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.