Waterborne Electrical Resistivity of the Hypersaline ...

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Apr 17, 2017 - The volcanic chain is the youngest part of Long Valley. Volcanic Region (LVVR), with more than 30 eruptions in the last 40,000 years (Bursik ...
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Waterborne Electrical Resistivity of the Hypersaline Mono Lake Nigel Crook1, and Dale F. Rucker2* * Corresponding author: [email protected]

ABSTRACT A short, waterborne streamer resistivity survey was conducted on Mono Lake in California. The lake, located on the eastern side of the Sierra Nevada Mountains, contains a significant amount of dissolved solids due to being a terminal lake with no outlet. The survey was conducted to determine if the method could be used to define subbottom sediments and other geological features to answer questions related to recent-past volcanic and tectonic events. The survey used a 15-meter dipole-dipole array towed at approximately 2 mph behind a small flat bottomed boat. The survey was limited in scope to collecting approximately 6,000 m of data within the western cove, where substantial core sampling has taken place. Comparison of the methods indicates the towed array can obtain good quality data despite the lake’s conductivity being in excess of 84,000 uS/cm. The resistivity models reconstructed geological material upwards of 400 ohm-m to depths of 35 m, which likely represent hard rock below the lake. We conclude that the method could be used to map most of the shallow areas of the lake, where the water column is less than 20 m. The deepest areas of the lake, south of Paoha Island, would require streamer resistivity cables with an electrode spacing of at least 60 m.

Introduction The hypersaline Mono Lake in California is home to a unique set of both biota and geology. The lake contains bacteria that are known to respire selenium and arsenic (Blum et al., 1998). Invertebrates in the lake include brine shrimp, Artemia monica (Dana et al., 1990), and alkali flies, Ephydra hians (Herbst, 1990), which feed on abundant phytoplankton in the lake (Jellison and Melack, 1993); migratory birds rely on these invertebrates for food. Geologically, the lake lies at the north end of the Mono-Inyo Craters volcanic chain, which has been active through at least the last glacial cycle (Zimmerman et al., 2006; Colman et al., 2014). The volcanic chain is the youngest part of Long Valley Volcanic Region (LVVR), with more than 30 eruptions in the last 40,000 years (Bursik and Sieh, 1989; Peacock et al., 2015). Many geophysical investigations have been conducted in and around the lake, and within the LVVR as a whole. These studies were undertaken to better define the lakebed geology and volcanic origins of the area. For example, Peacock et al. (2015, 2016) used the magnetotelluric method to map the electrical structure of the LVVR as it relates to magmatic features to depths that 1 2

Hydrogeophysics, Inc., Irvine, CA Hydrogeophysics, Inc., Tucson, AZ

exceed 30 km. Within Mono Lake itself, magnetic, gravity, and seismic methods have been deployed to map sediment thickness at the bottom lake, many of these surveys are described in Jayko et al. (2013). Raumann et al. (2002) completed a bathymetric survey of the lake bottom. Colman et al. (2014) completed a seismic reflection survey by collecting 150 km of survey lines. Their work incorporated a compressed high-intensity radar (CHIRP) system operating at frequencies of 4–20 kHz, which allowed them to image up to 50 meters (m) below the water level. One geophysical method that is absent from the literature for geophysical methods applied to Mono Lake was DC electrical resistivity. Waterborne DC electrical resistivity is a common method to map electrical structure of subbottom geology (e.g., Rucker and Noonan, 2013; Xu and Dunbar, 2015). However, applying it in saline to hypersaline water bodies can be difficult due to the electrically conductive nature of the water. Goto et al. (2008) applied DC resistivity successfully in the Japan Sea (q ¼ 0.35 ohm-m), where the cable was towed 5m above the sea floor. Passaro (2010) used electrical resistivity in the Tyrrhenian Sea (q ¼ 0.25 ohm-m) to map archeological targets. In this work, we are applying waterborne DC electrical resistivity in a hypersaline water body with q ¼ 0.25 ohm-m to map subbottom geology as it relates to

