THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS 
TRANS. SOCIETY PETROLEUM GEOPHYSICISTS, VOL. V (MARCH, 1935), PP. 132-153, 12 FIGS. 
MAPPING OF GEOLOGICAL STRUCTURE BY THE 
REFLEXION OF ELASTIC WAVES! 
F, GOLDSTONE? 
Houston, Texas 
SUMMARY 
The utilisation of reflected elastic waves in structure mapping is an adaptation of 
the older technique developed for depth-sounding in water. The geological problem is 
more complex due to the inferior qualities of rock contacts as reflecting planes and the 
existence of transverse waves which are not present in a fluid. The principal waves 
generated by a disturbance are the longitudinal, transverse, and surface waves; time- 
distance curves for these waves in the two layer problem are presented. Refraction 
seismic prospecting utilizes the first wave front to reach any point remote from the 
disturbance by any path, whilst reflexion seismic prospecting utilizes the reflected 
longitudinal waves which must arrive later than the same wave fronts propagated 
directly along the surface. The energy contained in the wave train reflected from a slate- 
granite contact has been computed as being less than 4% of the incident wave for 
angles of incidence below the critical angle. A reflexion unit must be capable of record- 
ing clearly these weak wave trains which arrive superimposed on other disturbances; 
this can be achieved by selective amplification of the dominant frequency of the re- 
flected wave train, which is usually between 30 and 60 cycles per second. The com- 
ponent elements of a reflexion unit are described. A typical reflexion seismogram is 
reproduced showing the normal succession of events, namely, the instant of explosion, 
arrival of the surface longitudinal wave, arrival of a reflected event and arrival of the 
sound wave through the atmosphere. The usual field procedure is discussed. Two 
radically different methods of interpretation are possible: the first is by correlation 
which requires that a particular burst of reflected energy can be identified over large 
areas; the second, by dip, requires that an event can be identified over only a short 
distance. Examples of typical seismograms permitting mapping by correlation are 
reproduced. The three possible sources of error inherent in this method are the correla- 
tion of events between seismograms obtained at adjacent locations, in the measurement 
of time intervals and in the computation of depths to reflecting planes; the first of these, 
which may lead to major errors, is governed by geological conditions and instrument 
design, the second can be eliminated by sufficiently careful technique and the last 
largely reduced by accurate determination of surface corrections. The results of such 
a survey in Oklahoma are reproduced. Examples of typical seismograms from which 
interpretation must be derived by the dip method are reproduced, and a graphical 
method of obtaining the reflecting plane explained. The principal source of error in this 
method is in deriving the correction for the low velocity surface zone. An example of 
a profile across a deep-seated salt dome derived by this method is reproduced. The con- 
clusion is drawn that many structural problems confronting the petroleum geologist 
can be solved by the two methods of reflexion shooting outlined. 
Seismology, in its application to geological problems, has as its 
tools two measurable quantities, time and distance, and one physical 
property, velocity. All methods involve the measurement of time 
intervals and can be broadly grouped into (a) those in which distance 
1 Reprinted from Proc. World Petroleum Congress (London), Vol. I (1934), PP- 
155-62, with the permission of the editors and the author. 
? Shell Petroleum Corporation. 
792 ise 
