24 OLAF F. SUNDT 
Perhaps the best marked are the parallel maximum and mimimum 
through Galveston, Brazoria, and Matagorda counties into Calhoun 
County, Texas. Sections across this system show gravity increases 
ranging from 10 to 15 millidynes from the minimum to the maximum. 
In general, there is an increase of gravity out to sea along the coast 
line. 
Salt domes seem to be situated on the flanks of these regional 
features rather than on the top of the maximum or in the minimum 
troughs. Little may ever be known as to the meaning of these remark- 
able gravity features on the Gulf Coast, since these effects are ob- 
viously the expression of basement features of great depths beyond 
reach of the drill. They are unlike anything found in other salt-dome 
regions, including East Texas, North Louisiana, and Germany. 
The effect of these regional gravity maxima and minima is that 
a number of salt domes, even of the relatively shallow intrusive type, 
like Danbury and Markham, are not easily distinguished except by 
deducting a regional gradient. The deeper domes are masked alto- 
gether except for a slight bulge in the isogam lines, a slight shortening 
and lengthening of the gradients respectively against, or following, 
the regional gravity dip. 
A knowledge of the extent and intensity of these regionals over 
relatively large distances is essential in attempting to isolate a deep 
gravity anomaly dome from the regional effect. Therefore, local 
torsion-balance surveys are not of much use without definite knowl- 
edge of the regional trend obtained by at least one long traverse into 
the surrounding territory, except with previous knowledge of general 
regional conditions in the area under examination. 
The torsion balance is ideal for exploration work, because from 
a distance anomalies can be followed until they are definitely local- 
ized. Then they can be checked if so desired by reflection shooting 
for further geophysical proof, to determine if uplift is present within 
reach of the drill or if the minimum represents an arrested type salt 
dome with no uplift in the commercially productive horizons. 
As to local gravity elongate closures in the bottom of gravity 
minimum trends (a good many of which have been drilled, even in 
recent years), most competent geophysicists will condemn them if 
they are familiar with regional conditions. Such minimum trends, of 
course, should not be confused with elongate lines or groups of salt 
domes at various stages of evolution, such as Lost Lake, Moss Bluff, 
and Hankamer. Prospects in or near the bottom of regional trends 
gain in value, if at least on one side a good closure is obtained along 
the axis of the trend. Discrimination is, therefore, necessary and should 
be the basis on which the competent geophysicist bases his conclu- 
sions. 
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