A LOW‐NOISE SEISMIC METHOD FOR USE IN PERMAFROST REGIONS

Abstract

Dynamite charges of ten lb or greater detonated in permafrost regions at depths less than fifty ft below the ground surface tend to create frost breaks, a phenomenon of strong seismic energy from an outward flow of minute cracks in the permafrost caused by the detonation. The frost breaks obliterate reflected energy arriving at the same time. A seismic method was devised which minimizes frost breaks and allows a reduction of the massive equipment currently required either to drill shotholes or for the application of a nondynamite surface energy source. Advantages expected for the method are a cost reduction of 10 percent or more by reduction of drilling equipment and the ability to employ light‐weight portable seismic crews in remote areas without large mileage programs (300 to 500 miles) being necessary to justify the cost of introducing a crew. Also, light‐weight portable crews could be used to determine optimum operational parameters in virgin areas. Since dynamite is the most concentrated available energy source, a seismic experiment was conducted by detonating individual one‐lb dynamite charges at depths 5 ft below the permafrost surface. To maximize the number of small charges, they were detonated at each geophone location (156 one‐lb shotpoint locations per mile). On a total energy per mile basis, the energy from the small charges compares favorably with the energy expended by the conventional method of [Formula: see text] 25‐lb shotpoint locations per mile or [Formula: see text] lb. The reflected energy was recorded digitally and processed by applying vertical summing plus 12‐fold horizontal CDP stacking. The total profile distance was sufficient to allow for one‐half mile of fully stacked subsurface coverage. The experimental results compare favorably with those from the conventional method, and the small charges detonated at shallow depths were safe both from the point of view of human safety and ecological care.