Paul W. Kasameyer

Geological, Geophysical, And Thermal Characteristics Of The Salton Sea Geothermal Field, California

Abstract The Salton Sea Geothermal Field is the largest water-dominated geothermal field in the Salton Trough in Southern California. Within the trough, local zones of extension among active right-stepping right-lateral strike-slip faults allow mantle-derived magmas to intrude the sedimentary sequence. The intrusions serve as heat sources to drive hydrothermal systems. We can characterize the field in detail because we have an extensive geological and geophysical data base. The sediments are relatively undeformed and can be divided into three categories as a function of depth: (1) low-permeability cap rock, (2) upper reservoir rocks consisting of sandstones, siltstones, and shales that were subject to minor alterations, and (3) lower reservoir rocks that were extensively altered. Because of the alteration, inter granular porosity and permeability are reduced with depth. Field permeability is enhanced by renewable fractures, i.e., fractures that can be reactivated by faulting or natural hydraulic fracturing subsequent to being sealed by mineral deposition. In the central portion of the field, temperature gradients are high near the surface and lower below 700 m. Surface gradients in this elliptically shaped region are fairly constant and define a thermal cap, which does not necessarily correspond to the lithologic cap. At the margin of the field, a narrow transition region, with a low near-surface gradient and an increasing gradient at greater depths, separates the high temperature resource from areas of normal regional gradient. Geophysical and geochemical evidence suggest that vertical convective motion in the reservoir beneath the thermal cap is confined to small units, and small-scale convection is superimposed on large-scale lateral flow of pore fluid. Interpretation of magnetic, resistivity, and gravity anomalies help to establish the relationship between the inferred heat source, the hydrothermal system, and the observed alteration patterns. A simple hydrothermal model is supported by interpreting the combined geological, geophysical, and thermal data. In the model, heat is transferred from an area of intrusion by lateral spreading of hot water in a reservoir beneath an impermeable cap rock.

Seismic velocity and attenuation structure of the Geysers geothermal field, California

The Geysers geothermal field is located in northern California and is one of the worlds largest producers of electricity from geothermal energy. The resource consists of primarily dry steam which is produced from a low, porosity fractured graywacke. Over the last several years steam pressure at the Geysers has been dropping. Concern over decline of the resource has prompted research to understand its fundamental nature. A key issue is the distribution of fluid in the matrix of the reservoir rock. In this paper we interpret seismic compressional-wave velocity and attenuation data at the Geysers in terms of the geologic structure and fluid saturation in the reservoir. Our data consist of approximately 300 earthquakes that are of magnitude 1.2 and are distributed in depth between sea level and 2.5 km. Using compressional-wave arrival times, we invert for earthquake location, origin time, and velocity along a three-dimensional grid. Using the initial pulse width of the compressional-wave, we invert for the initial pulse width associated with the source, and the one-dimensional Q structure. We find that the velocity structure correlates with known mapped geologic units, including a velocity high that is correlated with a felsite body at depth that is known from drilling. The dry steam reservoir, which is also known from drilling, is mostly correlated with low velocity. The Q increases with depth to the top of the dry steam reservoir and decreases with depth within the reservoir. The decrease of Q with depth probably indicates that the saturation of the matrix of the reservoir rock increases with depth.

Development and application of a hydrothermal model for the Salton Sea Geothermal Field, California

A simple lateral flow model adequately explains many of the features associated with the Salton Sea Geothermal Field. Earthquake swarms, a magnetic anomaly, and aspects of the gravity anomaly are all indirect evidence for the igneous activity which is the ultimate source of heat for the system. Heat is transferred from this area of intrusion by lateral spreading of hot water in a reservoir beneath an impermeable cap rock. A two dimensional analytic model encompassing this transport mechanism matches general features of the thermal anomaly and has been used to estimate the age of the presently observed thermal system. The age is calculated by minimizing the variance between the observed surface heat-flow data and the model. Estimates of the system age for this model range from 3,000 to 20,000 years.

