High-resolution geophysical surveying at the Springfield Fault, New Zealand

Abstract

To trace the active Springfield Fault (South Island, New Zealand) and map its character at shallow depths on a terrace where it exhibits no surface expression, we recorded 3-D georadar data across an approximately rectangular 110 x 40 m survey area. In addition, we carried out multi-electrode geoelectric measurements along a 198 m long profile that crossed the georadar survey area. Although the georadar depth penetration was limited to only ~5 m, the processed images revealed the presence of a prominent reflecting horizon disrupted by three main discontinuities. Semi-continuous subhorizontal reflection patterns were interpreted to represent sedimentary units within the fluvial deposits, whereas three detected discontinuities were interpreted as fault traces with small near-vertical offsets (~0.4 m). This interpretation was supported by vertical and lateral changes visible on the final inverted resistivity model indicating lithological boundaries and fault branches.