Geophysical Methods
Allred, B., V. Adamchuk, R. Rossel, J. Doolittle, R. S. Freeland, K. Grote, and D. Corwin.  2016.  Encyclopedia of Soil Science, Third Edition, Chesworth, W. (Ed.). (2008). Encyclopedia of soil science (Vol. 207). Berlin: Springer.

Abstract:
Near-surface geophysical methods have become an important tool for agriculture. Geophysical investigations for agriculture are most often focused on the first 2 meters directly beneath the ground surface, which includes the crop root zone and all, or at least most of the soil profile. Resistivity, electromagnetic induction, and ground-penetrating radar are the three geophysical methods most commonly employed for agricultural soil investigations; however, optical reflectance and γ-ray spectroscopy are increasingly becoming more widely utilized. Temporal and spatial variation of conditions and properties in the soil profile are important considerations when conducting a geophysical survey within an agricultural setting. Geophysical methods have been applied to soil surveys, precision farming, soil water content measurement, and soil salinity monitoring; with new agricultural geophysics applications continuing to evolve. Future development of multi-senor platforms and the use of unmanned aerial vehicles will dramatically improve geophysical soil investigation capabilities with respect to field accessibility and data interpretation.