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GEOPHYSICAL WELL TESTING^ 



1 HE ESSENCE of geophysical well testing is a determination of physical 

 rock properties in situ. Its purpose is (1) to correlate wells by using 

 physical formation characteristics which either are more significant or can 

 be obtained more readily than their petrographic and geologic character- 

 istics; (2) to detect commercial minerals (oil, gas, coal) or other media 

 which are of significance in the process of drilling (water, cement, and the 

 like); (3) to determine data required in the interpretation of geophysical 

 surface measurements (seismic wave velocities and the like). Somewhat 

 removed from our field is a fourth application, the determination of 

 crookedness of holes (by seismic measurements). 



In regard to procedure, geophysical well-testing methods may be divided 

 into (I) electrical logging, (II) temperature measurements, (III) seismic 

 measurements, and (IV) miscellaneous measurements (magnetic, radio- 

 active, and so on). 



I. ELECTRICAL LOGGING 



Electrical logging in the most general sense is the examination of the 

 electrical properties, electrical reaction, and geometric disposition of sub- 

 surface formations by electrical measurements in wells. It involves a 

 determination of the following quantities, in order of present commercial 

 importance: (A) resistance or impedance of formations, (B) spontaneous 

 potentials (porosities), (C) resistance of drilling mud, (D) dip and strike, 

 and (E) casing depth. 



A. Determination of Resistance or Impedance of Formations 



In respect to the determination of impedance or resistance in (uncased) 

 wells, electrical logging is equivalent to resistivity mapping with fixed elec- 

 trode separation (see page 708). Hence, a variety of electrode arrange- 

 ments is possible. In fact, most of those described at pages 710-711 have 



^ The symbols in this chapter are the same as in Chapter 10, except where other- 

 wise noted. 



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