Distributed Acoustic Sensing Technology for Seismic Exploration in Magmatic Geothermal Areas

Abstract

Seismic methods are a cornerstone for the exploration of the subsurface. In comparison to seismic surveys at the surface, downhole measurements can help to gather more detailed information about rock properties as well as potential fluid pathways within geothermal reservoirs. Temperatures within geothermal wells, especially in magmatic environments, however, often exceed the temperature limitation of conventional seismic sensors. One way to overcome the lack of seismic downhole data for geothermal exploration is the application of the novel fiber optic distributed acoustic sensing (DAS) technology. For DAS, an optical fiber is used as seismic sensor. Lowering a fiber optic cable into a well, dynamic vibrations can be measured along the entire fiber with a high spatial resolution. As all electronics can be operated at the surface, the temperature tolerance for the measurement set-up, is defined by the operating temperature range of the fiber optic cable. Special optical fibers can be operated at temperatures up to several hundred degrees centigrade, therefore, closing the gap between the operating temperature limit of conventional seismic sensors and the requirements for a downhole application within geothermal wells. In this paper, the requirements for an installation of a fiber optic cable within a high-temperature well will be discussed, DAS data will be compared to conventional geophone data, and the benefit of applying the DAS technology for seismic exploration within the framework of the Iceland Deep Drilling Project (IDDP) will be outlined.