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Within this presentation, fully defined synthetic models have been used to illustrate various features of the GRM. The benefits of this approach are obvious. Very few field examples have sufficient drillholes or other forms of sampling to guarantee the same control. Furthermore, models can be designed so that one aspect is emphasized, while other factors are held constant or are removed entirely.

However, like all methods of seismic refraction interpretation, the GRM will ultimately be judged on its ability to accommodate complexities of the real geologic environment. The seismic refraction survey at the Welcome Reef dam site (Hatherly, 1977) provides data which permit an assessment of the GRM, because field methods were appropriate. These methods included:

  1. The use of geophone separations considerably smaller than the depth to the deepest refractor;

  2. the measurement of arrival times to an accuracy of about one percent, despite the problems of introducing sufficient energy into the ground; and

  3. the use of sufficient shotpoints to resolve the ambiguity problem for the majority of layers.

A listing of locations and first arrival times for the refraction survey of the Welcome Reef dam site is given in Appendix B.

The proposed Welcome Reef damsite is on the Shoalhaven River, approximately 50 km east of Canberra in southeastern Australia.

Outcrop in the area consists of interbedded, steeply dipping, Ordovician metasediments,w hich are mainly quartzitesa nd phyllites. Weathering, particularly in the phyllites, can be extensive. In drillholes, the top of the weathered bedrock surface can often only be distinguished from the unconsolidated sediments by color changes,

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