The Importance of Survey Aperture for Imaging High-Temperature Geothermal Systems with Magnetotellurics

Abstract

Geothermal exploration of high-temperature geothermal systems commonly includes broadband magnetotelluric (MT) measurements, which can be sensitive to subsurface electrical resistivity structures at depths of 10 km or more. At shallow levels (~1-3 km depth), models of these MT data generally image a low-resistivity ‘cap’ (associated with hydrothermal brines and clays) embedded in a more resistive background. At greater depths (3 – 10 km), recent regional MT resistivity models (developed as part of GNS Science’s Deep Geothermal Research Program in the southeastern Taupo Volcanic Zone) have revealed vertical low-resistivity anomalies (plumes) that may indicate the locations of deep upflows.

These low-resistivity ‘plumes’ are key to refining conceptual models of geothermal systems. However, the process of inversion modeling (in 2D and 3D) requires that surface measurements cover a spatial extent that is at least twice the intended depth of investigation. With interest increasingly shifting to deep (i.e. > 3 km depth) exploration, we use an existing industry-acquired MT survey of the Ohaaki geothermal field as a case study to assess: 1) suitability of the industry MT survey for imaging resistivity structures at depths greater than 3 km; 2) potential limitations associated with the relatively short recording time (overnight nominal) of the industry MT survey compared with the two-night recording time used for the regional MT survey.

By generating a series of 2D MT inversion models, we show that only when the regional MT data are added to extend the aperture of the industry MT survey, is the deep resistivity structure reliably resolved. We also show that while a single-night recording time does not preclude imaging the deep structure, the data quality can be noticeably improved with two-night recording. The lessons from this work are equally applicable to any geothermal system where MT surveys exist with limited spatial aperture.