STRUCTURE OF THE EARTH^ — HODGSON 



301 



is buried in a hole, drilled usually 50 feet deep; and the recording 

 is done on very rapidly moving paper with time marks recorded 

 each hundredth of a second. Space fails here to do more than say that 

 this method uses electrical seismographs which feed their impulses 

 through wave filters and amplifiers so that the direct gromid motion 

 is killed out and those waves enhanced which have been reflected 

 from a selected subsurface stratum which is thought to have oil- 

 bearing possibilities. 



The average velocity within the upper strata above the reflecting 

 layer being studied can be measured by the refraction method or, 

 where dry wells exist in the area, by measuring "up-hole time" from 

 a shot in the well. KJnowing the average speed and the elapsed 

 time, and the distance from shot to seismograph, we have sufficient 

 data to solve the conditions shown in figure 12 and learn the depth 

 to the reflecting surface. 



The crews work over the networks of country roads, finding the 

 depth from point to point to the selected layer or "horizon" within 



FiGUEB 13. — Section of a typical six-instrument seismogram. 



the earth, until they find some place where it up-bulges about the 

 same way as a large carpet would up-bulge if you were to put an 

 inverted soup plate in its center. Imagine a carpeted dining room 

 with a table on the center of the carpet and with an inverted plate 

 under the carpet and under the table. The table top represents the 

 earth's surface — the carpet represents the oil-bearing reflecting layer. 

 The geophysicist seeks the invisible, up-bulged part represented by 

 that hump in the carpet pushed up by the inverted plate. In prac- 

 tice, the layer may be at any depth up to a couple of miles. The 

 bulge may be only 50 feet above the general level of the layer. But 

 if such a bulge exists in those invisible depths, to learn of it is worth 

 money ; for that is where the oil will be — if there is any. 



Plates 1, 2, and 3 show reproductions from photographs taken in 

 the field, and figure 13 shows a typical record. To deal adequately 

 with this subject would take a whole series of lectures, for it is a 

 very live business ; and new technique and new discoveries are being 

 reported every month. Some idea of the magnitude of the business 

 may be given when it is stated that more than 200 seismograph crews 



