Agricultural ground penetrating radar response to deep cultivation across a fault scarp after the 4 September 2010 Darfield earthquake, Canterbury, New Zealand

The 4 September 2010 Darfield earthquake, New Zealand, produced a >; 28 km long approximately east-west surface rupture trace through high intensity arable and pastoral farmland. The sense of motion was mostly dextral strike-slip with a component of reverse faulting at depth. We acquired ground penetrating radar (GPR) images before and after specialist land rehabilitation equipment was used to reduce the relief across a 1.5 m high fault scarp and to fill in surface “cracks”. The upper 1-2 m was calibrated using a dynamic cone penetro meter (DCP). The effects of the deep tilling caused an increase in the shallow velocity, due to reworking of the soil, and a consequent “pull up” of the reflectors immediately beneath the tilled zone due to the reduced two-way travel time. The deeper structure and features in the profiles, in contrast, were largely unaffected. We also examined the effects of the system settings on the response. We used a Sensors & Software pulseEKKO Pro with 100 MHz antennas, and both 500 and 1000 V transmitting power. The 1000 V transmitter yielded greater subsurface reflection energy than for the 500 V transmitter, but the direct ground wave was “clipped”, i.e. the top of the radar wavelet was truncated. The clipped waveform adversely affects migration and deconvolution, because the waveform top has a sharp “corner” rather than a smooth wavecrest. In general, similar subsurface information can be obtained using more stacking of a lower power signal. The rapidly deployed GPR survey was useful in earthquake response and recovery. It partially revealed what practices were successful for treating land affected by fault rupture, providing peace of mind to farmers that land was safe for people and livestock It also provided an insight into how modern agriculture may modify transient geological disturbances in gravelly soils.