Geophysical Reconnaissance for Siting Dryland Critical-Zone Monitoring Experiments in Southern New Mexico, USA

A dryland critical-zone observatory is planned on a piedmont setting of the Jornada Experimental Range northeast of Las Cruces, New Mexico, near a ∼10-yr eddy flux covariance tower and vegetation monitoring experiment and a 2-yr old water-uptake rainfall infiltration experiment. We carried out several geophysical surveys to help select sites that minimize geologic complexity for follow up hydrologic and biogeochemical studies that will be conducted by other researchers. First, we conducted a review of regional topography, gravity, and magnetics prior to a site visit and then collected reconnaissance magnetic and electromagnetic data to aid in planning more detailed geophysical site characterization surveys. Our initial topographic analysis using 1/3 arc-second digital elevation models (DEMs) showed the proposed area had an out-of-equilibrium curvature pointing to active erosion and possible faulting. Short-wavelength step-like topographic anomalies in the DEMs were confirmed in LiDAR elevations, and are consistent with erosionally resistant soil horizons in the old alluvial fan deposits. Comparison of 2-D density and susceptibility models based on nearby (3-8 km) hydrostratigraphic studies established that the observed regional gravity and magnetic anomalies were larger than could be modeled with the 2-D structural constraints, and established the station spacing our reconnaissance surveys would require to sample shallow soil variations. Our first site visit confirmed the general fault locations and we identified three outcropping caliche horizons distinct to alluvial channel, proximal splay and distal splay deposits in a several hundred-meter traverse that are consistent with the short-wavelength topographic features. In order to plan additional seismic, radar, gravity, and electrical surveys within a region of such high potential variability, we collected magnetic field and magnetic susceptibility measurements along two profiles at 10-50 m spacing. We found anomalies consistent with two projected faults, as well as other bedrock structures, a result significantly more complex than prior regional hydrostratigraphic mapping had suggested. We also conducted a more limited 0.5 km long ground conductivity survey with 5 m spacing that traversed the rainfall infiltration experiment site and found anomalies that aligned with one of the projected faults. The results showed deep (>6 m) 50 mS/m (milliSiemens/meter) values, indicating moister soils, on the footwall side, dropping to 20 mS/m after crossing the fault, consistent with previous observations that normal faults in the Rio Grande Valley asymmetrically influence fluid flow.