Assembly of a Crustal Seismic Velocity Database

Assembly of a Crustal Seismic Velocity Database

for the Western Great Basin

John N. Louie

Seismological Laboratory and Dept. of Geological Sciences

University of Nevada, Reno

Key Words: regional indicators, data base, crust, mantle, seismic velocity, Moho, heat flow, faulting, seismic refraction, PASSCAL, EarthScope.

ABSTRACT

This project is assembling a three-dimensional reference seismic velocity model for the western Great Basin region of Nevada and eastern California. Exploration for hidden resources requires a realistic crustal and upper-mantle model to understand the deep sources of geothermal heat. The type of rule-based representations developed by the Southern California Earthquake Center (SCEC) are very appropriate to defining velocity on the spatial scales of this application, particularly for the western Great Basin. Crustal properties and thickness are known only at wide spacing, but the structure of the urban basins and certain geothermal regions is known at some detail.

We are compiling velocity information from sources in the literature, results of previous seismic experiments and earthquake-monitoring projects, and data donated from mining, geothermal, and petroleum companies. We also collected (May 2002) one new crustal refraction profile using blasts at Barrick’s Goldstrike mine, across western Nevada and the northern Sierra to Auburn, Calif. This section had not been characterized previously.

The resulting seismic velocity model consists of simplified rule-based representations of some of the region's geothermal areas and sedimentary basins. Very shallow velocities are constrained by geotechnical data; seismic receiver-function and refraction analyses constrain deep Moho depths. The model is specified in a form compatible with computer codes developed for SCEC and EarthScope. We are developing from the seismic velocity database a geographic index of the likelihood of geothermal resources. The likelihood index will contribute to regional databases of economic geothermal indicators.

INTRODUCTION

This project will assemble a three-dimensional reference seismic velocity model for the western Great Basin region of Nevada and eastern California (Figure 1). This model will be rule-based and distributed as software as well as maps on the Internet, similar to the SCEC Community Velocity Model (CVM) of Magistrale et al. (2000). These qualities will make the model useful for multidisciplinary research activities including geothermal exploration, mineral exploration, earthquake-hazard assessment, high-precision microearthquake location and source-parameter estimation, and crustal-structure imaging.

A faculty geophysicist and a graduate student are assembling this model almost entirely from existing results. The project includes literature review and examination of gray literature and data donated by the geothermal, mining, petroleum, and geotechnical industries. We are assembling especially detailed information on the area's major geothermal resources (e.g., Figure 2) and sedimentary basins. We conducted in May 2002 a 500-km-long seismic refraction survey, using 20,000-40,000-kg blasts at Barrick’s Goldstrike mine, recorded on 199 instruments extending over the northern Sierra Nevada and Walker Lane to Auburn, California (Figure 1).

Exploration for hidden resources requires a realistic three-dimensional crustal and upper-mantle model to understand the deep sources of geothermal heat in the crust of the western Great Basin (e.g., Figures 3 and 4). Rule-based representations of velocity models have been developed by the Southern California Earthquake Center (SCEC; ) and are being adopted by the NSF Earthscope effort ( for its study of North American continental architecture. Such rule-based models are very appropriate to defining velocity on the spatial scales of this application, particularly for the western Great Basin. Crustal properties and thickness are known only at wide (100 km) spacing, but the structure of the urban basins and some geothermal regions (e.g., Coso, Dixie Valley) is known at some detail (0.2 km spacing).

Crustal thickness and velocity are closely related to a region's thermal and tectonic history. Known geothermal resources in north-central Nevada are closely associated with thin crust and an uplifted Moho (Savage and Sheehan, 2000; Ozalaybey et al., 1997; Fliedner et al., 1996; Humphreys and Dueker, 1994). By assembling a velocity model for the entire western Great Basin (Figure 1), we will be able to look for crustal features, similar to those under known geothermal resources, that may be closer to Southern California power markets.

RESEARCH METHODS

Seismic velocity information is being compiled from sources in the literature, results of previous seismic experiments and earthquake-monitoring projects, and data donated from mining and petroleum companies. We have collected a new crustal refraction profile between mine blasts near Auburn, Calif., and Battle Mountain, Nev. (Figure 1). This new profile will assess crustal seismic velocities across the northern Walker Lane, poorly known at present.

The assembled seismic velocity model consists of simplified rule-based representations of some of the region's geothermal areas (e.g., Dixie Valley, Steamboat, Rye Patch, Lake City, Empire, Fish Lake Valley, Coso, Tecopa) and sedimentary basins (e.g., Las Vegas, Reno, Carson Sink, Railroad Valley, Indian Wells Valley, Death Valley). Available details from geothermal fields and basins are embedded in a 3-d crust over a variable-depth Moho, as developed by SCEC for southern California. Very shallow velocities are constrained by geotechnical data, while seismic receiver-function and refraction analyses constrain deep Moho depths. The model will be specified in a form compatible with computer codes developed for the SCEC Community Velocity Model; and contributed as geocoded map coverages to the Great Basin Center for Geothermal Energy ( ) to assemble geographic databases of geothermal indicators.

