Magnetotelluric monitoring of coal seam gas depressurisation
The depressurisation of coal seam gas (CSG) formations causes in-situ fluids to migrate through pores and fractures in the earth. The removal of large volumes of water from coal seams has the potential to affect water table levels and groundwater flow in surrounding aquifer systems. Magnetotellurics (MT) is a passive electromagnetic technique that uses the natural fluctuations of electric and magnetic fields at the Earth’s surface to determine the Earth’s conductivity structure. The bulk movement of fluids during CSG depressurisation causes a conductivity change in the subsurface and this change can be monitored using MT. An electromagnetic survey was conducted at a CSG production scientific test site. Electric and magnetic field instruments were deployed, measuring continuously at 651 Hz across three months. New processing software was developed to generate MT responses in the bandwidth of 100–0.1 Hz across the experiment. The theory of monitoring subsurface fluid movement using MT is presented, as well as instrumentation and a case study to demonstrate the potential of the magnetotelluric method. Results from this approach can provide an inexpensive means of monitoring CSG depressurisation, as well as an improved understanding of the potential impacts on the subsurface environment during CSG production.