CSAMT Geophysical Survey K2 Groundwater Project

Transcription

1 CSAMT Geophysical Survey K2 Groundwater Project Strawberry, Arizona Prepared for: Pine Water Company January 30, 2008 by Zonge Engineering & Research Organization, Inc E Fort Lowell Rd. Tucson, Arizona, USA Phone: FAX: Zonge Engineering Strawberry K2 Site, p. 1

2 TABLE OF CONTENTS Survey Results Page 3 Project Logistics Page 9 Survey Summary Page 9 Field Instrumentation Page 9 Cultural Contamination Page 10 Data Quality Page 10 Data Presentation Page 11 Smooth-model Inversion Page 11 Sounding Plots Equipment Specifications CSAMT Background and References Appendix A Appendix B Appendix C List of Plates and Figures General Map Figure 1 Transmitter Location Map Figure 2 Pseudo-cross section plot of CSAMT data Figure 3 Zonge Engineering Strawberry K2 Site, p. 2

3 CSAMT Geophysical Survey Strawberry K2 Site Project for Pine Water Company by Zonge Engineering & Research Organization, Inc. Survey Results On November 10 and November 15, 2007, Zonge Engineering and Research Organization, Inc. acquired Controlled Source Audio-frequency Magnetotellurics (CSAMT) geophysical data on the K2 Groundwater Project near Strawberry, Arizona, at the request of John Breninger of the Pine Water Company. Mark Reed of Zonge Engineering was the field crew chief on site. The goals of the survey included determining, if possible, the likelihood and depth of water in the deep aquifer (known as the R aquifer) and the presence or absence of water in shallow zones (primarily for planning purposes in the drilling activities). As originally planned, the survey would have included several lines of data through and near the K2 site to provide cross sections of resistivity through the site. Permission to cross property outside the K2 site could not be obtained, however, and as a result, the survey activities were restricted to the K2 parcel itself. Data were acquired at a single station south of the storage tank on the K2 parcel, one station north of the storage tank, one station west of the tank, and two stations east and southeast of the tank. This small data set limits the ability to interpret lateral changes across the site (fractures, faults, perched zones), as well as limiting the ability to evaluate the effects of cultural contamination on the data (man-made features such as power lines, fences, and utilities). Zonge Engineering Strawberry K2 Site, p. 3

4 Figure 1 shows the general location of the survey site in Strawberry relative to the Mogollon Rim and nearby Pine, Arizona, and Figure 2 shows the location of the two transmitter dipoles used for this survey. Figure 3 shows a pseudo-cross section of results from the two stations that provided the most realistic modeling results. The two stations acquired using Transmitter # 1 provided moderately good data; the resistivity ranges and changes with depth from these two stations are in good agreement with the downhole geophysical resistivity log from the Strawberry Borehole (located approximately 1.5 miles west of the K2 site) and with prior CSAMT surveys by Zonge in the general area of Strawberry/Pine. The three stations acquired using Transmitter # 2 were noisier, and resulted in models with unrealistically high and low resistivities relative to the Strawberry Borehole and prior Zonge results. This is most likely the result of cultural noise that affected the north-south oriented readings more strongly than the east-west oriented stations. Our interpretation is based on the measurements made using Transmitter # 1; the model results for these stations are shown in Figure 3. Assuming that cultural affects are not influencing the final processed data, these results suggest that the geologic environment at the K2 site is consistent with expectations. For example, there are no anomalously high resistivity values at the target depths that might suggest tight, crystalline rock that would be non-productive. Although it is not possible from this limited data set to estimate a depth to water, our interpretation is that the R aquifer at the K2 site is not expected to be any less productive than at any other randomly selected site in the Strawberry/Pine area. It is possible that the differences in resistivity below approximately 700 feet between the stations north and south of the storage tank indicate fracturing, which would make the K2 site more attractive with respect to groundwater production. This should be considered tentative, since it assumes that cultural effects are approximately the same on the two stations. It should be noted that a lineament map (generated by Mike Plough) included in Pine Water s RFP for this project suggests a W-SW to E-NE trending lineament through this area which may be correlated to fracturing. In addition, mapping by Clay Conway (also provided by Pine Water) suggests that multiple faults may cross the general area of the K2 site. The independent interpretation of lineaments and faults, combined with the changes seen between the southern and northern station suggests the possibility that the K2 site may indeed be more fractured than is typical for the general area. Zonge Engineering Strawberry K2 Site, p. 4

