Using shallow seismic techniques to determine structure in the regolith

Example Research Project supported by the ASEG Research Foundation

Chris Leslie: Using shallow seismic techniques to determine structure in the regolith.

Host Institutions: Australian National University and Geoscience Australia

Supervisor: Eva Papp and Tony Eggleton

Mentors: Leonie Jones, Geoscience Australia

Project Summary

Seismic methods were used to determine structure and depths in the regolith near West Wyalong, NSW. The area is known for gold-bearing 'deep leads' and thus geological interpretations were to assist in further studies to determine possible mineralisation migration paths resulting from ground-water movement.

Two paleochannels were chosen as targets for the project but at that time only the spatial definition of the paleochannels was apparent from aerial magnetic images. Two reflection seismic survey lines, using a Minivibe seismic truck, were subsequently shot orthogonal to the flow direction of the paleochannels. The expectations were that the surveys would provide structural detail on the channel profiles and possibly other regolith structure.

As the paleochannel depths were unknown but possibly shallow, the seismic acquisition parameters were designed to enhance shallow features. Shot spacing was as tight as 1 m and vibroseis frequencies were as high as 500 Hz. Notorious with such shallow seismic work is interference of useful reflection and refraction data by coherent noise such as ground roll. Filtering out dominant noise frequencies, followed by spectrum equalisation and J/K filtering, proved to be effective in enhancing useful data in the shot records. Reflection events in the 50 to 100 m depth range consequently became more apparent.

Refraction first-break picks from the shot records, and values calculated during reflection static processing, were used to determine very shallow layers. Beneath one of the

lines the refraction layer shape suggests an apparent typical paleochannel profile with cut-bank asymmetry at about 2 to 9 m deep and about 200 m wide. Drill chips from shallow holes drilled over that line contained a significant amount of maghemite-rich nodules at corresponding depths. The drill chip constraints provide further evidence that the refraction layer represents a paleochannel on the assumption that the nodules provided a refractive density contrast.

The reflection seismic sections over both lines indicate deeper paleosurfaces at 50 to 200 m depth and constrained by borehole data as the interface between unconsolidated material and underlying solid sandstone. The interpretation is of a profile of an ancient and eroded landscape that has since been filled with transported material. The apparent dip directions may help in determining local ground-water movement.

The project demonstrated that reflection seismic methods were useful in assisting to determine structure in the regolith. The refraction component of the data, inherent in the shot records, enabled interpretation of very shallow structures, while the processed seismic sections enabled deeper structural interpretations.

A pertinent outcome of the project in terms of shallow seismic methods was that resolution was critically dependent on appropriate acquisition parameters such as geophone spacing, vibroseis energy and output frequencies.

Chris Leslie, Email: chris_leslie@telstra.com

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