A study was undertaken in Big Chino Valley, north of Prescott, Arizona, to assess the groundwater resource potential. This valley forms an elongate basin, extending some twenty-five miles northwest from the headwaters of the Verde river. Big Chino Valley is located in a physiographic and tectonic transition zone between the Colorado Plateau and the Basin and Range province. The Big Chino Valley basin is a graben bounded to the northeast by the Big Chino fault. The basin fill, extending to some 2500 feet depth, consists of clays, silts, sands, and some gravels. Tertiary basalts flowed into the valley at several locations.
The development of the hydrogeologic potential of this basin requires an understanding of geologic structure, stratigraphy, and lithologic properties of aquifers for groundwater production. The field studies consisted of geologic investigations, surface geophysical work, drilling of boreholes, and borehole geophysical logging. The surface geophysical work in the area included gravity profiles obtained during an earlier study in an effort to outline the basin structure.
These gravity profiles were supplemented with magnetic profiles to help define the extent of basalt layers in specific areas of interest. Surface resistivity soundings were obtained to def ine the extent of a clay zone postulated by earlier investigators as an impermeable boundary or clay plug. Finally a seismic reflection line was laid out to determine depth-to-basement and aid in locating the first of three drillholes.
Borehole geophysical logs obtained in drillholes provided valuable information to refine and recalibrate surface geophysical data. Quantitative evaluation of the borehole geophysical data provided parameters such as resistivity for a tie to surface resistivity soundings, velocity logs for the generation of synthetic seismograms for a tie to seismic reflection data, detailed lithologic information, and effective porosities, useful in groundwater evaluation modeling.
In summary, the study provides an example of a case study for the development of groundwater resources in a sedimentary basin. Furthermore this study serves to illustrate the use of integrated geophysics to groundwater development.
This study was undertaken by the Bureau of Reclamation for the City of Prescott, Arizona, in an effort to look at additional sources for the City water supply. The results of this study are intended to become part of the input for preliminary groundwater models of the Big Chino Valley area. Furthermore, this study is part of an effort by Reclamation to evaluate the groundwater resources of the Big Chino Valley and upper verde river areas.
Of major importance in this evaluation are: the extent and nature of basin fill materials as an impermeable barrier, and the influence of a well-field on the discharge of groundwater into the Verde River. A significant lowering of water levels in the Verde river is thought to impact the survival of some species of fish that make the Verde river their habitat.
As part of the geophysical program in Chino Valley, 21 resistivity soundings were conducted. The primary objective for most of the sounding locations was to delineate of fine-grained (clay-silt) sediments within the basin fill of Chino Valley. Some of the sounding locations were also intended to develop a cursory picture of resistivity trends and structural information within the limitations of resistivity-depth determinations. A number of desirable sounding locations had to be moved or abandoned due to cultural interference such as power lines, metal fences, or pipelines. Nevertheless, locations were found for three deep soundings of a 1000 meter Via" spacing, allowing some increased depth capabilities for structural interpretations. A few soundings were obtained merely in an effort to obtain representative resistivities for the major lithologies of interest found within the project area, such as basalt, limestone, alluvium, and clay.
Resistivity soundings were obtained with the Wenner array. Out of 21 soundings three were obtained with an tia" spacing of 1000 meters, one with an "a" spacing of 100 meters, and the remainderwith an "a" spacing of 300 meters.A restriction for the fieldworkwas the contractor request to limit the soundings to existing roadways.
The resistivity field data were subjected to an interactive computer algorithm that compares field data to sounding data calculated from a hypothetical distribution of horizontal layers over an infinite half-space. This process allows the interpreter to start a model layer distribution by assuming a number of layers bounded by hypothetical boundaries. As the process continues, the number of layers and the location of the individual boundaries are adjusted to increase the fit of the model sounding with that obtained in the field.
