Methodology:

The three gathers were oriented at 60° to one another to constrain azimuthal variation in seismic properties centred on a single site on the axis of the divide. Each gather used four shot points, each with 48 receivers at 10m intervals. The offset to the first receiver ranged from 30 to 1440m, producing traces out to 1940m offset, or incidence angle at the bed up to 58°. To achieve three-component recordings, the receivers were oriented to measure either vertical, radial or transverse motion. The CMP gather along the divide axis (K01; true bearing 043°) was shot twice: once with all sensors orientated vertically using 150g of high explosive, and once with horizontal sensors alternating radial and transverse orientation, using 600g shots; the latter resulting in an effective 20m receiver interval for the horizontal components. The oblique CMP gathers (K02: 60° east of K01; K03: 60° west of K01) were shot once only, each with 600g shots and alternating vertical, radial and transverse receivers, resulting in an effective 30m receiver interval for all three components. The larger shot is required to generate sufficient S-wave energy at the source. Although an explosive source in a homogeneous isotropic medium will theoretically not generate S-wave energy, the non-uniform nature of the shallow firn results in significant S-wave energy. The acquisition geometry results in a mid-point basal reflection ellipse with a length of 235m.

Data collection:

Instrumentation:

Recorded on 2 x 24 channel Geometrics Geode with 48 x 40Hz Georods; Source: 150-600g high explosive shots at 20 m depth.

Resolution:

The Fresnel zone is less than 80 m (minimum 150 Hz dominant frequency).

1940m maximum offset.

Data quality:

8000Hz sample rate with GPS shot trigger timing; 24 bit recording