BELLE ISLE TORSION-BALANCE SURVEY, ST. MARY 
PARISH, LOUISIANA: 
DONALD C. BARTON? 
Houston, Texas 
ABSTRACT 
The geological prediction was made that the Belle Isle salt dome might be much 
larger than supposed. The torsion balance was used to corroborate that prediction. 
From the data of the torsion-balance survey, calculations were made of the probable 
depth, thickness, and edge of the cap rock. Seven sulphur tests were drilled on loca- 
tions made on the basis of the calculated conformation of the cap. The predictions 
in regard to depth were shown to be 73 per cent correct, and in regard to the thick- 
ness of the cap, 58 per cent correct. The causes of the errors were probably the un- 
favorable marshy terrane, the assumption of a slightly too small relative density for 
the cap rock, and the irregularity of the conformation of the dome. 
INTRODUCTION 
This paper records an actual ordinary torsion-balance survey and 
the degree of success which was obtained in an attempt to base quantita- 
tive predictions on the torsion-balance data. The choice of this survey 
for publication was not made on the basis of exact verification by sub- 
sequent drilling. The accuracy of the predictions may seem somewhat 
disappointingly poor, if the actual cap rock which was encountered in the 
subsequent test well is compared with the predicted cap rock (Fig. 3). 
The survey, however, accomplished successfully the qualitative and semi- 
quantitative tasks assigned to it. This survey is more or less representa- 
tive of the degree of success attained by the average successful geo- 
physical surveys, although it was made under adverse terrane condi- 
tions; most of the stations were on unstable marsh, and a lake precluded 
stations in a wide central area. 
The cap rock as a whole is thinner, deeper, and steeper than was 
predicted and the inner part of the cap was very much thinner than was 
shown in the calculated west-east and southwest-northeast sections. 
These errors are in part caused by two factors: the absence of stations in 
tRead before the Association at the San Antonio meeting, March 21, 1931. Man- 
uscript received, March 20, 1931. Data released for publication through the courtesy 
of the Freeport Sulphur Company and Belle Oil Corporation. 
2Consulting geologist and geophysicist. 
1335 
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