Abstract: Bright Spots, Milligals, and Gammas

Alan O. Ramo, James W. Bradley

Spatially discontinuous high-amplitude seismic reflections were encountered in seismic data acquired in the early 1970s in northeast Louisiana, north-central Louisiana, and southwest Arkansas. Large acoustic impedance contrasts are known to result from gaseous hydrocarbon accumulations. However, amplitude anomalies also may result from large density and velocity contrasts which are unrelated geologically to hydrocarbon entrapment.

A well drilled on the northeast Louisiana amplitude anomaly encountered 300 ft (91 m) of rhyolite at a depth of 6,170 ft (1,880 m). Subsequent gravity and total-field magnetic profiles across the feature revealed the presence of 0.2-mgal and 17-gamma anomalies, respectively. The measured magnetic susceptibility of the rhyolite was 0.0035 emu and the measured density contrast was 0.3 g/cc. Model studies based on the seismic areal extent of the anomaly and the measured thickness of rhyolite corroborated the observed gravity and magnetic anomalies.

In north-central Louisiana, total-field magnetic profile data were acquired over two additional anomalies. Observed magnetic anomalies were characterized by amplitudes of 6 gamma or less. An interpreted spatial distribution of intrasedimentary igneous material was found to correlate well with the location of the seismic amplitude anomalies.

In southwest Arkansas, a southeast dipping, sheetlike amplitude anomaly was located. Two north-south magnetic profiles exhibited tenuous 4-gamma anomalies which appeared to be spatially correlated with the interpreted north edge of the seismic anomaly. Subsequent well information revealed 200 ft (61 m) of salt at about 7,500 ft (2,300 m). Reassessment of the north-south gravity profiles suggested a spatially correlatable 0.2 to 0.3-mgal anomaly.

Present amplitude analysis technology would treat these seismic data with suspicion. However, gravity and magnetic data acquisition can provide relatively inexpensive means for the evaluation and verification of amplitude anomalies and thus should be an adjunct for on-land seismic exploration utilizing amplitude analysis.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma