In data from a high-resolution seismic survey conducted over a near-surface environment consisting of homogeneous soft clay, we consistently observe a distinct seismic phase arriving between the direct compressional wave and the Rayleigh wave. This phase is characterized by high amplitudes at near offsets and a phase velocity corresponding to about twice the shear wave velocity. Based on analytical and numerical analyses, this signal could be unambiguously identified as a non-geometric wave, which is excited if the source is located near the Earth's surface and the Poisson's ratio in the vicinity of the source is unusually high. To date there are only a few speculative observations of this particular non-geometric seismic wave phenomenon. However, given the commonly very high Poisson's ratio of surficial materials, we expect this phase to be present, albeit unidentified, in many near-surface seismic surveys. The presence of this non-geometric wave increases the complexity of the seismic record, and failure to identify it may result in misinterpretations, particularly of high-resolution seismic reflection data as well as of shear wave and surface wave data. Potential applications of this seismic phase may arise from its high sensitivity to the shear wave velocity in the immediate source regio