The next process in the resistivity interpretation is the generation of sounding profiles showing resistivity as a function of interpreted depths. For the purpose of this report layer thicknesses were converted to total depth in feet. The resistivities were converted to the logarithm of the resistivity of 10, a contour interval of 2 corresponds to a resistivity of 100, and so on. Contouring a number of resistivity soundings in a given resistivity profile allows the interpreter to tie in geologic constraints or other pertinent information. This in turn may lead to additional adjustments in the individual model for a given resistivity sounding. Trend analysis from resistivity profiles also tends to aid in the visualization of resistivity soundings. This interpretive technique was found to be helpful even with the sparse coverage of field data in the Big Chino Valley.
One profile is shown for this interpretation (see Figure 2). Profile Z16-Z-ll-ZlO-DH3-Z7-Z4-Z5 runs NW-SE along the axis of Big Chino Valley. The length of the profile is 12.3 miles(65,000 feet). The center of the x-axis is at sounding Z10 (see Figure 1 for location). The heavy dashed line shows the location of this resistivity profile. The profile also includes resistivity data from the long normal log of drill hole CVDH-3. The log was converted to a blocked resistivity log (i.e. resistivities were averaged over large intervals, corresponding to major lithologic entities).
One must guard against over-interpreting resistivity data, however, some general relationships may be established when relating resistivities to lithology. In general, a contour of 1. 2 or less represents a clay-rich zone, while area between contour 1. 2 to 1. 6 tend to relate to clays with increased amounts of silt, sands, and gravel. it should be understood that transition zones between clay and clay-rich zones are not detectable from cores and may take place over several hundred feet. Contours between 1.8 and 2.6 may correlate to limestones, basalts, or sandstones. Contours above 2.6 have been interpreted as resistive basement, most likely granites or metamorphics. These contour boundaries must be adjusted by other information where feasible and are intended merely as a guide. These interpretations have been used with other data to construct the main body of the report.
The details of the findings from the resistivity soundings are discussed in the framework of lithology and structure in the geologic portion of this report. One must keep in mind that resistivity boundaries may cross over different lithologies and may not coincide with lithologic boundaries or geologic formation boundaries.
A number of seismic investigations were undertaken as part of the Chino Valley Project. This included 2.2 miles of seismic reflection, 13 refraction profiles, and a checkshot survey in one accessible borehole. Prior to locating the first deep drill hole (CV-DH-1) in Chino Valley, it was necessary to obtain more precise depths to basement or bedrock. Deep resistivity soundings yield only coarse depth estimates. it was determined to test the feasibility of the seismic reflection method in the general area of proposed CV-DH-1 in order to obtain improved estimates to depth of bedrock.
In addition, in an effort to gain an understanding of representative seismic velocities of major lithologies such as limestones, basalts, weathering velocities, and thickness and subweathering velocities of the alluvial material, 12 short refraction spreads were obtained. Also one long refraction line (4800 ft) was obtained over the central portion of the deep reflection line. Alluvial velocities were also determined through the use of a checkshot survey in the Weber Well, located in the southern part of the basin.
Seismic Refraction Surveys
Seismic refraction spread No. 1 was laid outon top of an outcrop of DevonianMartin limestone. The velocity obtained for the Martin is quite low (13,911 ft/sec) for a limestone. Fracturing and Mugginess coupled with the knowledge that the outcrop is above the water table may explain this result. For seismic refraction spread No. 2 the basalt velocity is low (5889 ft/sec) . Again the deep water table and weathering may account for the low velocity. Spread 3, laid out on blocky basalt next to borehole SLl in close proximity to the Verde River Canyon shows much higher velocities for this basalt (9857 ft/sec). From spread numbers 4 to 12, data were obtained right over the reflection line in order to obtain data on weathering velocities and thicknesses. It is of interest to note that weathering velocities (or velocities in the weathering zone) are about 1700 ft/sec with a thickness of about 43 feet.
In an effort to determine shallow structure in connection with the proposed location of CV-DH-1, a refraction spread was obtained over a surface distance of 4800 feet. Subweathering velocities are 5600 ft/sec.
Checkshot Survey
A checkshot survey was conducted in the Weber Well at the southern end of Chino Valley in conjunction with geophysical borehole logging. The purpose of the checkshot survey was to obtain velocities for the clayey alluvial material thought to be present throughout much of the basin and to confirm the presence or absence of bedrock. The average velocity is 7560 ft/sec for the first 378 feet in this well.
