Geophysical investigation of Jersey Valley geothermal field using magnetotelluric and gravity data, A

Abstract

Jersey Valley in Nevada hosts a 15MW geothermal power plant that has been operating since late 2010. Unfortunately this power plant is unable to run at its design capacity due to problems injecting geofluids after their use in power generation. A number of elements are required for a geothermal system to be a viable site for power generation, for a given system a description of these elements commonly comes in the form of a conceptual model. Conceptual models provide a basis for understanding of the field and quantitative modelling. The inability to inject fluids suggests a misunderstanding of the system at Jersey Valley and therefore an inadequate conceptual model. Two current models of the field exist along with an extensive data set including geophysical and geologic data. Poor justification of how the data relates to the models and continued injection problems suggest the existing models may be inadequate. I discuss the major structural elements of the two models and introduce a third model. These three models are examined using the geologic and geophysical data. From previous surveys geophysical data concentration near the power plant was dense, but initial observations suggested complicated structure to the west which was poorly sampled. This lack of data justified further geophysical data sets to be acquired. Data was acquired over 3 trips expanding data concentration to the west. I examined geologic data along with magnetotelluric and gravity data which were chosen due to their relatively high depth of investigation. Analysis of the two geophysical data sets provided complimentary and supplementary information. Gravity analysis was performed through a number of maps and a two-dimensional modelling program. Magnetotelluric analysis was performed using a number of interpretation tools which examine layer thickness, structure, and dimensionality, these tools included a one-dimensional modelling program. Through the analysis major structural aspects of two of the models are shown to be consistent with the data whereas the major structural aspects of one of the models is shown to be inconsistent with the data. In addition, the data analysis lead to information regarding previously unmapped or incorrectly mapped structures. These conclusions will help provide a basis for further conceptual models and lead to a better understanding of the field.