JEEG, Month 2017, Volume 22, Issue 2, pp. 193–198

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DOI: #10.2113/JEEG22.2.191

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Figure 1. Mono Lake geology in eastern California (adapted from Bailey, 1989). Dashed lines mark known or suspected faults. The inset map defines the area for geophysical study. superficial sediment accumulation and deeper hard rock. The work was a feasibility study to determine whether the resistivity method could be used to acquire data in a highly conductive environment. Setting Mono Lake (Fig. 1), located on the eastern side of the Sierra Nevada Mountains, contains a significant amount of dissolved solids due to being a terminal lake with no outlet. Several surface water sources feed the lake from the mountain drainage to the west. These sources have been diverted over the years to supply the city of Los Angeles, thus the lake is artificially low and has seen dramatic increases in salinity since the 1940’s. Currently, the lake level is at an approximate elevation of 1,946 m, but has been shown to be as high as 2,155 m during glacial periods of the Pleistocene (Zimmerman et al., 2011). Most of the exposed geology around Mono Lake consists of Quaternary lacustrine and glacial deposits of the Holocene and Pleistocene epochs, as part of the Wilson Creek Formation (Zimmerman et al., 2006, 2011). Material from the Wilson Creek Formation served as the basal geological layer in the seismic interpretation of Colman et al. (2014). Much of Paoha

Island is composed of lake sediments that were uplifted from the lake bottom, probably by intrusion of a rhyolite dome. Uplift caused slumping and sliding of sediments away from the center of the island, further causing sediments to build on the lake’s floor from landsides. Newer material found in the lake, as collected from coring on the lake’s margins, is composed of interbedded lake sediments and volcanic tephras formed during a sequence of local eruptions that deposited pumice and tephra into the lake over the last 2,000 years (Colman et al., 2014). Relatively older volcanic material can be seen at the margin of the lake’s basin, contributed by eruptions occurring at the northern end of the Mono-Inyo chain, south of Mono Lake. In Mono Lake, several small dacite cinder cones and flows form Negit Island and the northeast corner of Paoha Island (Bailey, 1989). Furthermore, a partly submerged rhyolite flow forms a small group of islets northeast of Negit (Bailey, 1989). Methodology A continuous-profiling, waterborne DC electrical resistivity survey was conducted on the western shore of the Mono Lake; Fig. 1 shows an inset map area that highlights the location of the survey area. The survey

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Figure 2. Inset map from Fig. 1 showing tracks taken along the western shore of Mono Lake for the electrical resistivity survey. Lake bathymetry was acquired online from http://pubs.usgs.gov/mf/2002/2393/monobathy.zip. The mapped faults are from Bailey (1989) and the projected fault was derived by the authors. was collected using a 168-meter marine resistivity cable with 15 m dipole separation of electrodes. Two transmitter electrodes constructed from graphite accompanied nine receiver electrodes constructed from stainless steel. A modified SuperSting R8 (Advanced Geosciences, Inc., Austin, TX) controlled power transmission and data acquisition of voltage on the eight receiver dipoles. The SuperSting is a fairly popular resistivity meter for use in environmental and engineering surveys (e.g., Minsley et al., 2011; Dunbar et al., 2015; Johnson et al., 2015; Xu and Dunbar, 2015; Swarzenski et al., 2016; Woodbury et al., 2016). The cable was towed at the lake’s surface at a rate of 0.9 m/s (2 mph) and an average output current of 2040 mA. Figure 2 shows the two GPS tracks taken by the boat for data acquisition, recorded with a marine-grade Lowrance GPS. Due to the uncertainty in data quality and imaging depths, the tracks were kept near the shore in shallow water. The first track cruised by the tufa-

dominated southern shoreline of the inlet for the first 1,200 m, before heading north near material identified by the geological map as lake terrace deposits. The second track continued north to image younger alluvial material. The survey was halted on Track 2 when the cable became entangled in rocky tufa formations in the middle of the lake. The resistivity data were then processed to remove spurious data and inverse modeled with RES2DINVx64, bathymetry from Raumann et al. (2002) and a lake water resistivity of 0.12 ohm-m were incorporated into the inverse model. Results The modeled resistivity results are shown in Fig. 3. The models for both tracks converged within five iterations, with an RMS less than 7%. The noisiest data came from the two receiver dipoles furthest from the transmitting dipoles at n ¼ 7 and 8. The resistivity data

