Seismological Laboratory and Dept. of Geological Sciences

University of Nevada, Reno

This project will assemble 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 three-dimensional crustal model to understand the deep sources of geothermal heat in the crust. 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 some geothermal regions is known at some detail.

We will compile 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 will also collect one new crustal refraction profile between mine blasts across the northern Walker Lane, poorly characterized at present. The proposed seismic recording equipment will capitalize Mackay's ability to continue to gather such basic information.

The resulting seismic velocity model will consist of simplified rule-based representations of some of the region's sedimentary basins. Shallow velocities will be constrained by geotechnical data; seismic receiver-function and refraction analyses will constrain Moho depths. The model will be specified in a form compatible with computer codes developed for SCEC; and contributed to the associated projects by Shevenell and Garside, UNR-NBMG, if funded, to assemble geographic databases of geothermal indicators.

A faculty geophysicist and a graduate student will assemble this model almost entirely from existing results. The project will both include literature review and examination of gray literature and data donated by the geothermal, mining, petroleum, and geotechnical industries. We will assemble especially detailed information on Nevada's major sedimentary basins. We will also conduct an inexpensive, 3- to 5-day seismic refraction survey, 500 km long, between mine blasts and crossing the northern Sierra Nevada and Walker Lane.

Exploration for hidden resources requires a realistic three-dimensional crustal model to understand the deep sources of geothermal heat in the crust of the western Great Basin. 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 (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 Basin and Range (fig. 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.

The acquisition under this project of 25 Reftek RT-125 "Texan" seismic recorders will assist our effort to acquire the 500-km-long refraction profile across the northern Sierras and Walker Lane. An additional 150 recorders we will borrow from the IRIS/PASSCAL Instrument Center for the experiment, for the cost of shipping and batteries. However, these instruments are in great demand, and scheduling of our experiment will be constrained by other projects. We have used the PASSCAL Texans at the UNR Seismo Lab several times in the past two years, and feel certain that we will be able to borrow the instruments and complete the experiment within the project year. The 25 instruments we propose to acquire will enable UCCSN geophysicists to continue smaller-scale refraction experiments regardless of the PASSCAL schedule.

Velocity information will be compiled from sources in the literature, results of previous seismic experiments and earthquake-monitoring projects, and data donated from mining and petroleum companies. We will also collect one new crustal refraction profile between mine blasts near Auburn, Calif., and Battle Mountain, Nev. (fig. 1). This new profile will assess crustal velocities across the northern Walker Lane, poorly known at present.

The resulting seismic velocity model will consist of simplified rule-based representations of some of the region's sedimentary basins (e.g., Las Vegas, Reno, Carson Sink, Dixie Val., Railroad Val., Indian Wells Val., Death Val.). Available basins will be embedded in a 3-d crust over a variable-depth Moho, as developed by SCEC for southern California. Shallow velocities will be constrained by geotechnical data; seismic receiver-function and refraction analyses will constrain Moho depths. The model will be specified in a form compatible with computer codes developed for the SCEC Community Velocity Model; and contributed to the associated projects by Shevenell and Garside, UNR-NBMG, if funded, to assemble geographic databases of geothermal indicators.

Harder and Keller (2000) demonstrated that a 150-km-long deployment of 300 PASSCAL Texan instruments could be completed in a single day, with the efforts of seven crews of 2 or 3 people. They observed crustal P, Pg, PcR, PmP, and SmS phases from a single ripple-fired mine blast. The proposed profile is three times as long as Harder and Keller's (2000), and will be reversed between mines in the western Sierra Nevada and west of Carlin (figure 1). However, the profile is centered on Reno and entirely accessible from Interstate 80. Thus, the proposed budget only needs include wages for three undergraduate helpers in addition to the PI and graduate assistant, and funds for 5 vehicles for the 3-5 days we anticipate for layout and pick-up. The profile will use 150 Texans; the PI as well as students have been trained in their use during two completed experiments.

Aside from the single inexpensive 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; 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 (figure 1) with long-period receiver functions.

Pn velocities– Thompson et al. (1989) review regional constraints on Pn velocities. Our proposed 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).

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 will find 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). The proposed 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.