Seismic Reflection Survey
A seismic reflection survey was conducted by Reclamation. Field parameters were determined on the basis of available drilling, recording equipment, and permitting limitations.
Other considerations included projected depth-to-basement from existing gravity and resistivity interpretations and proposed geologic models for the area.
For the seismic source, 3-pound 2-component explosives were placed in shotholes drilled to a depth of 8-12 feet.Shot holes were subsequently tampedwith soil. Shotpoints were placedat 50-foot intervals. Off-end shooting was utilized to obtain 24-fold coverage with 100-foot group intervals. Group spacing of a 100 ft was used throughout the survey.
Offset distances of 600 ft to the first geophone were used. It was decided not to spend the time to conduct wave tests, but use the stack array criterion instead.
In order to satisfy the stack array concept, geophones were laid out at constant increments per group. Two strings of 12 geophones (10 Hertz) each were used per group with geophone increments of about 5 feet.
A seismic recording system with 24 channels was available for the seismic survey. Data were recorded at 1 msec sample intervals for a total record length of 2 seconds. A commercial contractor was used for quality control providing in-field processing of the data to obtain a brute stack. It was from the brute stack, coupled with stacking and interval velocities obtained in the field, that the feasibility of drilling in this location was confirmed.
The seismic reflection data were processed by a commercial contractor in consultation with Reclamation personnel. The data were filtered, corrected for NMO, stacked, gain corrected and deconvolved (see Figure 3).
Interpretation of Seismic Reflection Data
The seismic processing contractor supplied tables of interval velocities and time-depth charts calculated from velocity scans for NMO corrections. There is insignificant lateral change in velocity over the area of the seismic reflection line.
Depth conversion was accomplished from data collected from a velocity log obtained in drill hole CV-DH-1.
Four horizons were interpreted from the seismic time section (see Figure 4). A synthetic seismogram, constructed from the velocity log, was used to tie seismic reflection and well-log information. The horizon of foremost importance (Layer 3) is the top of the Devonian Martin limestone. This boundary reflects the transition between unconsolidated Tertiary basin fill alluvial deposits and Devonian sedimentary section. Velocities for the Martin tend to be rather high both from surface core data and well log data and indicate possible dolomitization of this formation.
The next horizon of interest is the top of the granitic basement (Layer 4). Since the drill hole never penetrated basement, the top of the granite was inferred from estimated limestone thicknesses in the area.
Another horizon was included for mapping the seismic in the area. This horizon (Layer 1) shows pronounced continuityacross the reflection section.This third horizon correspondslithologically to the transitionzone between clay and sandy and gravelly clays in the lower portion of the alluvial basin fill. This transition zone can be seen well developed on the resistivity log obtained in drill hole CV-DH-1. This transition zone is only poorly defined from samples obtained during drilling.
An additional horizon (Layer 2) to be interpreted on the seismic section corresponds to a pronounced velocity break interpreted from velocity scans of the reflection data and subsequently from the velocity log obtained in drill hole CV-DH-1. Drill samples show this reflection to contain calcite cement and an increase in gravel in a matrix of clay.
The interpreted cross-section shows a deepening of the alluvial section towards the NW. In addition, faults are indicated in the vicinity of station Nos. 82, 112, and 120. These faults have been interpreted to cut through the lower part of Tertiary alluvial sedimentary fill (Layer 2 and 3). The fault in the vicinity of station 82 appears to be crossed by the seismic line at an oblique angle. Additional seismic lines would have to be obtained to determine the strike of the fault. The throw on the faults is interpreted to be as high as 150 feet. Maximum alluvial thickness is about 2000 feet. Maximum sedimentary section above basement is about 2400 feet. Better definition of basin structure can undoubtedly be obtained through the increased use of seismic reflection data in the Chino Basin.
Gravity and Magnetic Investigations
The testing of the magnetic method in Chino Valley was limited and performed only to demonstrate the usefulness of the technique. It was thought that magnetic profiling might help to outline the areal extent of shallow layers of basalt. For this purpose a magnetic profile (50 feet spacing) was obtained in the area north of Pine Creek where basalt had been reported in water well logs. Part of the survey paralleled a gravity survey conducted earlier by Water Resources Associates, Inc. (WRA, 1989).
