Detailed mapping for tunnel lines is generally done on a scale of 1 inch = 200 feet, and detailed mapping for dam and power station sites, tunnel portals and quarries on a scale of 1 inch = 50 feet.

Very precise and detailed contour maps prepared by ground photogrammetry are often used as base maps. These are especially valuable in rugged terrain where the normal methods of surveying can only produce very sketchy and generalized contour maps, of limited value as bases for geological mapping. The original photo pairs can be studied in conjunction with the maps produced from them.

Mapping of excavations is carried out on a scale of 1 inch = 10 feet or 20 feet. Use of the photo-theodolite also enables photographic records, from which contours may be plotted, to be obtained of excavations during progress of the work and also finally before they are covered with structures.

Diamond Drilling:

Diamond core-drilling is regularly used to explore sub-surface conditions at the sites of dams, tunnels (particularly at portals and areas of low cover), deep shafts, underground power station sites and quarries. The drilling programmes are closely integrated with the surface geological mapping and other forms of exploration.

Diamond drilling is inevitably a very costly procedure. In addition to the cost of the drilling itself, there must also be the cost of making and maintaining access to the drilling sites, which in the Snowy Mountains are often in very rugged terrain without any existing means of access.

In the case of dams, power stations and tunnel portals, it often happens that more than one hole is required at the site, and the costs of making access to the site can be spread over all the holes drilled. Furthermore, access to such places may later be required if the site is adopted for construction purposes. In the case of tunnel exploration, it is common for only one hole to be required at each particular drilling location scattered along the tunnel line, and very high costs for making access are likely to be incurred by such drilling. Diamond drilling as a means of tunnel exploration, therefore, calls for an especially careful assessment of the value of the data likely to be obtained.

In diamond drilling, in consequence of the nature of the drilling process, there is an ever-present tendency not to recover core from clay seams, zones of crushed, weathered and softened rock, and from closely jointed hard rocks. That is, the core lost is most probably from those parts of the rock section which are of the greatest engineering significance. Unfortunately, it cannot be assumed that, because core has been lost, that the rock is weak, since good rock can be ground away and lost by inexpert or careless drilling, or by the use of unsuitable equipment. Drill cores from which much core has been lost are always difficult to interpret and may even be misleading.

Considerable attention is paid, therefore, not only to the planning of the drilling and to the siting of the drill holes, but also to the selection of diamond drilling equipment.