A coupled seismic-geotechnical approach to site-specific strong motion

At Lawrence Livermore National Laboratory (LLNL) we have developed a new approach to provide estimates of incident strong ground-motion time-histories at bedrock, which are specific to a site. The estimates involve the recording of small earthquakes at the site, both at bedrock and at the surface, from specific faults in the region. The method also involves the definition of rupture scenarios for those faults. The estimates are obtained for earthquakes of specified moment magnitude (energy release). The new approach is a coupled seismic-geotechnical method. Besides its advantage of giving site-specific time-histories, this approach has the added benefit of providing an optimized definition of the dynamic geotechnical site properties which will be used to calculate shear wave propagation in the soils, from the estimated bedrock motions. This method is intended to replace state-ofpractice approaches in which the site motion estimation may involve data from distant locations and faults having nothing to do with the specific site. The procedure is demonstrated at the Painter Street bridge site in Rio Dell, CA, for which we provide a range of surface motions corresponding to an earthquake of magnitude 7 on the subducting plate underlying this region. These calculated motions bracket the records of the Petrolia event ( M = 7) measured near the site.

The vent of the 1912 Katmai Eruption: Gravity and magnetic measurements

This study is aimed at defining the boundary and internal structure of the Novarupta vent area, the source of the 1912 Katmai eruption. A geodetic grid consisting of 145 stations was occupied for gravity and magnetic measurements and an aeromagnetic survey was flown at 1100 m above sea level. The combined gravity and magnetic data indicate that there are probably two intrusive bodies within the vent, not just one at Novarupta, as was previously thought. The outline of the overall vent at the surface is well defined by the aeromagnetic survey, and the vent appears to dip inwards indicating that it flares from depth. This is in contrast to the piston-like collapse thought to have taken place at Mt. Katmai in 1912.

Time‐domain electromagnetic soundings in the vicinity of Novarupta, Katmai National Park, Alaska

A time-domain electromagnetic survey was conducted near the rhyolite dome Novarupta. We found a continuous (60 to 100 m thick) high-resistivity surface layer, which represents the unsaturated or unaltered tephra and whose base is probably the water table. This layer is thinnest near fumaroles; the largest thin areas lie under the tephra ring surrounding Novarupta, where the altered zones extend to a depth of more than 100 m. A deeper conductive layer is associated with the top of the underlying sedimentary rocks and is inferred to indicate the depth to the pre-eruptive surface.

Results from Shallow Research Drilling at Inyo Domes, Long Valley Caldera, California and the Salton Sea Geothermal Field, Salton Trough, California

A goal of the thermal regimes sector of the U.S. Continental Scientific Drilling Program is to understand the intrusion of magma into the crust, the release of heat and volatiles from these intrusions, and the development of associated hydrothermal systems. These processes result in the formation and modification of continental crust. Magmatic processes of interest include tectonic constraints on intrusions, intrusion mechanisms, cooling behavior, geochemical evolution, interaction with crustal rocks and fluids, degassing, volcanic eruption mechanisms, and coupling of the hydrothermal system to the heat source. Hydrothermal processes of interest include interaction between magma and near-surface fluids, fluid flow pathways, circulation patterns, flow rates, barriers to flow, rock/water interactions, and development of mineral deposits.

3‐D tomographic imaging of the geologic structure in the Salton Sea Geothermal Field

A three‐dimensional tomographic reconstruction of the Salton Sea Geothermal Field is presented. This reconstruction is developed from data gathered in the course of one year between 15 September 1987 and 30 September 1988 using a microearthquake network. This geothermal field is important not only due to understanding potential energy sources but also because it is the result of a tectonic spreading zone bounded between two transverse fault systems: The San Andres system to the North and the Brawley fracture zone (BFZ) to the South. Here magma has penetrated into the crust to a depth of at least 8 km. This magmatic source is responsible for the microearthquakes generated along the BFZ as well as providing the thermal source for the geothermal activity. Using techniques for both blind source estimation as well as blind deconvolution, a travel time tomographic algorithm is applied to these data. The objective is to characterize the subterranean geological structure and estimate the fracturing that supports ...