadband acoustic signal. This device, suitable for marine sub-bottom profiling is a unique variation of the multi-electrode sparker source. Calibration experiments performed in "free field" conditions have shown that the output is tunable by varying the power supplied to the source. Typical output has a 4.2 kHz bandwidth and a 3.0 kHz centre frequency, measured 3 dB down, at an operating power of 480 Joules. INTRODUCTION Resolution of two seismic reflectors depends on the length of the acoustic pulse. Simply put, the shorter the pulse, the finer the temporal resolution. Taken to the extreme, the ideal pulse would be a spike with all the energy concentrated at a single point in time. Since bandwidth increases with decreasing pulse length, the desired pulse is broad band, containing high frequencies to permit resolution of thinly spaced reflectors, as well as low frequencies which achieve greater sub-surface penetration. [1] For a given pulse length, the desired shape is 'front end loaded'; having its energy maximally concentrated at the onset. [2] Such a pulse is known as minimum phase. Digital statistical deconvolution processes assume that the source wavelet is minimum phase. The quality of the output from these deconvolution processes degrades as the pulse departs from minimum phase. An acoustic source which delivered the same acoustic signature for an indefinite period of time would reduce the possibility of interpreting a deviation in source signature as a lithologic change in the subsurface. Finally, an ability to control and alter the length and shape of the emitted pulse would permit a tuning of the source to a particular seabed. In summary, good quality seismic data depend, among other things, on the length, shape and consistency of the downgoing acoustic pulse, as well as on an ability to alter the pulse in a controlled manner. We have developed an acoustic source based on the common sparker source routinely used in sub-bottom profiling. The conventional multi-electrode sparker has many advantages including its ease of operation, versatility and low cost; however, it does suffer from a degradation in pulse shape through the course of a survey due to the deterioration of electrodes. [3] Our efforts have concentrated on engineering a multi-electrode sparker that emits a minimum phase, reproducible signal with a higher frequency and a broader bandwidth than the conventional sparker. This device was developed specifically for the unique requirements of a stationary high resolution geophysical site surveying tool. [4][5] However, calibration of the source in a towed mode is planned for the next phase of our research. This paper describes the source and shows examples of field data. SPARKER THEORY The term sparker is misleading, it tends to imply that the mechanism is the same as that of a spark plug. However, a spark plug is a device which includes a pair of electrodes with an insulator in the inter-electrode space.