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Figure 3. Resistivity results along Tracks 1 and 2. The solid white line indicates the lake bottom surface used in the resistivity model and the water column resistivity was fixed in the inverse model based on the measured conductivity value. show remarkable detail that can likely be attributed to late Holocene deposition or earlier competent volcanics. For the first 1,200 m of Track 1, along the southern shoreline of the inlet survey area, the upper subbottom materials are fairly resistive for the first 600 m before transitioning to conductive material. We interpret this as a thickening of the sediments atop deeper tufa or other bedrock, similar to the results reported by Colman et al. (2014). The deepest portion along this first section shows data becoming highly resistive. The next section from about 1,400 m to 3,100 m shows a thicker sequence of these conductive sediments, as the basement bedrock dives below our depth of investigation. Coincidentally, at about 1,800 m, there is a break in the contours that fits with an interpretation of an extension of the fault mapped by Bailey (1989); the projected fault is indicated in Fig. 2. At 3,100 m, the resistive bedrock shows another sharp break in the contours, which could indicate another fault. The beginning of Track 2 is approximately 250 m beyond the end of Track 1 and the resistivity indicates a more conductive bedrock than that at the end of Track 1. This could be from a modeling artifact, with the resistive material between 0 to 400 m an extension of the resistive feature at the end of Track 1. From the geological map, it is possible that the competent resistive bedrock is

granodiorite. Towards the end of Track 2 the survey traverses more tufa, with the model results showing similar thinning of conductive sediments as at the beginning of Track 1. One last feature to the resistivity data is the variability in the values attributed to the lake water after modeling. There is a correlation between the lake bed’s depth and conductivity. The resistivity ranges from about 0.125 to 0.35 ohm-m, and could reflect some interesting limnological phenomena outside of our knowledge area. Given the successes of this feasibility study to acquire electrical resistivity in a hypersaline environment, we recommend that more waterborne DC electrical resistivity be acquired to help define sediment thickness, bedrock topography, and structural features that could enhance the understanding of this unique environment. The electrode spacing of 15 m allowed us to image a water column and subbottom material thickness of approximately 35 m, and likely could be used in water column thicknesses of up to 20 m. The deepest portion of the lake is approximately 50 m, south of Paoha Island. Here, Colman et al. (2014) describe mudflows up to 18 m thick and it is likely that a different cable, with electrodes spaced 60 m apart, would be needed to successfully map below the mud.

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197 Conclusions A waterborne DC electrical resistivity survey was completed in Mono Lake, CA. The survey was conducted as a feasibility study to determine whether the method could collect data of sufficient quality to help refine the geological understanding of the lake’s bottom. This work showed that data quality from a waterborne resistivity survey was sufficient to produce coherent models that could be attributed to geological features described by existing maps. We interpreted conductive layers beneath the water column as sediments that have accumulated since the late Holocene. Deeper resistive bodies could be related to older intrusive igneous rocks. We determined that the survey was a success and recommend that additional electrical resistivity data be acquired to accompany other large seismic, magnetic, and gravimetric data sets. Acknowledgements This work was originally presented at SAGEEP 2017 and subsequently reformatted for submission to the Journal of Environmental and Engineering Geophysics as a Near Surface Geophysical Letter.

References Bailey, R.A., 1989, Geologic map of Long Valley caldera. MonoInyo Craters volcanic chain, and vicinity, eastern California: US Geological Survey Miscellaneous Investigations Map I-1933, scale, 1(62,500), p.11. Blum, J.S., Bindi, A.B., Buzzelli, J., Stolz, J.F., and Oremland, R.S., 1998, Bacillus arsenicoselenatis, sp. nov., and Bacillus selenitireducens, sp. nov.: two haloalkaliphiles from Mono Lake, California that respire oxyanions of selenium and arsenic: Archives of microbiology, 171, 19– 30. Colman, S.M., Hemming, S.R., Stine, S., and Zimmerman, S.R.H., 2014, The effects of recent uplift and volcanism on deposition in Mono Lake, California, from seismicreflection (CHIRP) profiles: Journal of Geophysical Research: Solid Earth, 119, 3,955–3,970. Dana, G.L., Jellison, R., and Melack, J.M., 1990, Artemia monica cyst production and recruitment in Mono Lake, California, USA. In Saline Lakes. Springer Netherlands. pp. 233–243. Dunbar, J., Allen, P., White, J., Neupane, R., Xu, T., Wolfe, J., and Arnold, J., 2015, Characterizing a shallow groundwater system beneath irrigated iugarcane with electrical resistivity and radon (222Rn), Puunene, Hawaii: Journal of Environmental and Engineering Geophysics, 20, 165–181. Goto, T.N., Kasaya, T., Machiyama, H., Takagi, R., Matsumoto, R., Okuda, Y., Satoh, M., Watanabe, T., Seama, N., Mikada, H., and Sanada, Y., 2008, A marine deep-towed

