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 region.