Localization of a coal seam fire using combined self-potential and resistivity data

Abstract

The remediation of coal seam fires requires, as a first step, the precise localization of the burning front. Various geophysical methods can be employed for this purpose but not all of them can provide an unambiguous localization of this type of target. We propose a combined inversion of self-potential and resistivity data and the use of joint attributes (characterizing the target) to localize the burning front of a coal seam fire. This new methodology is applied to the 3D reconstruction of a shallow coal seam fire in Boulder, Colorado. The coal seam fire is located in the Gorham subbituminous coal formation at a depth comprised between 10 and 15m. The self-potential survey comprises 160 stations (5 profiles and 32 stations per profile) located at the ground surface with self-potential anomalies amounting +70 to −50mV with respect to an arbitrary reference located further away from the burning front. The resistivity survey comprises 5 resistivity profiles with 118 measurements per profile (590 measurements in total). The burning front is associated with very low value of the electrical resistivity (<15Ωm) and source current density probably of thermoelectric nature. The combined inversion and interpretation of the self-potential and resistivity data indicates clearly the position of the coal seam fire, which is corroborated by a thermal anomaly observed at depth of 30cm and indicating that the minimum depth of the thermal source is approximately 9m. This methodology can be easily extended to greater depths using resistivity derived from galvanometric or airborne electromagnetic methods and large-scale self-potential surveys.