Harder and Keller (2000) observed crustal P, Pg, PcR, PmP, and SmS phases from a single ripple-fired mine blast, over a 150-km line of portable seismometers. The profile we obtained is three times as long as Harder and Keller's (2000), but the Barrick Goldstrike and Florida Canyon mine blasts we recorded from the east end of our profile are much larger at 20,000 to 40,000 kg of explosive charge. We will use the 20 portable Reftek “Texan” recorders funded by the DOE for this project to collect a reversal of the profile from smaller mine blasts in the western Sierra (Figure 1), over the course of a year.

We recorded the eastern mine blasts along the profile in May 2002, and we will show data samples and preliminary crustal models at the September 2002 GRC meeting. First-arrival time picks of the refraction seismograms will be optimized for a tomographic seismic velocity section using the method of Pullammanappallil and Louie (1994). With a maximum offset of 500 km, we expect to derive crustal and lithospheric velocities as deep as 100 km. Regionally, heat flow from the mantle through the crust should correlate inversely with crustal thickness, crustal seismic velocity, and mantle Pn velocity (the velocity of the P-wave refraction at the Moho).

Our profile passes near known geothermal resources at Steamboat (Reno), Stillwater (Fallon), Dixie Valley, Rye Patch, and the Battle Mountain heat-flow high (Figure 1). It also passes over the northern Sierra, where known geothermal areas are few. The experiment thus characterizes the crust near a large proportion of the geothermal resources in the western Great Basin. Other important resources such as Long Valley and Coso have already been well characterized at a crustal scale. We will then be able to compute, across the region, a geothermal indicator index that is maximized for a thin (<35 km), low-velocity (<6.0 km/s average) crust over a hot, low-velocity (<7.9 km/s Pn) mantle. With the improved coverage from this project, we will be able to test whether this regional indicator index has any correlation with the locations of all the known geothermal resources.

Aside from the single refraction survey, most of the proposed effort will be to assemble velocity information of several types and at several scales to define the model at different depths:

Upper mantle— The tomographic image of Humphreys and Dueker (1994) provides a starting framework for mantle velocity in the western Great Basin (Figure 3), although their coverage north and east of Reno (Figure 1) is poor. Dueker and Sheehan (1997) tracked upper-mantle discontinuities across the Snake River Plain with long-period receiver functions (Figure 4). In computing the geothermal indicator index it is important to see whether low Pn velocities continue to at least 50 km depths. Continuing velocity lows indicate high-temperature mantle, while lows that are confined to within a few kilometers of the Moho are more likely geometric effects on seismic refraction propagation.

Pn velocities— Thompson et al. (1989) review regional constraints on Pn velocities. Our profile across the northern Sierra and Walker Lane will provide some of the constraints available for the southern Sierra and Death Valley from Fliedner et al. (1996). Hot, buoyant mantle shows a slower Pn velocity. Thus, crustal heat flow and regional geothermal potential may be inversely dependent on the Pn velocity.

Moho depth— Mooney and Braile (1989) and Kaban and Mooney (2001) reviewed all available constraints on Moho depth for the western Great Basin (Figure 1). We are finding out if Dueker and Sheehan (1997) also made shorter-period receiver functions more suitable for estimating mantle depths in the northern Great Basin. For the central Great Basin constrained receiver-function analyses are available from Ozalaybey et al. (1997). Our refraction survey between mine blasts will provide Moho depth information across the northern Sierra and Walker Lane (Figure 1), where constraints are poorer than to the south near Death Valley. Since the Moho is the top of the hot, adiabatic mantle, smaller Moho depths should be associated with higher crustal temperature gradients, and an increase in regional geothermal potential.

Middle & lower crustal velocities— Mooney and Braile (1989), Thompson et al. (1989), and Fliedner et al. (1996) provide reviews of crustal velocity information that are forming a basis for our 3-d velocity model. This model is parameterized as functional profiles at the locations of control points, with interpolation extending the model laterally between controls. In the northern and eastern Basin and Range control may be sparse enough that we will need to employ the CRUST 5.1 global model of Mooney et al. (1998). Ozalaybey et al. (1997) constrained crustal velocity profiles at several locations in the central Great Basin, establishing low-velocity zones exist at very few. We are also assembling published and unpublished studies of joint aftershock relocation and velocity inversion such as by Asad et al. (1999) for the Eureka Valley sequence north of Death Valley (Figure 1). Deep-crustal seismic velocities may be reduced in areas of high geothermal potential due to volcanic intrusion and partial melting. We are looking for such prominent velocity anomalies outside of the known anomalies at Coso and Long Valley.

