Subsurface Methods as Applied in Geophysics 1041 



presence or the absence of a given mineral or group of minerals. He is 

 able, however, to indicate geologic conditions that are favorable to the 

 accumulation of a given mineral or group of minerals. For example, it 

 is impossible to predict the presence of petroleum by surface geophysical 

 measurements; geophysics does, however, indicate favorable geologic 

 structure for the accumulation of petroleum. Several exceptions to the 

 foregoing statement should be noted. Gravitational methods will indicate 

 accumulations of salt, peridotite, serpentine, etc., in certain areas where 

 good geologic control is available. Under the same conditions the magnet- 

 ometer will indicate the presence of serpentine plugs, concentrations of 

 "black sand" in placers, and hematite deposits. The refraction seismograph 

 on the Gulf Coast presents unmistakable evidence of salt uplifts. The self- 

 potential method shows accumulations of the heavy sulphides. 



The potential methods, magnetic and gravitational, although rapid 

 and relatively inexpensive, are essentially reconnaissance tools. For a 

 given magnetic or gravity map any number of subsurface conditions may 

 exist that would produce the observed surface-intensity distribution; pre- 

 dications based on these methods may, therefore, be ambiguous. In the 

 seismic and electrical methods control is maintained over the depth of 

 penetration by operational technique, leading to a unique interpretation 

 of subsurface geologic conditions. 



Planning a Geophysical Program 



Assuming that the prospective area is potentially petroliferous, geo- 

 physical programs generally follow a well-integrated plan. The "struc- 

 tural grain" may first be determined by one of the rapid reconnaissance 

 methods. This may be accomplished by a number of air-borne-magneto- 

 meter profiles or by rather widely spaced magnetic or gravity observations 

 (station density of two or three to the township) . In the event that the 

 "grain" is fairly evident and competition for leases is not keen, a more 

 complete survey is generally begun at once with either the gravity meter 

 or both magnetometer and gravity meter working the area. On a survey 

 of this nature, gravity observations will be made with a density of about 

 85 stations to the township. Magnetometer observations are commonly 

 spaced one mile apart or with a station density of about 48 stations to 

 the township. In promising areas, more detailed work is usually desirable 

 to substantiate an anomaly. 



Several advantages accrue from the use of such reconnaisance instru- 

 ments: (1) Prospects may be leased in a minimum of time, (2) Leases in 

 the less-promising areas may be immediately dropped, and (3) Areas to 

 be worked by more expensive seismic or core-drilling methods are reduced 

 in size, with a better chance of locating structure for the exploration 

 dollar spent. 



With the location of either interesting magnetic or gravity anomalies 

 the next step in the geophysical program is to shoot the magnetic or 

 gravity anomalies with the seismograph. The general procedure today is 

 to shoot continuous profiles spaced as closely as is deemed necessary. 