DC resistivity survey in a methane hydrate area, Japan Sea: Exploration Geophysics, 39, 52–59. Herbst, D.B., 1990, Distribution and abundance of the alkali fly (Ephydra hians) say at Mono Lake, California (USA) in relation to physical habitat: Hydrobiologia, 197, 193–205. Jayko, A.S., Hart, P.E., Childs, J.R., Cormier, M.H., Ponce, D.A., Athens, N.D., and McClain, J.S., 2013, Methods and spatial extent of geophysical Investigations, Mono Lake, California, 2009 to 2011 (No. 2013-1113). US Geological Survey. Jellison, R., and Melack, J.M., 1993, Algal photosynthetic activity and its response to meromixis in hypersaline Mono Lake, California: Limnology and Oceanography, 38, 818–837. Johnson, C.D., Swarzenski, P.W., Richardson, C.M., Smith, C.G., Kroeger, K.D., and Ganguli, P.M., 2015, Ground-truthing electrical resistivity methods in support of submarine groundwater discharge studies: Examples from Hawaii, Washington, and California: Journal of Environmental and Engineering Geophysics, 20, 81–87. Minsley, B.J., Burton, B.L., Ikard, S., and Powers, M.H., 2011, Hydrogeophysical investigations at Hidden Dam, Raymond, California: Journal of Environmental & Engineering Geophysics, 16, 145–164. Passaro, S., 2010, Marine electrical resistivity tomography for shipwreck detection in very shallow water: A case study from Agropoli (Salerno, Southern Italy). Journal of Archaeological Science, 37, 1,989–1,998. Peacock, J.R., Mangan, M.T., McPhee, D., and Ponce, D.A., 2015, Imaging the magmatic system of Mono Basin, California, with magnetotellurics in three dimensions: Journal of Geophysical Research: Solid Earth, 120, 7,273–7,289. Peacock, J.R., Mangan, M.T., McPhee, D., and Wannamaker, P.E., 2016, Three-dimensional electrical resistivity model of the hydrothermal system in Long Valley Caldera, California, from magnetotellurics: Geophysical Research Letters, 43, 7,953–7,962. Raumann, C.G., Stine, S., Evans, A., and Wilson, J., 2002, Digital bathymetric model of Mono Lake, California: US Geological Survey, Miscellaneous Field Studies Map MF-2393. Rucker, D.F., and Noonan, G.E., 2013, Using marine resistivity to map geotechnical properties: A case study in support of dredging the Panama Canal: Near Surface Geophysics, 11, 625–637. Swarzenski, P.W., Johnson, C.D., Lorenson, T.D., Conaway, C.H., Gibbs, A.E., Erikson, L.H., Richmond, B.M., and Waldrop, M.P., 2016, Seasonal electrical resistivity surveys of a coastal bluff, Barter Island, North Slope Alaska: Journal of Environmental and Engineering Geophysics, 21, 37–42. Woodbury, B.L., Eigenberg, R.A., and Franz, T.E., 2016, Development of non-collinear arrays for use near wastewater holding ponds: Journal of Environmental and Engineering Geophysics, 21, 231–236. Xu, T., and Dunbar, J.A., 2015, Binning method for mapping irregularly distributed continuous resistivity profiling data onto a regular grid for 3-D inversion: Journal of Environmental and Engineering Geophysics, 20, 1–17.

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198 Journal of Environmental and Engineering Geophysics Zimmerman, S.H., Hemming, S.R., Kent, D.V., and Searle, S.Y., 2006, Revised chronology for late Pleistocene Mono Lake sediments based on paleointensity correlation to the global reference curve: Earth and Planetary Science Letters, 252, 94–106.

Zimmerman, S.R., Hemming, S.R., Hemming, N.G., Tomascak, P.B., and Pearl, C., 2011, High-resolution chemostratigraphic record of late Pleistocene lake-level variability, Mono Lake, California: Geological Society of America Bulletin, 123, 2,320–2,334.

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