Upper crust— Louie and Qin (1991) and Louie et al. (1997) used surface waves and COCORP reflection surveys to constrain upper-crustal velocities west of Death Valley (Figure 1). The optimization methods of Pullammanappallil and Louie (1993; 1997) have proved effective in obtaining velocities to 5 km depth from reflection surveys. Figure 2 shows an example of how this method has in many areas been able to identify the limits of geothermal production zones, by their bounding velocity discontinuities. Upper-crustal velocity lows (P velocity < 5.5 km/s) are likely correlated with highly fractured areas that may provide pathways for the deep heating of meteoric waters.

We will pick and optimize first-arrival and reflection times where needed from available COCORP and industry data. In addition to crustal thickness, much of the work reviewed by Kaban and Mooney (2001) constrains P velocities as well to 5-10 km depth. Additional constraints are reviewed by Thompson et al. (1989) and Fliedner et al. (1996); many of them come from long COCORP surveys extending from the northern Sierra to the Ruby Mountains, and in the Death Valley region (Figure 1). We are compiling and will report on all available information from reflection stacking velocity analyses, and seek to examine copies of commercial spec surveys, abundant in the Carlin gold trend.

Basin depths and velocities— Honjas et al. (1997), Chavez-Perez et al. (1998) and Abbott et al. (2001) estimated basin depths and velocities for Death Valley and Dixie Valley (Figure 1) from the first-arrival times recorded in reflection surveys. Jachens and Moring (1990) summarize relations between density and depth in Nevada basins from oil-well logs, mostly from Railroad Valley (Figure 1). Langenheim et al. (2001) and Abbott and Louie (2000) used these relations together with some borehole and seismic data to detail the depths and density profiles of Nevada's urban basins in Las Vegas, Reno, and Carson (Figure 1). A thick sedimentary basin may be a negative geothermal indicator, since any hot bedrock is far below the surface. On the other hand, many significant resources such as at Dixie Valley, Carson Valley, Black Rock Desert, and Lake City appear to lie on shallow basin margins, where the basin-bounding faults may have channeled hot fluids to the basin edges. Seismic velocities within basins can indicate whether the sediments are highly permeable gravels, impermeable shales, or fractured diatomites (common in the northern Great Basin). Thus the sedimentary basin data provide important refinements on the regional crustal characterization, possibly allowing more specific targeting within areas of regional high geothermal potential.

We have already developed rule-based velocity models for the Las Vegas and Reno basins, from the published depth and density data, viewable at . These models are expressed in Java code. One measurement of shear velocity to the basement has been done in Reno (Louie, 2001), using the method of Horike (1985), Liu et al. (2000), and Satoh et al. (2001a). COCORP stacking velocities from basins will provide a few P-velocity constraints for basins between Reno and Carlin, and near Death Valley; spec seismic data we are able to view will gain us some data for the central Great Basin.

Unlike how Magistrale et al. (2000) created the SCEC CVM with rules for formation depths and velocities gained from oil wells, Nevada's sedimentary basins have far too few deep boreholes. However, SCEC's CVM must be accurate for basins under compression, with kilometers of thrust deformation. All of Nevada's sedimentary basins are principally extensional, and almost all are still receiving sediment. We propose to form the western Great Basin model using instead rules for depth within a basin, and possibly the basin's proximity to Tertiary volcanic centers, and its age and subsidence rate. Controlling for these factors, as far as is possible, will enable us to predict velocities within all the basins in the region, from the Railroad Valley density profiles, and from the Death Valley and Dixie Valley velocity optimizations.

Geotechnical— Louie (2000) published some shallow geotechnical velocity information on the Reno basin. We have continued to apply our refraction microtremor technique in and around this basin, resulting in a complete profile of the shallow basin, and in measurements at a dozen rock sites around the basin. Many of these measurements were made at borehole sites with assistance from local engineering consultants. We are seeking out geotechnical data from consultants working in both Las Vegas and Reno, as well as shallow geophysical data such as the study between Death Valley and Las Vegas (Figure 1) by Shields et al. (1998). Very low shear velocities in the shallow geotechnical layer suggest less permeable clay playa deposits, which may force geothermal fluids laterally toward sands and gravels with higher geotechnical shear velocities. Only a few basins (Las Vegas, Reno, Carson) will have detailed enough characterizations for the geotechnical data to contribute toward detailing the geothermal indicators.

EXPECTED RESULTS

(1) We will complete a 500-km-long by 100 km-deep tomographic analysis of P velocity across the northern Walker Lane from refraction data recorded in May 2002.

(2) We will develop and document the form of the velocity database, and render it as map coverages to cooperating GIS projects. We are developing trial maps of the economic geothermal potential from the velocity database, for scrutiny by the industry. The "geothermal potential index" mapped may be tied to indicators in our velocity database such as mantle velocity, crustal thickness, lack of significant sediment thickness, and relatively low shallow-crustal velocity.

(3) Technical papers detailing the features of the assembled seismic velocity model, and their implications for the distribution of geothermal resources, are being prepared for submittal to peer-reviewed scientific journals. In addition, we are creating a web site at where interested parties may access the database, yielding map and cross-section products. We will evaluate the effectiveness of our web site in meeting the exploration and assessment needs of the geothermal industry, and improve it in response to their suggestions.