5 With respect to shallow perched water, the survey results suggest that a zone centered at a depth of approximately 400 feet may be fractured and/or saturated below the southern station, but this zone is not evident on the northern station. If cultural effects are influencing the two stations similarly, this suggests that the low resistivity zone is relatively small, or at least not extensive across the site. It is very important to note that the interpretation of the K2 site as an average or better-thanaverage drill site for R aquifer production, with a possible shallow perched or fracture zone centered at about 400 feet, is tentative due to the limited number of stations and the cultural noise in the area. Normally, multiple stations, usually in a line, provide a way to evaluate the repeatability of data and the effects of cultural features, since these effects vary with orientation and distance relative to the culture. The individual components of the data have been evaluated on a block-by-block basis, the data are admittedly limited and noisy, but given the data as it is, the K2 site appears to be an average or better-than-average location for deep R aquifer groundwater production. Zonge Engineering Strawberry K2 Site, p. 5

6 Survey Area Figure 1: General location of K2 survey area in Strawberry, Arizona. Zonge Engineering Strawberry K2 Site, p. 6

7 Figure 2: Location map showing Transmitters 1 and 2 relative to the survey site. Zonge Engineering Strawberry K2 Site, p. 7

8 Figure 3: Pseudo-cross section plot of CSAMT data from the north transmitter site. Zonge Engineering Strawberry K2 Site, p. 8

9 PROJECT LOGISTICS Survey Summary: The geophysical survey was designed to map subsurface changes in resistivity, which can be related to changes in pore space and pore fluids. Bedrock is often high resistivity relative to overlying material, and fractured, saturated bedrock is often lower resistivity than un-fractured bedrock. Areas of high TDS in the groundwater should appear more conductive than equivalent areas of low TDS. Variations in depth to bedrock, faulting, and other structural changes are often also evident as changes in resistivity. The geophysical method used was controlled source audio-frequency magnetotellurics (CSAMT). CSAMT is a resistivity sounding method used commonly in the minerals, geothermal, and groundwater exploration industries. This method typically has higher lateral resolution than other resistivity methods, and is usually logistically more efficient. The CSAMT data were acquired on two days in November of The CSAMT stations were acquired using an electric-field receiver dipole size varying from 80 to120 feet (due to property restrictions), and the transmitted frequencies were in binary increments from 4 Hz up to 8192 Hz (i.e., 4Hz, 8 Hz, 16 Hz, etc.). For each electric-field measurement, a magnetic field measurement was made simultaneously at the center of the dipole. Hand-held GPS measurements for the station locations are as follows (in NAD 83 UTM feet, Zone 12): Sounding Easting Northing K K K K K These locations should be considered approximate, since the GPS locations were acquired using a hand-held GPS unit, and vegetative cover, structures, and topography most likely affected satellite reception. Field Instrumentation: The receiver used for the CSAMT survey was a Zonge GDP-32 multipurpose receiver. This receiver is a backpack-portable, 16-bit, microprocessor-controlled Zonge Engineering Strawberry K2 Site, p. 9

10 receiver capable of gathering data on as many as 16 channels simultaneously. The electricfield signals were sensed using non-polarizable porous pot electrodes, connected to the receiver with 16-gauge insulated wire. The CSAMT magnetic-field signal was sensed with a Zonge Ant/1B magnetic field antenna. The signal source for the survey was a Zonge GGT-30 transmitter, which is a current-controlled transmitter capable of 30 kw output. The transmitter was controlled with an XMT-32 transmitter controller, which contains a quartz oscillator identical to the one in the receiver. Each morning prior to data acquisition, the two oscillators were trimmed and synchronized in order to allow the crew to acquire accurate phase data. For each CSAMT receiver setup, the transmitter generates a square-wave signal at discrete frequencies from 4 hertz to 8192 hertz in binary increments (i.e.,4 Hz, 8 Hz, 16 Hz, etc.). Electric and magnetic field values were recorded at each of these primary frequencies, as well as the odd harmonics for all frequencies up to 1024 Hz. Results from the electric and magnetic field measurements are then used to calculate resistivity and impedance phase values at each measured frequency, from which depth vs. resistivity sections can be generated. At all frequencies, the receiver recorded the received electric field and magnetic field magnitude and phase components, as well as the data at the odd harmonics (3 rd, 5 th, 7 th, and 9 th ) of the transmitted frequencies, providing a very large data set for modeling purposes. The 1 st and 3 rd harmonics were used for modeling purposes. Cultural Contamination: Cultural contamination refers to any man-made electrically conductive or electrically noisy objects that may influence the geophysical measurements. These include passive objects like metal fences, pipelines, power lines, or large metal structures, and active (electrically) objects such as active power lines, pipelines with cathodic protection, and radio transmitters. The K2 site is in a residential area and has numerous sources of electrical noise, including power lines, fences, and pipelines, as well as suspected buried utilities that were not obvious to the field crew. Data Quality: Data quality was only poor to fair on this project, due to the cultural noise discussed above. Standard Zonge field procedure requires that the receiver operator make multiple measurements of each data point while monitoring real-time standard-error values displayed on the screen of the receiver. For CSAMT, multiple blocks of the data are also displayed graphically as resistivity-versus-frequency curves (plotted on a log-log scale), with Zonge Engineering Strawberry K2 Site, p. 10

