236 DEEP BOREHOLE SURVEYS AND PROBLEMS 



When speaking of natural quakes it should be noted 

 that about 3,000 km. is considered an average distance 

 (epicentral) earthquake. Now the study of these waves, 

 their accompanying waves and features, the resultant 

 reflected waves from different boundaries in the earth, 

 and above all the course-times of all these, depends on 

 individual rock characteristics; hence we may by their 

 aid learn certain rock structures and borehole deviations 

 in the earth which could not otherwise be revealed. The 

 chief regions of application are those in which we wish to 

 obtain the thickness of overburden resting on older beds. 

 This has been successfully carried out in the preliminary 

 work of some Swedish electrical surveys.^ 



Moreover, the seeking of dislocations, the determination 

 of deeper strata directions and therewith also saddles and 

 basins and borehole data are located by this means. These 

 waves make themselves felt at their points of emergence 

 at the earth's surface in slight impulses and movements, 

 the waves themselves being in the nature of harmonic 

 vibratory motions, which, in the case of uniform and 

 homogeneous rocks, are shown in a correspondingly 

 uniform and harmonic, and indeed characteristic, motion 

 of the transmitted waves. But since the crust of the earth 

 is decidedly heterogeneous not only in chemical constitu- 

 tion but in physical formation and deformation, the waves 

 are hampered in transit and their vibratory properties 

 defaced in a manner which often is quite bewildering, 

 but the changes are always proportional to the changes 

 in the material traversed. The consequent alteration 

 in the velocity, frequency, and amplitude of the motion, 

 and the arrival time factors at reception stations have then 

 to be investigated as they occur. On the waves striking 

 the limits between strata of different elasticities, etc., 

 broken waves are set up in the second medium and at the 

 same time a part of the wave energy is reflected, and, 

 indeed, according to the kind of wave being discussed, 

 i.e., longitudinal or transversal, corresponding condensa- 



^ SuNDBERG, K., "Electrical Prospecting in Sweden," p. 31. 



