. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. 122 E. E. ROSA I RE AXD JOSEPH L. ABLER COMPUTATION OF SUBSURFACE DIP FROM REFLECTIONS Figure i illustrates how dips are measured by reflections. By plotting the image of the shot point for the recording geophones, the problem is resolved into one of triangulation upon this image point by means of sound-wave paths. The distance between the recording geophones serves as a base line, and the other two sides of the triangle are the path lengths of the recorded reflections. Since only the travel times f
. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. 122 E. E. ROSA I RE AXD JOSEPH L. ABLER COMPUTATION OF SUBSURFACE DIP FROM REFLECTIONS Figure i illustrates how dips are measured by reflections. By plotting the image of the shot point for the recording geophones, the problem is resolved into one of triangulation upon this image point by means of sound-wave paths. The distance between the recording geophones serves as a base line, and the other two sides of the triangle are the path lengths of the recorded reflections. Since only the travel times for these reflections are determined directly, the triangle is completed by multiplying these travel times by the appropriate veloc- ity with which the sound waves have traveled. This computation is. 3P=SUol Poinl ^Poml S P 1 <SllolPa'inl ImjAe FIGURE I. Fig. i.—Geometrical solution of dip determination. modified by certain corrections necessitated by a thin zone of low- velocity material almost invariably present at the earth's surface. The methods of correcting for this zone, known as the "aerated layer," are familiar to seismologists, but are too involved to describe here. The velocity of the sound waves below the aerated layer can be measured in any of several ways. One method is to lower a recorder down a well, and record the travel time from an explosion at the sur- face. A second method is the analysis of refraction data from a profile of appropriate length and recorder density. Thirdly, the appropriate velocity may also be determined empirically by an analysis of the re- flection times themselves, and due to the general lack of available refraction profiles or well recordings, this is the most widely used method in the Gulf Coast. ACCURACY AND CONTROL Dip reflection surveys for the purpose of local preliminary recon- naissance consist essentially of unclosed traverses, in general made up 644. Please note that these images are extracted from scann
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