Injection of waste materials into deep subsurface formations is becoming an increasingly popular method of waste disposal. Attendant to this growing practice is an increasing possibility of accidental damage to the subsurface and surface environments. An effective method of monitoring the movement and distribution of these injected waste materials is needed.

In most cases, the acoustical properties of the receiving formation material (i.e., its density and velocity of propagation of compressional sound waves) will be changed upon contacting the injected waste materials. These changes subsequently will change the reflection coefficient encountered by a vertically traveling sound wave at the receiving formation. This change in acoustical properties suggests the application of modern seismic-reflection and data-processing techniques to this monitoring problem.

The seismic-reflection technique involves introduction of acoustical energy into the earth from the surface, and recording of signals at or near the surface. The signals are indicative of the travel time and amount of energy reflected from each of many closely spaced points along each subsurface formation. The use of the seismic method therefore is proposed on a periodic basis to detect changes of acoustical properties in the receiving formation and surrounding formations to monitor effectively the movement and position of the waste materials.

The assumed seismic field system for this monitoring system includes the following components: (1) permanent arrays of velocity geophones buried a short depth below the surface of the earth; (2) a multichannel digital recording system of the instantaneous-floating-point, binary-gain type; (3) truck-mounted seismic surface energy sources; and (4) use of a digital seismic data-processing center with special software.

The feasibility of the monitoring system has been studied by means of an acoustical model derived from well-log information for a typical Gulf Coast injection well. This model and the characteristics of the seismic field system determine the smallest detectable lateral

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change in reflection coefficient. This information then is related to the range of expected changes in reflection coefficient due to the injected waste materials, and subsequently to the feasibility of the seismic monitoring system.

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