Geophysicists, looking for new exploration tools, have studied the coupling between seismic and electromagnetic waves in the near-surface since the 1930s. Our research explores the possibility that electromagnetic-to-seismic (ES) conversion is useful at greater depths. Field tests of ES conversion over gas sands and carbonate oil reservoirs succeeded in delineating known hydrocarbon accumulations from depths up to [Formula: see text]. This is the first observation of electromagnetic-to-seismic coupling from surface electrodes and geophones. Electrodes at the earth’s surface generate electric fields at the target and digital accelerometers detect the returning seismic wave. Conversion at depth is confirmed with hydrophones placed in wells. The gas sands yielded a linear ES response, as expected for electrokinetic energy conversion, and in qualitative agreement with numerical simulations. The carbonate oil reservoirs generate nonlinear conversions; a qualitatively new observation and a new probe of rock properties. The hard-rock results suggest applications in lithologies where seismic hydrocarbon indicators are weak. With greater effort, deeper penetration should be possible.