Strategies for detecting hydrocarbon microseepages using airborne geophysics integrated with remote sensing data

Abstract

Areas affected by light hydrocarbon microseepages present chemical and physical changes in soil, rocks and vegetation. These alterations result in at least six possible indicators which can be effectively detected by airborne geophysics and remote sensing: (i) potassium depletion; (ii) increase of the uranium concentration, in relation to the potassium concentration (DRAD index); (iii) accumulation of magnetic minerals (micromagnetic anomalies); (iv) increase of kaolinite and (v) Fe≤ + ion concentration; (vi) geobotanic anomalies. This study shows the strategies to identify these typical attributes in some portions of Remanso do Fogo area ‐ MG, using airborne gamma ray spectrometry and magnetic data, integrated with remote sensing data. The geophysical data were applied to detect subtle variations in radiometric and magnetic data by minimizing the influence of recent sediments coverage and deeper magnetic signatures, respectively. Therefore, the gammaray spectrometry data were normalized by thorium and yielded low residual potassium and high DRAD values estimates. In the magnetic data, the amplitude of analytic signal of the anomalous magnetic field, using the second derivatives, was the main product to interpret shallow signatures. These magnetic features were related to possible gas pathways. The main geophysical products were integrated using fuzzy logic classification and most potential areas were confirmed by indicators founded in the remote sensing images. Also hydrocarbon isoconcentration curves and known occurrences of microseepages validated the final results.