Water-Resources Investigations Report 01-4220

Hydrogeologic Framework of Antelope Valley and Bedell Flat, Washoe County, West-Central Nevada

GEOPHYSICAL METHODS

Geophysical methods can be used to determine indirectly the hydrogeologic framework of an area based on the physical properties of the subsurface. The combination of gravimetry and seismic refraction is well-suited for use in west-central Nevada. Among the different hydrogeologic units in this area, differences in rock densities and seismic velocities can be very pronounced. The gravimetric method allows for a large area to be surveyed rapidly to produce a general description of the hydrogeologic framework. When more detail about the subsurface is required, the seismic-refraction method can be used. Brief descriptions of the gravimetric and seismic-refraction methods follow; for additional information see Grant and West (1965), Telford and others (1976), and Dobrin (1976).

Gravity

The objective in gravity exploration is to associate variations in measured gravity with differences in the distribution of densities in the subsurface, which correspond to different rock types. Differences in rock densities produce small changes in the Earth's gravitational field that can be detected by a gravimeter. In the study area, the contrast in density between Cenozoic deposits and pre-Cenozoic basement produces distinctive gravity anomalies. These anomalies can be used both to determine the thickness of Cenozoic deposits and to infer the structure and geometry of the basement.

Density information from hydrogeologic units within the study area (table 1) is essential to understanding the relationship between gravitational anomalies and their causative sources. From 25 rock samples collected in the study area, the saturated bulk density of pre-Cenozoic basement (plutonic and metamorphic rocks) was found to range from 2.28 to 3.03 g/cm³, with an average of about 2.71 g/cm³. Samples of Cenozoic volcanic rocks ranged in density from 2.11 to 2.36 g/cm³, with an average of 2.24 g/cm³. The density of basin-fill deposits beneath valleys in Nevada has been determined to range from about 1.60 to 2.20 g/cm³ (Jewel and others, 2000, p. 6; Berger and others, 1997, p. 21). The density of Cenozoic deposits, particularly basin fill, generally increases with depth as a result of compaction. In this study, a layered-density model (Jachens and Moring, 1990, p. 14) was used to describe the relation between density and depth of Cenozoic deposits (table 2). Additional physical property data, locations of samples, description of gravity reduction procedures used, and an isostatic anomaly map of the study area are presented by Jewel and others (2000).

The thickness of Cenozoic deposits beneath Antelope Valley and Bedell Flat was determined by an iterative gravity inversion method that uses isostatic gravity anomalies (Jachens and Moring, 1990). Isostatic gravity was used in this analysis because these anomalies reflect density variations in the shallow (depth of less than about 4,000 ft) mid-crust of the Earth from which the structure of the subsurface can be inferred (Simpson and others, 1986). The inversion method separates the isostatic gravity field into two components: the gravity field generated by pre-Cenozoic basement and the gravity field generated by less-dense overlying Cenozoic deposits. Gravity data were interpolated with a minimum curvature method (Briggs, 1974) from the 351 point values (pl. 1) to a regular 656-foot grid (Webring, 1981).

The inversion process is started by using an initial basement gravity field determined from gravity data collected on outcrops of pre-Cenozoic rock exposed in the mountains. This initial field is a first approximation because gravity measured on basement outcrops is influenced by the gravity effect of low-density deposits in adjacent basins, especially for those measurements nearest the edge of a basin. The difference between the isostatic and basement gravity fields represents an initial basin gravity field. The effects of this gravity field are removed from gravity measurements made on the basement rock, essentially removing gravitational effects caused by low-density basin material, and thus creating an improved measure of the basement gravity field. This process is repeated until successive iterations produce small changes in the basement gravity field. Inversion of the final basin gravity field yields the final estimate of depth to pre-Cenozoic basement.

The inversion process is based on the density contrast between Cenozoic deposits and pre-Cenozoic basement. Density assignments of basement rocks are constrained by the lateral (horizontal) extent of mapped units, contact between basin fill (Cenezoic units) and consolidated bedrock (pre-Cenezoic) units (Lydon and others, 1960; Bonham, 1969; and Stewart and Carlson, 1978). The density of Cenozoic deposits were allowed to vary according to the density-depth function of Jachens and Moring (1990; table 2). Reported depths to consolidated rock from drillers' logs in Antelope Valley and Bedell Flat and data from five seismic-refraction profiles were used as independent constraints during the inversion process.