11 error bars denoting data scatter for the operator in the field. The data quality for these five sets of measurements is considered poor (at Station 5 for example) to fair (at Station 1). Data Presentation: The results of processing and modeling the data are shown as a color pseudo-cross section for stations 1 and 2 (south and north of the storage tank, respectively), which were acquired from the northern transmitter. In this plot, decreasing elevation is shown down the side of the plot. Resistivity results are shown in ohm-meters, with warm colors (orange, red) indicating low resistivity and cool colors (green, blue) indicating high resistivity. The color shading and contouring is on a logarithmic scale. Raw data and sounding curves for each station are also included in the appendix of this report. Smooth-Model Inversion: Briefly, smooth-model inversion mathematically back-calculates (or inverts ) from the measured data to determine a likely location, size and depth of the source or sources of resistivity changes. The results of the smooth-model inversion are intentionally gradational, rather than showing abrupt, blocky changes in the subsurface. For the CSAMT data, a 1D smooth-model inversion program was used for modeling this data due to the inability to acquire multiple stations in along a line. This program is a robust method for converting CSAMT measurements to profiles of resistivity versus depth. Cagniard apparent resistivities and impedance-phase data for each station are used to determine the parameters of a layered earth model. Layer thicknesses are fixed by calculating source-field penetration depths for each frequency. Layer resistivities are then adjusted iteratively until the model CSAMT response is as close as possible to the observed data. The algorithm for calculating the CSAMT response of a layered model includes the effects of finite transmitter-receiver separation and a three-dimensional source field. Accurate impedance magnitude and phase values are calculated for all frequencies and transmitter-receiver separations for the 1-D models. The result of the smooth-model inversion is a set of estimated resistivities which vary smoothly with depth, giving the gradational result seen in the color data plots in this report. The smooth-model inversion does not require any a priori estimates of model parameters, thus the results are unaffected by any data processor's bias. Zonge Engineering Strawberry K2 Site, p. 11

12 Norman R. Carlson Chief Geophysicist Cris Mayerle Geophysicist Zonge Engineering & Research Organization, Inc E. Fort Lowell Rd. Tucson. Arizona, USA Zonge Engineering Strawberry K2 Site, p. 12

13 Appendix A Sounding Plots Zonge Engineering Strawberry K2 Site, p. 13

14 Apparent Resistivity (ohm-m) Frequency (hertz) Model Resistivity (ohm-m) Model Depth (ft) Strawberry Line K2 1, Station 1 Scalar CSAMT data from K21.scs dxweight: 1.00 dzweight: 2.00 Residual: 2.70 Zonge Engineering Smooth-Model CSAMT Inversion Plotted at 12:05:01, 15/01/08 Zonge Engineering Strawberry K2 Site, p. 14

15 Apparent Resistivity (ohm-m) Frequency (hertz) Model Resistivity (ohm-m) Model Depth (ft) Strawbeery Line K2 2, Station 1 Scalar CSAMT data from K22.scs dxweight: 1.00 dzweight: 2.00 Residual: 3.65 Zonge Engineering Smooth-Model CSAMT Inversion Plotted at 12:06:17, 15/01/08 Zonge Engineering Strawberry K2 Site, p. 15

16 Apparent Resistivity (ohm-m) Zonge Engineering Frequency (hertz) Model Resistivity (ohm-m) Model Depth (ft) Strawbeery Line K2 3 N, Station 45 Scalar CSAMT data from K23.scs dxweight: 1.00 dzweight: 2.00 Residual: 2.91 Smooth-Model CSAMT Inversion Plotted at 12:06:34, 15/01/08 Zonge Engineering Strawberry K2 Site, p. 16

17 Apparent Resistivity (ohm-m) Zonge Engineering Frequency (hertz) Model Resistivity (ohm-m) Model Depth (ft) Strawbeery Line K2 4 N, Station 45 Scalar CSAMT data from K24.scs dxweight: 1.00 dzweight: 2.00 Residual: 2.33 Smooth-Model CSAMT Inversion Plotted at 12:07:03, 15/01/08 Zonge Engineering Strawberry K2 Site, p. 17

