Present State of the Long Valley Caldera/California Derived From DC-Resistivity Imaging and Self-Potential Measurements

The geoelectrical methods make sense to apply in volcanic areas since porosity, permeability, ionic mobility, ionic concentration in rock fluids, and cation exchange capability are modified by volcanic - magmatic activity directly and processes generated by volcanism (geothermal systems). Resistivity of volcanic rocks is a meaningful indicator for changes in porosity and ionic mobility, ionic concentrations in fluids, mobility of fluids caused by changes in temperature. A large-scale Direct-Current (DC) imaging investigation of the Long Valley system included both an active 21 km DC-survey line across the caldera and the mapping of natural self-potential (SP) anomalies in the western and central part. The deep sounding Electrical Resistivity Tomography (ERT) was applied successfully. The 21 km long dipole-dipole profile in east-west direction according to Wannamaker et al. (1991) was selected deliberately in order to compare the results. For the detection of the potential differences special stand alone transient recorders were applied. This kind of signal recording offers the possibility of statistical methods for signal enhancement. The recorded time series yield information not only about the DC-resistivity than special effects like induced polarization events and self-potentials. The first application of tomographic resistivity sounding in Long Valley is resulting in a recent 2-D model of resistivity distribution in subsurface after the resurgence of volcanic activity in the 80ies. The inversion result reveals numerous resistivity structures which are in very good correlation with known geologic and tectonic structures. The tomographical inversion of DC-data was carried out with the goal to study the geological structures of the Long Valley Caldera down to 3-4 km, especially in the part affected by the resurgent dome and the hydrothermal system by use of geoelectrical parameters. The important structural components include low-resistivity regions beneath the western half of caldera, which are originated by the shallow hydrothermal system. A large conductive segment under the Medial Graben, already described by Wannamaker et al. (1991), can also be seen, whereas the majority of resurgent dome appears highly resistive. The interpretation of the resistivity model is sustained by self-potential anomalies, measured in the western part of caldera, by the comparison with known fault zones and by implications for fluid flow inferred from geothermal data, whereupon the resurgent dome is not influenced by the present hydrothermal fluid flow.