The results of both the northern portion of gravity and magnetic surveys were interpreted with an interactive modeling program. The program calculates both gravity and magnetic effects for the same theoretical model. The use of both gravity and magnetic parameters tends to constrain the model better than either method by itself.
Susceptibilities were taken from values published in the literature (Applied Geophysics, Telford). Additional constraints applied to the modeling effort included information obtained from Reclamation's seismic reflection data, deep resistivity soundings, and exploration drill holes CV-DH-1 and CV-DH-3 and water wells CVML and CVM2. One should be careful not to over-interpret modeling efforts from potential field data. Modeling results are not unique and represents an approximate confirmation of postulated geological models.
The completed model (Figure 5) indicates two faults which are shown to offset the basement and an overlying layer of high density material which is thought to be largely dolomite overlying sandstone, i.e. Martin limestone over Tapeats sandstone. Layers of basalts have been incorporated into the model on the basis of drill hole information from drill holes CV-DH-3, CVML and CVM2. Densities of the alluvial material were adjusted to match known depths to bedrock from drill holes CV-
DH-1 and CV-DH-3. More definite information on the existence, location, and offset of the postulated faults would have to come from seismic reflection or drill hole data in the area of the projected offset.
The modeling shows that shallow volcanics have a sizeable impact on the total magnetic field. Therefore, it is concluded that the areal extent of shallow volcanics can be refined ith magnetic methods.
Borehole Geophysical Logging
s a part of the Chino Valley Geologic ramework Investigations, 15 drill oles were geophysically logged. The purpose of the geophysical logging was twofold: 1) to better characterize the lithologies encountered and; 2) to obtain quantitative values of resistivity and seismic velocity for incorporation into the surface geophysical investigations.
With the exception of the three borings drilled as a part of this investigation (CV-DH-1, CV-DH-2, CV-DH-3), the wells geophysically logged were selected on the basis of location, depth, accessibility from well owners, and if the hole was open (no pump).
Figure 6 is a copy of the geophysical logs run in each drill hole. Shown in Table 1 are the interpreted lithologic units encountered in each drill hole. These units are interpreted from a combination of information obtained from the geophysical logs, surface mapping of geology, cuttings, and well driller's logs.
Table I
Description of Map Units:
|
Qt |
Quaternary terrace deposits - variable response on resistivity and natural gamma - generally high to moderate resistivity (10-100 ohm meters) and low to moderate natural gamma counts (15-40 cps). |
|
Tsf |
Tertiary basin fill sediments, generally fine-grained - low resistivity (1-5 ohm-meters); low to moderate natural gamma counts (15-40 cps). |
|
Tsfg |
Tertiary basin fill sediments fine- to coarse-grained - moderate to high resistivity (10-200 ohm meters); moderate to high natural gamma (40-75 cps). |
|
Tsg |
Tertiary basin fill sediments, generally coarse grained - moderate to high resistivity (10-200 ohm-meters), low to moderate natural gamma (15-40 cps). |
|
Tb |
Tertiary basalt, moderate to high resistivity, (50-1000 ohm meters); low natural gamma, (10-20 cps); seismic velocity (ranges from 8000 to 13,000). |
|
Dm |
Devonian Martin Limestone - moderate to high resistivity (200-1000 ohm meters); low natural gamma, (10-20 cps); seismic velocity (ranges from 14,000 to 19,000 fps). |
A cluster plot shows lithologic separation and is displayed in figure 7. The clustering technique represents a multicomponent plot, where each component is represented by a log. If we represent the magnitude M of a multicomponent vector by
then we can define the sine of angle F by: (see also Figure 8)
This allows the mapping of n logs into a plane using magnitude and angle. Separation of log-derived rock types is generallyenhanced by using five or more logs.In the plot shown in Figure 7 the following logs were used:
The log interpretation proceeded by first arriving at a lithologic analysis using a general matrix solution. The resistivity, gamma ray, and neutron porosity logs were used for this approach. The result is shown in Figure 9. From left to right the components are as follows:
There is a gradual increase in sands and silt with depth until the top of the Devonian Martin limestone at about 1520 feet.
