An Unbiased Spiral Array for MASW Data Acquisition

MASW surveys generally use linear arrays, and the result is posted at the centre of the geophone array. A question is asked whether there is any bias related to the source offset and direction relative to the linear array. This paper first examines the existence of the biases by a receiver array with variable source and a source array to different receiver locations.

A new pattern of geophone array is proposed to attempt to eliminate these biases. It is an omnidirectional, variable offset array with a spiral pattern. Geometrically, there is no uncertainty in the centre of the array. An experimental survey was carried out to compare this spiral array with a linear array centred at the same point. A record by the spiral array can be analysed using conventional MASW software which assumes constant geophone interval. The records are compared in both the time-distance and the frequency-phase velocity domains. The result demonstrates the spiral array delivers comparable dispersion image to the linear array and it assures a more confident array centre location.

The spiral array also indicates viability of a MASW array arranged two-dimensionally. This is also a step forward to a 3D MASW.