Number: NS23A-1645 Mapping the edge of the Cerros del Rio volcanic field, : a piece of the puzzle to understanding a potential geothermal resource

Louise Pellerin, Green Engineering, Berkeley, CA, [email protected]; Madison Gallegos, DePauw University, IN, [email protected]; Meredith Goebel, University of California, Berkeley, CA, [email protected]; Ben Murphy, Pomona College, CA, [email protected]; John Smith, Brigham Young University, UT, [email protected]; Daniela Soto, University of Santiago, Chile, [email protected]@gmail.com; Jerlyn Swiatlowski, California State University, East Bay, CA, [email protected]; Christopher Volk, University of Utah, UT, [email protected]; Mark Welch, Cornell University, NY, [email protected];Danny Feucht, University of Colorado, Boulder, CO, [email protected]; Becky Hollingshaus, Colorado School of Mines, CO, [email protected]; Paul A. Bedrosian, U.S. Geological Survey, Denver, CO, [email protected] and Darcy K. McPhee, U.S. Geological Survey, Menlo Park, CA, [email protected]

0.4 Summer of Applied Geophysical Experience - SAGE Audiomagnetotelluric - AMT 50 1D Sensitivity Analysis 0.35 http://www.sage.lanl.gov/ 45 AMT data were acquired at eight stations, at 400-500 m spacing, using the Geometric Stratagem system recording from 1D sensitivity analysis allows con- A four-week-long, field-based program in exploration geophysics where 25-30 92 kHz to 10 Hz. Data were sorted with 0.9, 0.7 and 0.5 coherencies; 0.5 was found to produce the highest quality appar- 40 0.3 straints to be put on the depths and resistivities of layers. The depths to students study field problems that include mapping archaeological sites, and ent resistivity and phase curves, which were used in the inversion. 35 RMS studying subsurface structure, water and geothermal resources of the Rio 0.25 the top and bottom of the resistive Grande Rift near Santa Fe, New Mexico (ohm-m) Resistivity 30 layers are accurate to within about AMT 2D model 0.2 25 10 meters. The resistivities in the WESAGE data for the top and bottom 2W 20 1W 600 0.15 2000 0 150200 250 300 350 400 450 500 layers are reasonably well con- 1E 2E 3E Conductor Thickness (m) strained to within about 20 and 2 4E 5E 6500 ohm-m, respectively. 0.55 6000 reverse polarity flow ? 5500 0.5 Resistivity (Ohm-m) 1900 5000 0.45 1 10 100 1000 10000 300 1 Ancha Formation sediments 4500 0.4 4000 ρ Elevation (m) Elevation RMS 1 T 0.35 1 normal polarity flow 3500 10 0.3 1800 (ohm-m) Resistivity 3000

resistivity (ohm-m)resistivity 2500 0.25 ρ T 2 2

2000 (m) Depth 5 0.2 100 1500 ρ 0 0.5 1.0 1.52.0 2.5 3.0 3.5 4.0 4.5 5.0 0.15 3 10 15 20 25 30 35 40 45 Distance along profile (km) Resistor Thickness (m) 1000 Background & Motivation The Cerros del Rio volcanic field located west of Santa Fe, New Mexico, spans the southwestern part of the Espanola Basin with the to the west. Underlying the volcanics are Transient Electromagnetics - TEM the sediments that contain the Ancha Forma- tion, an important aquifer in the region. High temperature gradi- TEM quasi-2D Model TEM data acquired at ten stations with 200-400 m spac- ents in water wells reveal a potential geothermal prospect. W E ing along a ~5 km transect. We employed an in-loop 1W configuration with a square 100 m x 100 m transmitter In 2012 SAGE acquired transient electromagnetic (TEM), 0 1E 2E 2000 4E loop and both a Zonge receiver coil and a 5-m square audiomagnetotelluric (AMT), gravity and ground magnetic data 3E 5E 6E 8E 10E receiver loop. The 5-m loop allowed for the recovery of to determine the buried eastern margin of the volcanic field and 300 early-time data that was saturated when using the coil, the connectivity related to the underlying sediments. The rough- reverse polarity flow while the coil permitted greater depth of penetration. ly EW 5-km long transect was sited from USGS aeromagnetic 1900 100 Twenty layer-earth (or 1D) models were fit to the data at data to cross the boundary of the Cerros del Rio volcanic field. each station and stitched together along the profile to Ancha Formation sediments Elevation (m) Elevation create a pseudo 2D cross-section. The model indicates 30 normal polarity flow the resistive volcanics are thickest in the west and thin 1800 Gravity & Magnetics towards the east, with an upper and lower resistive unit