Some difficulties are generally experienced with the matrix approach in terms of quantifying effective porosities. Standard log evaluation equations were used to convert the neutron log to effective porosities by removing the clay volume (see Figure 10). The clay and non-clay volume percent is displayed to the right of the caliper log. The effective porosity log was subsequently converted to permeability using an experimental relationship which correlates permeability derived from injection pump tests to effective log porosity (Schimschal, 1981).
This relationship yields permeabilities in cubic meter per square meter per year and is of the form:
The result of this calculation was converted to millidarcy and is shown in Figure 11 in trace MDARCY.
A log-linear relationship was subsequently developed between resistivity and permeability in an effort to extrapolate permeability to the resistivity cross-section along the axis of the basin. This relationship is of the form
K = - 0.5 + 1.36 log Rt.
The result of this interpretation is shown in Figure 11. The granite basement in this figure is considered impermeable and has been darkened in. It can be seen that the permeabilities range from 0.2 millidarcy in the central section of the basin to about 3 millidarcy in the underlying bedrock sections. This tends to support that the basin is essentially impermeable with the bedrock showing some potential for minor fluid motion.
Summary
The overall exploration effort shows the advantages of borehole geophysical logs to calibrate the surface geophysical exploration. The geophysical data help to determine the structure of the basin, including the limits of basaltic aquifers. The basin fill is seen to provide a barrier to groundwater movement. Bedrock faulting, as indicated from seismic reflection data, further inhibits groundwater movement through the somewhat permeable bedrock. The geophysical data indicate that a shallow well field in the northern section of Chino Valley would have little impact on discharge into the Verde River if the conditions encountered in the center of the basin can be extrapolated laterally to the edges of the valley.
References
Anderson, P.L. , 1991, Analysis of gravity data from the Chino Valley area,Yavapai and Coconino Counties, Arizona: Flagstaff, Northern Arizona Univ., M.S. Thesis, 84 p.
Billingsley, G.H. , C.M. Conway, and L.S. Beard, 1988, Geologic map of the Prescott 30- x 60-minute quadrangle, Arizona: U.S. Geol. Surv. Open-File Report 88-372, XX p. plus map.
Cunion, E.J. , Jr. , 1985, Analysis of gravity data from the southeastern Chino Valley area, Yavapi County, Arizona: Flagstaff, Northern Arizona Univ., M.S. Thesis, 76 p.
Krieger, M.H., 1965, Geology of the rescott and Paulden quadrangles, Arizona: U.S. Geol. Surv. Prof. Paper 467, 127 p.
Ostenaa, D.A. , Schimschal, U., King, C. E. and Wright, J.W. , 1993, Big Chino Valley Groundwater Study-Geologic Framework Investigations: 2 vol., Geotechnical Engineering and Geology Division, Bureau of Reclamation, Denver, Colorado.
Schimschal, Ulrich, 1981, The relationship of geophysical measurements to hydraulic conductivity at the Brantley damsite, New Mexico: Geoexploration, v. 19, p. 115-125.
U.S. Bureau of Reclamation (USBR), 1974, Western United States Water Plan, State of Arizona, Chino Valley Unit, Appraisal Report: Bureau of Reclamation, 125 p.
Water Resources Associates, Inc. (WRA), 1989, Hydrogeology investigation, Big Chino Valley, Yavapai County, Arizona, Phase 1, Volumes I & II: consultants report for City of Prescott, City Attorney's Office, Prescott, Arizona, November 29, 1989, 2 volumes.
About the Author
Ulrich Schimschal holds a BS degree in geology from the University of Washington, an Ms degree in geophysics, and a PhD in geophysical engineering from the Colorado School of Mines. He spent 11 years with the petroleum industry, and 16 years with the U.S. Geological Survey and the U.S. Bureau of Reclamation. He is a registered geologist in the State of Wyoming. He has published numerous papers on surface and borehole geophysics and is the recipient of the best paper of the year award from the SPWLA. His research interests include seismic and acoustic anisotropy, electromagnetic modeling theory, and nuclear logging development.