Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 GEOLOGICAL APPLICATION OF SEISMOGRAPHV 1317 SHOT £>u/?r/KC£ KSCOPTDeR SHOT H>£:CORDER Fig. 3.—Shortest time paths (upper) in sediments and (lower) with salt dome present. the high-speed salt. For a dome a mile in diameter, which is a fair av- erage, the saving in time is approximately second; obviously, the pres- ence of the dome is easily detected. As the method was applied to progressively deeper domes, the time- anomalies to be expected became correspondingly less. It became
Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 GEOLOGICAL APPLICATION OF SEISMOGRAPHV 1317 SHOT £>u/?r/KC£ KSCOPTDeR SHOT H>£:CORDER Fig. 3.—Shortest time paths (upper) in sediments and (lower) with salt dome present. the high-speed salt. For a dome a mile in diameter, which is a fair av- erage, the saving in time is approximately second; obviously, the pres- ence of the dome is easily detected. As the method was applied to progressively deeper domes, the time- anomalies to be expected became correspondingly less. It became necessary to increase the accuracy of timing to at least second. Whereas the distance between shot and recorder points was previously determined by timing the air wave travelling from shot to recorder, ii now became necessary carefully to survey this distance, especially as, for the deeper domes, it was necessary to place the recorder farther away from the shot point. Though a mile or two had been sufficient for the very shallow domes, it was found necessary to use 6-8 miles for the deep domes. This, of course, meant larger charges of dynamite, charges as large as 1,000 pounds being used for a single shot. The large charge re- quired and the increased accuracy necessary in determining the distance between shot and recorder made the work increasingly expensive; also, the smaller time anomalies exhibited by the deeper domes made the results less reliable. However, in a period of 7 years many domes have been discovered by the seismograph. REFRACTION' METHOD; STRUCTURE DETERMINATION In the study of the comparatively simple problem of searching for salt domes, the attempt was cautiously made to extend the range of usefulness of the seismograph method. Having found a dome, it was desirable to define it in as much detail as possible. It was possible to contour the top of the dome and determine its top edge. 35
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