GRAVITY PROSPECTING ON GULF COAST 2B 
viously used. They are sensitive to about 3.10 * except for occasional 
lapses, when one station will suddenly fail for no apparent reason. 
The probable error is reduced by two observations with four com- 
parisons with two different base stations. It is then only about 
2 or 3.10-*. All direct gravity measuring instruments have the problem 
of daily and periodical drift and sudden jumps or shifts. Constant 
comparisons with the base stations are essential daily. 
These instruments, if operated to effect a network of stations a 
mile or two apart, are very valuable for regional gravity prospecting. 
Larger distances between stations are not advisable because interest- 
ing areas may be missed by the larger mesh. It should also be remem- 
bered that isolated pendulum stations do not give directional data. 
Interpolations in contouring are somewhat hazardous except with a 
close net of reliable stations. The Cleveland dome was found by the 
pendulum and checked by the torsion balance before the first loca- 
tion was made by the Gulf Production Company. 
Gravity meters are based more or less on the basic patent of 
Kenneth Hartley. They consist of a weight suspended on a delicate 
spring protected from temperature differences. Variations of the 
length of the spring due to small differences in gravity are read on a 
dial controlling a regulating device. The optical system is ingenious 
and projects two filar images that can be made to overlap and coincide 
by rotating the dial. There are two or three of these instruments in the 
field. Their sensitivity is similar to that of the pendulum about 2 or 
3.104 under the best conditions. They can be worked much more 
rapidly and need no timing device except an ordinary watch to note 
approximate time for the moon correction. On account of the delicate 
spring mechanism, the instrument needs great care in handling and 
must be kept constantly at an even temperature. The clamping device 
is so constructed as to avoid stresses in the spring while clamping 
and releasing the weight. Very accurate leveling is of prime impor- 
tance. These instruments cover a large area in a very short time, 
but can not always be expected to find domes of very little gravi- 
metric expression, where the total anomaly is about the same as the 
probable error in reading, or dependent on the additive or neutralizing 
effect of the probable error in enhancing or wiping out the gravity 
anomaly. . 
PROBLEM OF REGIONAL SURVEYS 
Since 1927 several of the larger oil companies have been tying 
in isolated torsion-balance surveys in order to get an idea of the region- 
al grain of the Gulf Coastal country. The regional maxima and min- 
ima are all nearly parallel with the coast line, that is, their axes ex- 
tend northeast and southwest or east and west. 
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