| Title | Land and Marine Magnetotelluric Exploration at the Salton Sea Geothermal Field |
|---|---|
| Authors | Kaspereit, Dennis; Berard, Brian; Gutierrez, Pablo; Nichols, Edward; Jiuping Chen |
| Year | 2006 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Salton Trough; Salton Sea geothermal field, marine magnetotellurics, MT, 2D inversion |
| Abstract | The Salton Sea geothermal field is one of the largest geothermal resources in the world. It was discovered in the 1950s but has only been developed on the southeastern lake shore and the actual field boundaries are not well known. In this paper we describe a combined offshore/onshore Magnetotelluric (MT) survey made over the known geothermal field and surrounding region to determine the formation resistivity signature of the geothermal field and to use this signature to map the external field boundaries and internal structure. The survey was made with land, marine and hybrid MT field systems. These instruments use a portable, low-power digital data acquisition system with sensors deployed on land, and on the shallow sea bed. The survey consisted of 70 sites arranged in 4 profile lines; 3 of these profiles cross the northeast trending geothermal field in a NW-SE direction and the 4th profile crosses in a NE-SW direction. The data from the sites were processed to provide apparent resistivity and phase as functions of frequency for each site. The sites were then grouped into profiles, and a 2D inversion code was applied to provide a resistivity versus depth section along each profile. The MT profiles show that the geothermal reservoir has a lower resistivity than the background. This difference is largely due to the higher temperatures and higher formation water salinity. Based on the low resistivity signature, we estimate that the field encompasses more than 200 km2, over half of which lies offshore. Within the field, the MT profiles match the known geology and borehole induction resistivity logs well. The general stratigraphic section can be divided into three vertical horizons: a shallow mud and silt cap rock, an upper reservoir zone consisting of high temperature sand and clays and a deeper, more continuous reservoir zone of consolidated sand and silt. |