Between two furrows: soil bulk density from non-invasive seismology

Soil is a critical resource for global food security. However, traditional physical analyses of soil samples and geophysical imaging techniques are often labour intensive and time-consuming. This study investigates the potential of ultra high-frequency (> 500 Hz) hammer-source seismology to characterise the physical properties of soil at the decimetre scale. We conducted experiments within a long-term field experiment near Harper Adams University (UK) aimed at comparing Conservation and Conventional agriculture. We surveyed two meter-and-a-half sections of each agricultural treatment with 16 geophones and collected soil samples with the same horizontal resolution. Our estimates of the P-wave velocity (vp) and bulk density in the upper 40 cm of the soil reveal a strong and statistically significant correlation. Consistent correlation of bulk density and vp throughout the depth profile were observed between the seismic images and interpolated bulk density data derived from physical soil samples. Our work demonstrates that ultra-high frequency seismic analysis is a promising, cost-effective tool for estimating soil bulk density, in support of agronomic and land-management decision making, and improving the accuracy of soil carbon stock quantification.