Soil Soundscapes from Seismic Arrays: Monitoring Bioturbation in Mima Mounds and Hillslopes

Seismic arrays are ubiquitous around the globe, with large volumes of data gathered continuously to monitor processes (typically earthquakes and explosions) that generate elastic waves and ground motion. However, these events occur only during a small fraction of the time and the data at other times, comprising the bulk of the dataset, are considered 'noise' and are largely ignored. However, recent work has demonstrated that low-energy elastic waves can be generated by processes in the atmosphere, hydrosphere, and critical zone, potentially opening up a new avenue in shallow geophysical research: utilizing this previously discarded 'noise' to characterize near-surface processes like bioturbation and soil creep in order to provide insight into connections between mechanistic drivers of change and source processes that act as catalysts for geomorphic evolution.

To test the feasibility of this potentially ground-breaking frontier that bridges geomorphology, geophysics, and biology, we utilize a variety of techniques in order to provide real-time monitoring of bioturbation and other processes that likely exert a first-order influence on the hillslopes of the Tennessee Valley, California and the Luquillo Mountains, Puerto Rico, in addition to geomorphic features known as Mima mounds in San Diego, California. We have installed an array of high-frequency, passive-source geophones at these sites in an attempt to monitor and characterize this biotic disturbance, primarily driven by pocket gophers or earthworms, to the very-shallow subsurface for the first time. This technique is strengthened by the use of ground penetrating radar and repeat active source surveying using a 40 kg propelled energy generator to image the subsurface and model the shallow velocity field for the California sites, respectively, in order to better understand morphological change over time. In Puerto Rico, we also installed an array of acoustic recorders alongside the seismometers to link sounds from earthworm bioturbation with observed vibrations in the soil. If successful, we further aim to compare our empirical data to recent granular hillslope models to track and quantify soil creep and the factors that influence it to better understand one of the most widespread, yet enigmatic, processes operating on soil-mantled hillslopes.