The 2D gravity forward model is based on the AMT model. Aeromagnetic data (Grauch et al., 2009) were used to site the EM pro- (ohm-m)resistivity 1 in the West separated by a thin conductor. Possible file. About 3500 m of ground magnetic data were acquired with a Geometrics Cs vapor G-858 magnetometer along the EM profile 0 0.5 1.0 1.52.0 2.5 3.0 3.5 4.0 4.5 5.0 explanations for the observed structure include multiple at a 0.5 second sample rate during SAGE 2012.Note the center region of the magnetic field strength plot is generally higher than Distance along profile (km) volcanic flows, fault control, and the presence of water. the overall trend of the data; this region corresponds to the lower volcanic flow in the AMT data. The magnetic data suggest that the flow is normally magnetized. Ground Data (Low Pass) Aeromagnetic Data Ground Data (High Pass) Trend Line 50.1x103 2D Gravity forward model along EM line Aeromagnetics Stratigraphy East of Line -240 Conclusions 50.0 0m -243 Cerros Surficial Sediment (Late & Holocene Two volcanic flows interbedded with Ancha Formation and overlying Santa Fe Group sediments were identified in both the TEM and AMT Ancha Formation (?) (Pleistocene) -246 del Rio 49.9 modeling. High surface resistivity zones (>300 ohm-m) with depths ranging from ~100 to 300 m define the volcanic flows and correspond to Basaltic Andesite of Twin Hills (early Pleistocene) (mGal) volcanic (nT) fieldMagnetic Strength or Basalt of Canada Ancha (Late high densities (2.3 to 2.55 g/cm3), while low resistivity zones (<30 ohm-m) correspond to lower densities (~2.1 g/cm3). High spatial-frequency incomplete -249 Observed Calculated Error 0.427 498 magnetic variations are a possible indication of different volcanic flows that are characterized by normal or reverse remnant magnetization.

Bouguer Anomaly Bouguer field The upper flow (normally polarized) is inferred to extend beyond the study area and, therefore, past the end of the Cerros del Rio volcanic field 2000 Ancha Formation (Pleistocene to ) LAYER 1 D=2.12 EM profile 0 500 1000 1500 2000 boundary defined by USGS aeromagnetic data. Distance from West End of AMT Line 100m 0 500 1000m Flow Remnant LAYER 2B Magnetization LAYER 2A D=2.55 AMT Stations References 1000 D=2.3 FR24 Basalt of (Pliocene) or Lower Unit of the Other Roads Polarity Basaltic of Canada Ancha Basalt of Arroyoy Calabasas (Pliocene) Normal Grauch, V.J.S., Phillips, J.D., Koning, D.J., Johnson, P.S., Bankey, V., 2009, GeophysicaI Interpretations of the Southern Espanola Basin, AMT model Reverse Basaltic Andesite of Twin Hills 0 New Mexico, That Contribute to Understanding Its Hydrogeologic Framework , U.S Geological Survey Professional Paper 1761, p. 13-23. Elevation

Ancha Formation (Pliocene) and Tesque formation (Miocene) -1000 LAYER 3 Andesite of Cerrita Portillo Acknowledgments D=2.33 200n

-20000 2000 4000 6000 Special thanks to the students and faculty of SAGE 2012, Zonge International, GM-SYS, Geometrics and Aarhus Geophysics for their work Basalt of arroyo Calabasas (upper unit) and support. Distance (m) Basalt of Caja del Rio from Aubele (1979) Basalt of arroyo Calabasas (lower unit)