CHAPTER XI 



BORE-HOLE INVESTIGATIONS 



During the past 15 years geophysical bore-hole logging has been uni- 

 versally accepted as one of the more important exploration tools. 



It is now generally known that the measurement of certain physical 

 properties provides a record of the formations traversed by the drill hole. 

 This record is complete, detailed, and highly useful and may give additional 

 information regarding the fluid and mineral content of the rocks. 



It has become general practice, when a hole has been drilled, or at 

 intervals during the drilling, to run a bore-hole survey. This recording is 

 of immediate value for the geological correlation of the strata, and the 

 detection and evaluation of possible productive zones or horizons. The 

 information of the log may at the same time be supplemented by side-wall 

 samples of the formations taken from the walls of the hole, or by cuttings 

 taken from the circulating mud. 



In common with surface exploration methods, drill or bore-hole ex- 

 plorations may be classified according to two principles. One basic classi- 

 fication is concerned with the application of an artificial field to the 

 formations penetrated by the drill hole. This field may be: (a) electrical; 

 (b) magnetic; (c) thermal; (d) seismic; or (e) neutron. The correspond- 

 ing properties of the formations which may be investigated are: (a) resist- 

 ivity, impedance, electrical anisotropy, and other electrical properties ; (b) 

 magnetic permeability and retentivity; (c) thermal conductivity and 

 temperature; (d) velocity, attenuation, and absorption of seismic waves; 

 and (e) radioactivity. The other basic classification is concerned with natural 

 phenomena. These include : (a) electrochemical effects due to differences in 

 the concentration of the fluids (electrolytes) in the hole and in the forma- 

 tions and electrofiltration effects due to flow of fluid through the pores of the 

 formations ; (b) magnetic anomalies associated with the formations pene- 

 trated by the bore hole; (c) variations of natural earth temperatures with 

 depth and with the formations ; (d) gravitational anomalies associated with 

 the formations; and (e) radioactive effects (gamma ray). 



The greatest commercial application and development in bore-hole in- 

 vestigations has been in the measurements of electrical resistivities, electro- 

 lytic and electrofiltration potentials, and the radioactive effects of neutron 

 and gamma rays. 



