Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 33 



mitter to the receiver. In the catagory of radio methods belong the so- 

 called "treasure finders." These are portable instruments for the location 

 of shallow metallic objects, pipe lines, and the like. 



VI. GEOPHYSICAL WELL TESTING 



Geophysical well-testing methods may be divided into four groups: 

 (1) electrical testing methods, known as "electrical logging"; (2) temper- 

 ature measurements; (3) seismic measurements; and (4) miscellaneous 

 measurements of radioactivity, rock magnetism, opacity of drilling fluid, 

 and so on. 



Electrical logging methods. Electrical logging methods fall into two 

 groups : (a) methods calling for the application of an electrical field, which 

 furnish the resistivities of the formations traversed; and (6) methods 

 in which spontaneous electrical potentials are observed. The latter will 

 give data regarding the porosity of formations. Numerous arrangements 

 have been proposed for the determination of resistivities in wells. The 

 system most widely used is the Schlumberger procedure. Three elec- 

 trodes are lowered into the well and the fourth is the casing at the surface 

 (see Fig. 2-15). Current is supphed to the casing and the lowest electrode, 

 and the potential difference between the two other electrodes is recorded. 

 The resistivities thus obtained are not true formation resistivities but 

 apparent resistivities which are functions of the electrode spacing and of 

 the absolute resistivities involved. Electrical logging makes possible the 

 correlation of formations by their resistivity parameters, and the loca- 

 tion of water- and oil-bearing strata. In some wells it has been found 

 that the resistivities are a true indication of the productivity of oil sands. 



In the second group of electrical coring methods spontaneous potentials 

 are observed. These are of two types: (1) electrofiltration potentials 

 (caused by movement of fluids through porous formations), and (2) con- 

 centration potentials (caused by difference in ion concentration of drilling 

 fluid and formation water). Spontaneous potential records are valuable 

 in connection with resistivity logs. A high resistivity may mean a lime- 

 stone or an oil sand, but high porosity will eliminate the limestone from 

 the picture. 



Temperature measurements. It is generally known that there is an 

 increase in temperature with depth of penetration into the earth's crust. 

 However, this temperature variation with depth is not uniform; it depends 

 not only on the local geologic and stratigraphic conditions but also on the 

 pre.- nee or absence of heat developed by physical and chemical actions 

 of the formation fluids. As the heat conductivity of beds is greater in 

 the bedding planes than at right angles thereto, uplifts of sedimentary 

 beds and salt domes will generally result in a rise of well temperatures. 