Middle & lower crustal velocities– Mooney and Braile (1989), Thompson et al. (1989), and Fliedner et al. (1996) provide reviews of crustal velocity information that will form a basis for a 3-d crustal velocity model. This model will be 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 will assemble as well 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).

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. 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 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 will compile 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).

We have already developed rule-based velocity models for the Las Vegas and Reno basins, from the published depth and density data, and viewable at http://www.seismo.unr.edu/ftp/pub/louie/reno/basinseis.jpg. 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 urban 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 urban basins are principally extensional, and 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 the Las Vegas and Reno basins from the Railroad Valley density profiles, and 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 the refraction microtremor technique in and around this basin, resulting 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. During both project years we will seek out geotechnical data from consultants working in both Las Vegas and Reno. We will seek out as well shallow geophysical data such as the study between Death Valley and Las Vegas (figure 1) by Shields et al. (1998).

(1) September 30, 2002: a brief report will be delivered to the funder outlining the sources of existing velocity information mined to date, reporting on the acquisition and performance of the new Reftek "Texan" instruments, and preliminary data from the refraction experiment.

(2) February 2003: a brief report outlining and referencing all data sources will be delivered to the funder. In addition, we will describe the form of the velocity database and its transmission to cooperating GIS projects. We intend to present an interpretation of this crustal model to industry at the 2003 GRC meeting.

(3) At the completion of the project: technical papers detailing the features of the assembled seismic velocity model, and their implications for the distribution of geothermal resources, will be submitted for publication in peer-reviewed scientific journals. In addition, a web site accessing the database and all results will be available, and a complete copy delivered to the funder on CD-Rs or DVD-R disc. The new Reftek recorders will be placed in a Seismo Lab facility, to be maintained for use by UCCSN geophysicists.

(4) Quarterly DOE Reports: will be completed as required by the PI.

(5) Apr. 26, 2002 GRC Meeting abstract: will show samples of sources of velocity data, and appeal to the industry for donations of data to help fill gaps in coverage.

J. Louie, Assoc. Prof. of Seismology, UNR: 21 days summer @ $450/day 9450

Graduate Student Stipend (Seismological Lab): 14,500

(9 mo @ 20 hrs/wk , $966.66/mo; 3 mo @ 40 hrs/wk, $1933.33/mo)

Casual Labor (Lab and Field Technician): 117 hours @ $15/hr 1755

Undergraduate Assistant Wages: 3 @ 40 hrs ea., $10/hr 1200

Total Salaries and Wages $26,905

4 % of Faculty Summer Salary $378

10 % of Casual Labor 176

2 % of Student Stipends and Wages 314

Total Fringe Benefits $ 868

Reftek RT-125 "Texan" Seismic Recording System, 25 channels @ $3000/chan. $75,000

Desktop and Laptop Computers for Reftek data-handling system, 2 @ $3400 each 6800

Total Equipment $81,800

University or Commercial vehicle rental, 5 trucks @ $75/day/truck, 5 days fieldwork $1875

Per Diem for fieldwork, 5 people @ $75/day/person, 5 days fieldwork 1875

Two to Amer. Geophys. Union meeting, Dec. 2002, $800 each 1600

Total Travel $5350

GRC 2002 Reno Meeting registration fees, 2 @ $500 ea. 1000

Field and computer supplies (including $350 for Reftek Texan D-cells) 2000

Telephone line and toll charges, postage, $850 shipping of 150 Reftek Texan recorders 1500

Computer systems hardware and software fees, system administration 3000

and network fees, computer repairs: to support data analysis and database formation

Publication costs, printing and copying charges and supplies 1500

Graduate student fees: 18 credits @ $94.75/credit 1706

Total Direct Costs $125,629

Modified Total Direct Costs (DC minus equipment minus student fees) 42,123

Seismological Laboratory 174, Mackay School of Mines

The University of Nevada, Reno, NV 89557-0141

(775) 784-4219; fax (775) 784-1833; louie@seismo.unr.edu

California Institute of Technology, Pasadena, California. Degrees: Ph.D. Geophysics, June, 1987; M.S. Geophysics, June, 1983.