THE ROCK VALLEY DIRECT COMPARISON (RV/DC) EXPERIMENT TESTBED

The Source Physics Experiment (SPE) has been ongoing since 2010 at the Nevada National Security Sites (NNSS) in southern Nevada. Phase I and Phase II focused on the source physics in contrasting geologies between hard and weak rock with the final chemical explosion occurring in 2019. The data from those two phases continue to be analyzed. SPE Phase III (RV/DC) is the final aspect of SPE that focuses on discrimination features between shallow earthquakes and underground explosions. In 1993, a shallow earthquake sequence occurred in Rock Valley at the NNSS. The largest event, M3.7, was followed by eleven M>2 events ranging in depth from 1-3 km. All events were well constrained due to the deployment of seismic stations early in the sequence. Comparison of these shallow events to nearby historic nuclear tests identified gaps in our ability to discriminate these event types. To advance our understanding about the physics of shallow earthquakes, the RV/DC Experiment was conceived. The main component of the experiment is to conduct two chemical explosions at hypocenters like those of the earthquake sequence to understand the distinguishing characteristics between these types of events. In order to carry out the experiment, the Testbed will consist of 4 boreholes with a planned depth of 2 km, 1 corehole, 2 observational boreholes, and 1 main experiment hole. The drilling activities will be staged, and boreholes will be instrumented to obtain microseismicity data, sample fault properties, and will record controlled and natural signals for up to 2 years prior to the main experiments. In addition, we have and will continue to collect geophysical and geologic data in Rock Valley. This includes reoccupying existing seismic stations and installing new ones, conducting controlled-source seismic surveys using an Accelerated Weight Drop source, installing new geodetic stations, performing geologic mapping, acquiring new dense gravity data, conducting material properties testing, and acquiring UAS data over the Testbed. We are presenting the status of the Testbed development, the characterization data acquired thus far, and future plans. This unprecedented experiment will address seismic waveform differences between earthquake and explosion sources supporting the underground explosion monitoring efforts.