862 GEOPHYSICAL WELL TESTING [Chap. 11 



time in days. It follows from this equation that the depth penetration 

 of the daily variation is l/\/365, or 1/19, of that of the annual variation. 



III. SEISMIC MEASUREMENTS 



Seismic measurements in wells are made (1) to determine the vertical 

 velocity distribution, (2) to extend the range of refraction exploration 

 vertically and laterally, and (3) to determine crookedness of holes. 



Seismic (deep) well shooting is applied widely to secure data on velocities 

 along refraction and reflection paths. To this end, shots are fired a certain 

 distance away from the top of a well, detectors are lowered to successive 

 depths, and travel times are recorded. This method was mentioned on 

 page 465 in connection with total and differential vertical velocity deter- 

 minations. On page 568 its relation to average reflection velocities was 

 discussed. Seismic well shooting detectors are usually reluctance seismo- 

 graphs of small diameter.*^ Several units may be arranged in tandem to 

 increase the sensitivity. Precautions are required to protect their interior 

 from the pressure of the drilling fluid. Shot instant and travel times are 

 recorded as usual; over-all or differential velocities can then be readily 

 calculated if the depth to the detector or detectors is known. For dis- 

 tances from the top of the well comparable to the detector depths (oblique 

 incidence) more elaborate calculations are necessary " in order to obtain 

 the vertical velocity distribution from travel-time records (see Fig. 

 11-32). 



In shallow holes the procedure is reversed and shots are fired at the 

 bottom of the hole while the detectors are set up at the surface. This is 

 the present practice in connection with weathered-layer procedure and for 

 securing average vertical travel times and velocities at the shot point 

 (see page 576). 



In the vicinity of salt domes, faults, and other vertical contacts which 

 are difficult to delineate by surface refraction measurements, deep wells 

 can be used to advantage to extend the vertical range of refraction observa- 

 tions.*' Detectors lowered into wells on the outside of a dome will help 

 to obtain more data on the flank formations. Wells used with shot points 

 on the opposite side of the dome are useful for determining overhang. 

 The velocities in the salt and surrounding formations at various depths 

 are usually well enough known to determine proportionate paths in the 

 salt and in the sediments. 



The crookedness of drill holes may be measured by seismic procedures 



" C. A. Heiland, Explosives Engineer, Dec, 1935, Fig. 14. 



«C. H. Dix, Geophysics, 4(1), 24-32 (1939). 



" B. McCollum and W. W. LaRue, Oil Weekly, June, 1931. 



