Industrial Research 



239 



As a background for this system of exploration there is, 

 first of all, the determination of the general law of 

 gravity, and, secondly, the development of instruments 

 that are delicate enough to be influenced by any small 

 variations In the distribution of the different rock layers. 

 The general principles were known to university physi- 

 cists long before any practical application was made in 

 the oil mdustry. When it was realized that such a 

 practical application coidd be made, the oil industry 

 established laboratories in which groups of physicists 

 were engaged in the work of making this type of explo- 

 ration practical. First it was necessary to calculate 

 from the law of gravity the results to be expected from 

 typical rock deformations that were known to exist in 

 oil-bearing districts. When it had been determined 

 that, because of anomalies in the rock structure, the 

 variations in the gravitational attraction for bodies on 

 the earth's surface were great enough to be read on 

 instruments, the next step was to develop instruments 

 that were sufficiently sensitive and rugged and suffi- 

 ciently quick in operation to be practical for field explora- 

 tion. An instrument that reads gravitational force to 

 1 part in 10,000,000 must also be rugged enough to be 

 carried on an automobile or a truck, and convenient 

 enough to be set up at any field location and to allow a 

 reading to be taken in a reasonable length of time. This 

 means that the apparatus, among other things, must be 

 insensitive to minor vibrations and to temperature 

 changes that are likely to be encountered in the field. 

 Many types of apparatus were developed which met 

 these requirements, and as a result of this work the 

 amount of gravitational exploration that had been 

 done in the last few years is many times as great as 

 that which has been done by all methods during all the 

 rest of the world's history. 



After these measurements have been made to deter- 

 mine gravity, it is necessary to map and to interpret 

 them in terms of subterranean structures. Again, a 

 very complicated application of physics and mathe- 

 matics, together with geology, is required. The physics, 

 taken together with the mathematical calculations, 

 describes the possible structures insofar as their densi- 

 ties and locations are concerned, and the geology 

 interprets the structure in terms of the likeliliood that 

 oil is present. These methods can be applied also to 

 exploration for other types of mmerals whenever they 

 are associated in any way with variations in densities 

 and vertical positions of rock layers. 



Magnetism has been known for many hundred years, 

 especially as applied in the use of a magnet as a com- 

 pass. University and other laboratories have been 

 studying the magnetic characteristics of materials over 

 a long period of time. It has been found that not 

 only iron and compounds of iron, but practically every 

 type of material has measurable magnetic character- 



istics. It is known that igneous rocks whicii form tlie 

 substratum under all sedimentary rocks are more 

 strongly' magnetic than the latter. Hence it is i)os- 

 sible to use sensitive magnetic apparatus to determine 

 api)roximate depth and slope of the upper surface of 

 the igneous substratum. The story of this tyi)e of 

 exploration is very similar to that mentioned above, 

 insofar as it is absolutely dependent on the develop- 

 ment of sensitive apparatus to make measurements. 

 An industrial physicist is employed to carry the 

 development on from the point at which his academic 

 brother left it. Apparatus is j)roduced which is 

 sensitive, rugged, and relatively unaffected by vibra- 

 tions and temperature variations. The physicist is 

 familiar with this type of development and has the 

 benefit of the work of many predecessors who have 

 overcome similar difficulties in other circumstances. 



Another procedure used in geophysical exploration 

 is the study of the transmission of mechanical waves 

 or of sound through the subterranean structures. A 

 charge of dynamite is exploded in a hole that has been 

 drilled to the necessary depth to give it adequate con- 

 tact with the rock layers. The compressional wave 

 produced by the exjjlosion is transmitted through the 

 earth and comes to the surface again in other neighbor- 

 mg locations after being bent because of the gradual 

 changes Ln wave speed m underlying rocks or after 

 being reflected at the surface separating rocks of one 

 structure from another having a different wave speed. 

 Here again it was necessary to develop sensitive 

 apparatus not only for recording the arrival of com- 

 pressional waves but also for recording the time that 

 elapses between the discharge of the dynamite and the 

 reception of the wave at a distant location. The data 

 obtained can be used in many ways to determine the 

 subsurface contours of various rock layers, w-hich, 

 together with geological knowledge about the neighbor- 

 hood, give even more direct and useful information 

 than that obtained in gravitational exploration. 



All three methods of exploration are used by the oil 

 companies at the present time. Although the forma- 

 tions discovered do not always contain oil, the prob- 

 ability of finding oil is considerably increased over that 

 which obtains when wells are drilled at random. The 

 cost of drilling is so great that even a small increase in 

 the probability of finding oil makes the exploratory 

 research carried out by the physicist worth many times 

 its cost. 



Of the many other applications of physics in the oil 

 industry we mention only two. Until a few years ago 

 it was a common experience that when a deep well was 

 drilled by the rotary method the hole would not be 

 straight. The drill would gradually veer in one direc- 

 tion or the other, so that the location of the bottom of 

 the hole was indeterminate. A physical study of the 



