SUMMARY

As part of an effort to understand a complicated shallow lowtemperature geothermal system in the Arkansas Valley, Colorado, we focused on Chalk Cliff Valley by acquiring a 3-D seismic data with a 240 m2 footprint. In order to optimize a reflection image of the site, we use refraction and wavefield modeling in an initial attempt to interpret the shot gathers. Our results show the bedrock dipping to the south by up to 31_, up to 110 m deep at the southern extent of the site. We also find a water table depth of 10 m and present evidence that the saturated sediments are terminating near the northern end of the site where the bedrock is shallowing. We also find that the velocity field may be anisotropic with the refraction model results estimating a 12 percent increase in the N-S direction. Our findings, and particularly the fractured bedrock and a termination of the saturated sediments on the northern end of our study area are consistent with geologic observations and Self- Potential data identifying a localized anomaly associated with up-welling (hot) water. The velocity model will guide reflection imaging to be performed shortly.

INTRODUCTION

Applying geophysical methods to geothermal systems is key for their characterization and development for geothermal power generation. In the Upper Arkansas Valley we are using near surface geophysical methods to characterize a shallow lowtemperature geothermal system. Imaging this shallow low temperature system will help develop new insights and techniques for exploring deeper and higher temperature systems. Since 2007, the Colorado School of Mines, Boise State University and Imperial College London have conducted a geophysics field camp to acquire a range of geophysical data sets at a geothermal site in the Upper Arkansas Valley of Colorado (see Figures 1 and 2). These data are being used to develop an understanding of the subsurface through imaging the structure, fluids and their movement. Self potential (SP) and resistivity data in conjunction with well log and geological mapping have been used to understand the movement of geothermal water in the near surface and where preferential pathways may exist in the underlying basement rock (Richards et al., 2010). This information has helped site selection for near surface 2-D and 3- D seismic surveys with the objective of mapping the water table and the physical structure of the near surface. Understanding the interconnectedness between the near surface structure and fluid flow, may help identify preferential pathways that are connected to the deeper normal faults of the Upper Arkansas Valley and ultimately the geothermal heat source. Near surface 3-D seismic surveys are challenging since many of the events are often located within the first 100 to 200 ms of travel time. The acquisition and processing methods used in deeper exploration seismic are typically scaled down however further attention is required to separate reflections from refractions and improve static corrections. In certain cases it may even prove to be impossible to satisfactorily process and interpret the data (Steeples and Miller, 1998).

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