Insights on deciphering the network of hidden internal soil pipes in agricultural fields using acoustic techniques

Soil piping significantly accelerates erosion in agricultural fields, contributing to substantial soil loss. However, quantifying its impact is challenging due to limited direct observations. Soil pipe locations are often inferred from surface features, such as flute holes and gully windows, or through methods like dye tracer testing, which involves injecting fluorescein dye into uppermost pipe collapse features and sampling downslope. This study investigates an acoustic technique for mapping soil pipe networks, leveraging the propagation of sound waves through air-filled pipes. These waves generate seismic vibrations in the surrounding soil, detected at the surface by geophones. Field measurements were conducted at the Goodwin Creek experimental site with extensive soil pipe networks supported by six gully windows. A speaker placed in various gully windows generated sound waves and surface vibrations were recorded along two lines. Recorded data were analyzed in the frequency domain, calculating energy content using the Riemann sum approximation. Signal-to-noise ratios (S/N), probability density functions (PDFs), and Z-scores were computed for each geophone. High S/N and Z-scores (>2) indicated proximity to large, shallow pipes, while moderate scores (1 < Z < 2) suggested smaller or deeper pipes. Results were validated with in situ investigations, enabling detailed mapping of primary and secondary soil pipe networks.