Velocity Measurements of Powdered Rock at Low Confining Pressures and Comparison to Lunar Shallow Seismic Velocity

Seismic methods will be useful for future lunar near-surface characterization, and high-fidelity elastic models will be required to aid interpretation of seismic observations. To develop an elastic lunar near-surface model, we performed ultrasonic velocity measurements of lunar regolith simulant at low confining pressure and developed a rock physics model calibrated to these measurements. Grain contact models based on Hertz-Mindlin theory produce accurate results at high confining pressure (i.e., several hundred meters or more burial depth) but historically fail to predict observed velocities in unconsolidated media at low pressure. Therefore, we heuristically modified existing models to fit our measured data over a range of porosities and confining pressures. To compare with Apollo 14 and 16 active seismic experiments, we used our new heuristic rock physics model to produce lunar subsurface velocity profiles. We performed ray tracing through our velocity profiles to calculate seismic traveltime, which results in good agreement with first arrivals interpreted from the Apollo experiments. Our model suggests a slightly higher velocity-pressure dependence than inferred from in situ measurements, which may be due to porosity reduction in the lunar regolith from impact-induced and natural vibrations. Seismic velocity is the speed at which mechanical waves travel through planetary materials and is controlled by the physical properties of those materials such as density and compressibility. We have created a model that relates seismic velocity to depth and porosity under lunar conditions and have calibrated that model with laboratory measurements of lunar soil simulant at low pressure (i.e., near-surface) conditions. Using this model, we can predict seismic velocity in the lunar subsurface and compare the modeled velocity with observations from Apollo active seismic experiments. Slight differences between our model and Apollo observations may be due to lower porosity in the lunar soil caused by vibrations from asteroid impact events and seismic quakes. We collected ultrasonic velocity measurements of lunar regolith simulant at low confining pressure to calibrate a rock physics model Our rock physics model predicts shallow lunar seismic velocity in agreement with observations from the Apollo active seismic experiments Our model may be used in future studies to predict lunar near-surface seismic velocity under variable resource scenarios