The inversion process used to determine the thickness of Cenozoic deposits is subject to a number of limitations, including: (1) the coverage of gravity data, especially for stations on basement outcrops; (2) the ability of the density-depth function to represent the relationship of increasing density and depth in the Cenozoic deposits; (3) the accuracy or scale of geologic mapping; and (4) the simplifying assumptions regarding concealed geology. A more detailed discussion of the limitations and accuracy of the inversion method is provided by Jachens and Moring (1990). The lines of equal thickness shown on plate 1 are contoured at an interval of 250 ft. Because of the limitations mentioned above and the inherent ambiguity in the gravity method, caution should be exercised when thickness values are interpolated beyond this interval.

Seismic Refraction

The seismic-refraction method is based on measured traveltimes of artificially generated waves of elastic energy as they propagate through the subsurface. When seismic waves encounter a velocity contrast in the subsurface, such as the water table or Cenozoic basement, they refract according to Snell's Law (Telford and others, 1976, p. 245). The refracting interface represents an increase in seismic velocity; the depth to this interface can be determined using wave-path geometry and recorded traveltimes. The objective of a seismic-refraction survey is to profile the velocity contrasts within the subsurface on the basis of depth and dip of each refractor encountered. For the seismic-refraction method to be successful, each successively deeper refractor must have a higher seismic velocity along with a considerable velocity contrast.

To determine seismic velocity, traveltimes of refracted waves are measured by an array of geophones and plotted against distance from the shot point along the profile (fig. 2). The inverse slope of the time-distance curve is equal to the apparent seismic velocity of the corresponding refracting interface. Traveltime data are collected from shot points at either end of the seismic profile (reverse profiling) to correct for dipping interfaces and to compute a true seismic velocity. Shot points are moved outward from the geophone array at successively greater distances to increase the depth of investigation beneath the profile. At greater depths only the central portion of the refracting interface is sampled by the seismic waves.

Seismic velocity depends on a large number of factors, including mineralogical composition, grain size, cementation, pressure, and direction with respect to bedding. Generally, the type of most hydrogeologic units can be inferred from seismic velocities, although ambiguities exist. Weathered near-surface sediments typically exhibit velocities between 400 ft/s and 700 ft/s (Haeni, 1988, p. 41). In basin-fill deposits beneath many valleys in Nevada, seismic velocity increases with depth mainly because of compaction, but also from partial cementation by mineral precipitation. Unsaturated basin-fill deposits exhibit seismic velocities that typically range from 1,200 ft/s to about 3,000 ft/s.

When it reaches the water table, the velocity of the seismic wave may increase by as much as 150 percent. This increase, which creates a considerable contrast in velocity at the interface between unsaturated and saturated basin fill, is represented as the first change in slope on the time-distance curve (fig. 2, profiles B-E). Seismic velocities in saturated basin fill range from about 5,000 ft/s to 8,000 ft/s depending on the depth and induration of the basin-fill deposits (fig. 2). The velocity increase resulting from saturation allows the refraction method to be generally successful in determining the depth to the water table. Volcanic rocks of Tertiary age in west-central Nevada typically exhibit seismic velocities between 7,000 ft/s and 10,000 ft/s. Because older basin-fill deposits may be semi-consolidated, a characteristic that increases seismic velocity, differentiating between older basin fill and Tertiary volcanic rocks based solely on seismic velocity generally is not possible. Consolidated rocks, such as granodiorite or metamorphic rocks, exhibit seismic velocities that range from about 10,000 ft/s to 23,000 ft/s, depending on the degree of weathering and extent of fracturing of the rock encountered (Haeni, 1988, p. 41). In this study, measured velocities assumed to represent pre-Cenozoic basement ranged from 10,300 ft/s to 19,900 ft/s. Pre-Cenozoic basement was not detected beneath seismic profiles B-B′ and C-C′ (fig. 2). Minimum depths to the basement were computed assuming that the latest recorded traveltime was refracted from an interface with a velocity of 12,000 ft/s and 17,000 ft/s.

For this study, seismic-refraction data were collected along four profiles in Bedell Flat and one profile north of Antelope Valley to help corroborate depth estimates obtained from the gravity data. Time-distance curves and the corresponding velocity-depth cross-sections for the five seismic profiles are in figure 2. No seismic-refraction data were collected in Antelope Valley because an adequate distribution of depths to consolidated rock was available from drillers' logs. The seismic-refraction data initially were interpreted in the field from time-distance curves using the intercept-time formula (Dobrin, 1976, p. 297). Subsequent interpretations were guided by an inversion algorithm that uses the delay-time method (Barthelmes, 1946; Pakiser and Black, 1957) to obtain a first-approximation depth model, then enhanced by a series of ray-tracing iterations (Scott, 1993).

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