18 Apparent Resistivity (ohm-m) Zonge Engineering Frequency (hertz) Model Resistivity (ohm-m) Model Depth (ft) Strawbeery Line K2 5 N, Station 45 Scalar CSAMT data from K25.scs dxweight: 1.00 dzweight: 2.00 Residual: 9.94 Smooth-Model CSAMT Inversion Plotted at 12:07:24, 15/01/08 Zonge Engineering Strawberry K2 Site, p. 18

19 Appendix B Equipment Specifications Zonge Engineering Strawberry K2 Site, p. 19

20 Zonge Engineering Strawberry K2 Site, p. 20

21 Zonge Engineering Strawberry K2 Site, p. 21

22 Appendix C CSAMT is a commonly-used, surface-based geophysical method which provides resistivity information of the subsurface, usually at greater depths and better lateral resolution than other resistivity methods such as Schlumberger soundings, dipole-dipole, or gradient arrays. CSAMT has been used extensively by the minerals, geothermal, hydrocarbon, and groundwater exploration industries since 1978 when CSAMT equipment systems first became commercially available. CSAMT Methodology: Controlled source audio-frequency magnetotellurics (CSAMT) is a highresolution electromagnetic sounding technique that uses a fixed grounded dipole as a signal source. For complete, published, peer-reviewed discussions of the CSAMT method and its common applications, see the Zonge and Hughes (1991) and Zonge (1992) references. Briefly, the CSAMT method can be described as follows: A CSAMT transmitter signal source usually consists of a grounded electric dipole one to two km in length, located four to ten km from the area where the measurements are to be made (see Figure App-1). Figure App-1: General field lay-out of a scalar CSAMT survey. Zonge Engineering Strawberry K2 Site, p. 22

23 At the receiver site, grounded dipoles detect the electric field parallel to the transmitter and magnetic coil antennas sense the perpendicular magnetic field. The ratio of orthogonal, horizontal electric and magnetic field magnitudes (e.g. Ex and Hy) yields the apparent resistivity: ρ a = 1 5 f where f is frequency of the measurement, Ex is the electric field along the observation line, and Hy is the magnetic field perpendicular to the line. E H x y The difference between the phase of the electric and magnetic fields yields the impedance phase, which we will often just call the phase or phase difference: ϕ = ϕ ( E ) ϕ( ) x H y Varying the frequency of the observations controls the depth of investigation using the CSAMT method. A concept used extensively in electromagnetic geophysics is the skin depth, which is the depth at which the amplitude of the field decays to 37 percent of the original value. The skin depth,, is given by the equation: where ρ a = 503 meters; f ρ a = apparent (measured) resistivity, and f = signal frequency. An estimate of the total depth of investigation is given by D, which is: D = δ or ρ a f meters. Therefore, depth sections can be generated using the CSAMT method by measuring the electric and magnetic fields over a range of frequencies. The ratio of the measured electric and magnetic fields provides information about the resistivity at depth and by making measurements at lower frequencies a greater depth of penetration can be attained. Zonge Engineering Strawberry K2 Site, p. 23

24 CSAMT Reference Material Cagniard, L. 1953, Basic Theory of the magnetotelluric method of geophysical prospecting, Geophysics, 18, pp Goldstein, M.A., and Strangway, D.W., 1975, Audio-frequency magnetotellurics with a grounded electric dipole source: Geophysics, 40, Hughes, L.J., and Carlson, N.R., 1987, Structure mapping at Trap Spring Oilfield, Nevada, using controlled-source magnetotellurics, First Break, Vol. 5, No. 11, the European Association of Exploration Geophysicists, pp Zonge, K.L., Ostrander, A.O., and Emer, D.F., 1985, Controlled-source Audio-Frequency Magnetotelluric Measurements, in Magnetotelluric Methods, ed. Vozoff, K., Geophysics Reprint Series No. 5, Society of Exploration Geophysicists, pp Zonge, K.L. and Hughes, L.J., 1991, Controlled source audio-frequency magnetotellurics, in Electromagnetic Methods in Applied Geophysics, ed. Nabighian, M.N., Vol. 2, Society of Exploration Geophysicists, pp Zonge, K. L., 1992, Broad Band Electromagnetic Systems, in Practical Geophysics II for the Exploration Geologist, ed. Richard Van Blaricom, Northwest Mining Association, pp Zonge Engineering Strawberry K2 Site, p. 24