A decade of successful magnetotelluric surveys for delineating shallow geothermal reservoirs beneath nonvolcanic hot springs in Thailand

Abstract

Magnetotelluric (MT) applications to probe the shallow reservoirs of nonvolcanic hot springs in Thailand have become ever more convincing during the past few decades. In 2013, the first modern MT survey was deployed over the Mae Chan hot spring. A shallow hot water reservoir within a network of fractured granite was imaged as a low-resistivity zone beneath the hot spring surrounded by a resistive granite heat source. With the success of the first MT deployment, in 2014 and 2015, MT measurements were expanded to five hot springs with the objective of 1 km deep drilling where the Muang Rae hot spring was chosen out of these five hot springs to assess the volume of hot fluid for a future geothermal power plant. The conductive zone was again detected at a depth of 200-600 m beneath the Muang Rae hot spring, which our team interpreted as a shallow hot fluid reservoir. However, others interpreted it as just the clay cap with no hot fluid inside, which occurs in most volcanic systems. Therefore, a 1 km well was drilled next to the conductor, instead of through it as we suggested. The well did not find any hot water flowing. In 2018, an MT survey with densely spaced stations was used around the Mae Chan hot spring to validate the shallow conductive zone beneath the hot spring through which five wells (<200 m) were drilled. The depth positions where the hot fluid was found match well with the conductor location at most wells. This is a good indicator that in the Thailand nonvolcanic system, the conductive zone beneath the hot spring is genuinely the shallow reservoir, and not the clay cap zone as in the volcanic systems.