Some Basics and Applications of the VIBROSEIS System of Exploration

Abstract

This paper was prepared for the Eastern Regional Meeting to be held in Columbus, Ohio, November 8–9, 1972. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract The "VIBROSEIS" system of exploration had its beginnings in 1950, its first application on a production basis in 1957, its first public demonstration in Ponca City on April 10, 1958, its first published article in the February 1960 issue of Geophysics and its introduction as a tool for use by the general exploration industry in 1961. While the principles have remained the same, the intervening years have seen tremendous changes in vibrator, recorder and correlator equipment as well as in processing techniques. The main result of these changes has been continued improvement in signal quality and in m reliable and effective field operations. The "VIBROSEIS" system is an engineer system that uses hydraulically operated vibrators to send sound signals (instead of shock waves) down through the earth. The "VIBROSEIS" system input signal is a swept-frequency sinusoid (called the "sweep") that can be made to last for any length of time but in general practice has been from 3 to 24 seconds. The entire length of the sweep is partially reflected at each rock formation or seismic interface. Each reflected event then is essentially the length of the sweep duration. The total recorded field record time must be equivalent to the desired geologic record length plus the sweep duration time (18 seconds for a plus the sweep duration time (18 seconds for a 5 seconds of data and a 13 second sweep). The raw field data bears little resemblance to a conventional explosive (or impulse) seismic record, since it consists of a composite of overlapping 13-second reflections from each reflecting horizon. The "VIBROSEIS" system field recording is converted into a conventional type of seismogram by the "VIBROSEIS" or cross-correlation process. The cross-correlation of each trace of the field record with the transmitted sweep signal effectively results in pulse compression of the reflected wave trains to produce a normal appearing reflection record. The produce a normal appearing reflection record. The cross-correlation wavelet is exactly the same shape as the well known Ricker wavelet, therefore "VIBROSEIS" system and conventionally recorded data look exactly the same after processing. processing. Without resorting to complex mathematics, it is easy to demonstrate how a seismic record is correlated from the sweep frequency signal of the "VIBROSEIS" system.