From the graph the speed of the seismic waves in various layers below the surface and the depth to each different seismic layer below the shot point and geophones can be calculated.

The different seismic layers may represent either different rock types or the same rock type in different degrees of weathering or in different degrees of compactness arising from differences in spacing or openness of joints. Seismic refraction surveys have been used in preliminary investigation to measure approximately the depth of weathering and the sound rock surface at surge tank sites and tunnels, and the thickness of gravel and soil deposits. In detailed investigations it has been used to supplement and extend the results of drilling. Not all sites are adaptable to the seismic refraction method. Where it is applicable advantages are that it is relatively cheap and rapid, and the equipment is very portable so that there is no need to make special access tracks into the sites. Depths obtained are approximate, but the accuracy is usually quite sufficient for preliminary surveys.

Examples of investigations to which the method has been applied are the following:

Kosciusko Dam Sites:

(a) Measurement of the thickness of glacial moraine and weathered granite (Fig. ll) overlying granite bed-rock, and determination of bed-rock contours.

(b) Measurement of the thickness of weathered granite for use as embankment material in earth dam construction.

Adaminaby Dam :

Measurement of the thickness of river gravels forming an alluvial flat, for possible use in the dam embankment. Some indication was also obtained of the variations in composition of the gravels, particularly of the presence of silt layers.

T.2 Project :

(a) Measurement of the thickness of granite bed-rock cover over the proposed T.2 Tailrace Tunnel where it passes at shallow depths under wide alluvial flats.

(b) Measurement of the thickness of the weathered zone and the depth to sound granite at several possible surge tank sites high up on the slopes of the Tumut River Valley.

Guthega Project:

Measurement of the thickness of weathered granite at the portals of Piper's Creek aqueduct tunnel (Fig. 2).

Petrographic Laboratory:

A small Petrographic Laboratory unit is maintained to supplement the field investigations. The work of this laboratory is chiefly of two kinds—

• (a) The examination of natural construction materials, chiefly concrete aggregates, to assess their suitability by means of a study of their mineral composition, structure, degree of weathering and physical soundness, and by comparison with materials of known performance. This work is complementary to tests made on the materials in the Concrete and Soil Mechanics Laboratories.
• (b) The identification and classification of rocks from the various projects to assist the geologists working in the field.

Notes on Typical Projects

Dam Sites:

Most of the dam sites for the scheme are either in granitic rocks or in highly folded metamorphosed sedimentary rocks--- quartzites and slates, chert, schists, or hardened siltstones and slates. It has been possible to find sites for all of these dams in narrow sections of the valleys, generally with rather steep walls and either no alluvial flood plain or a very narrow one. The beds of these streams are degrading and fresh rock forms outcrops at water level, although often concealed to a varying extent by boulders and gravel.