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DEPARTMENT OF NATIONAL DEVELOPMENT i
BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS.
GEOPHYSICAL INVESTIGATION AT THE ROSEBEi,y NO. 1 1. 1
DAMSITE, TASMANIA. ,
D. F. DYSON and B. J. BAMBER.
RECORDS 1959 NO. 85
GEOPHYSICAL INVESTIGATION AT THE ROSEBERY NO.1 DAMSITE,
D•F ~ DYSON and B.J. BAMBER
** **** ***************** CONTENTS Page Abstract
Methods A Resistivity B Seismic
1 2 2
Resistivity B Seismic A
Locali ty Map. Geological Plan Geological SectioB Resistivity Profiles Seismic Profiles.
ABSTRACT Seismic refraction and resistivity investigations were carried out in response to an application from the Hydro-Electric Commission of Tasmania to investigate a proposed dam site on the Pieman River near Rosebery.
At the site overburden, less than 100 feet thick, and consisting of talus, scree, till gravel and weathered rock overlies bedrock which consists of Permian Pyroclastics and lavas. The purposes of the surveys were to determine the depth to bedrock and some of the physical properties of the bedrock and overburden. Seismic velocities of 1,400 to 8,000 ft/sec. were recorded in the ; overburden and velocities of 11,000 to 18,000 ft/sec. in the bedrock. It is estimated that the thickness of the overburden has bee~ calculated, in general, with a maximum error of + 20 per cent. There is, however, one region where the estimate of overburden thickness may be 50 per cent in error. On the traverse there are no drillholes which oould be used for control of the seismic calculations.
The Hydro-Electric Commission of Tasmania proposes to construct a dam on the Pieman River about four miles north of the township of Rosebery. 0 The approximate latitude and longitude of the site are 41 43' S. and 0 145 30' E. respectively. The dam will be part of the Pieman River Scheme.
In response to an application from the Commission, the Bureau of Mineral Resources carried out a geophysical survey on the site to determinethe depth to bedrock and some of the physical properties of the overburden and bedrock. The seismic refraction and resistivity methods were applied. The survey was conducted from 21th Februar~ to 3rd March 1959, by a party consisting of D.F. Dyson (Party Leader), B. Bamber (Geophysicist) and J. Croger (Geophysical Assistant). The Commission provided an additional five assistants and carried out the topographical survey on the traverse line. 2.
The geology of the area is described by Ward (1908) and Bradley (1954, 1956). A detailed geological examination of the site was carried out by Mather (1958) and his geological map is reproduced on Plate 1.' This is a map of the area at the southern end of the traverse where the geo-physical measurements were taken. Although the topographical relief of 200 feet in the length of 2,200 feet is relatively slight, it was thought possible that the bedrock might be very deep at the northern end of the traverse where there is a topographical gully. The term "bedrock", as used in this report, refers to Cambrian pyroclastics and lavas, which have a seismic velocity ranging from 11,000 to 18,000 ft/sec. The term "overburden" refers to soil, talus, scree and weathered Cambri~ pyroclastics and lavas which overly the bedrock. The s.eismic velocity in the overburden ranges from 1,400 to 8,000 ft/sec. 3.
The resistivity and seismic refraction methods were applied.
Different rock types possess different electrical resistivity, as a result of variation in the lithology and physical properties of the rocks. Hard, non-porous, unweathered rocks generally have a high resistiVity. Shearing and fracturing cause localised weathered zones which, because of the resultant increase in porosity, water oontent and salinity of the pore solutions, produce a decrease in resistiVity. As a general rule it may be said that the resistivity of a rock is inversely proportional to the product of' porosity and salinity of the pore solutions. In the Wenner method of ,i,resistivi ty traversing ii , Which was used in the present investigations, four electrodes, equally spaced in a straight line are moved as a whole along a traverse and readings are taken at consecutive statibns~ The electrode spacing is of the same order of magnitude as the depth to which the resistivity is measured. The resistivity value measured is an apparent resistiVity and not a true value for any particular rock layer. In the interpretation, absolute values of the apparent resistivity are not as important as sudden changes from high to low apparent resistivity - such changes generall? indicate a change in rock type~
The total length of resistivity traverses surveyed was 2,200 ft. They were surveyed twice using 100 foot and 50 foot electrode spacings. A Tellohm Geophysical Meter, manufactured by Nash and Thomson (U.K.) was used. B. Seismio Method. The seismic method of exploration depends on the contrast in the velocity of seismic waves through different rock formations. Hard, unweathered rocks have higher velocities than their weathered counterparts, I while these in turn usually have higher velocities than soil and unconsolidated deposits. The method of differences was used (Heiland, 1946) and the following types of spread were shot. (i) Weathering Spreads. These were used to obtain seismic wave velocities in, and the thickness of, near-surface layers. The geophone intervals were 10 ft. and shot points were 10 ft., 50 ft., and 150 ft. from each end of the spread. (ii) Normal Spreads. The geophone intervals were 25 ft. and 50 ft. and shot points were 50 ft. and 200 ft. from each end of the spread. The equipment used in the seismic survey was an SIE 12-channel refraction seismograph Type PR0-11-6 with Teohnical Instrument Co. geophones of natural frequency about 20 cycles per second. The total length of seismic traverses surveyed was 2,200 ft. 4. A.
The results of the resistivity survey using electrode spacings of 100 and of 50 foot are shown in profile on Plate 2. "
The only relatively high values of resistivity correspond to the topographical high between pegs 30 and 36. This may mean that the bedrock in this region is closer to the surface although the same effect could bu caused by the better drained and drier overburden which may be expectud on such a topographical high.
- 3 Provided tha thickness of overburden alohgmost of the traverse is not more than the electrode spacing, and this is indicated as fact by the seismic work, then the lack of contrast in apparent resistivity values along the traverse suggests that the overburden does hot change very much in thickness or resistivity along the traverse. B.
Plate 2 shows the interpretation of the results of the seismic survey in the form of a profile, indicating the thickness of the overburden. Table 1 shows a tentative interpretation of velocities in terms of rock type. TABLE 1.
Seismic velocity (ft/sec)
1400 - 1600
Scree and Talus Weathered Cambrian Pyroclastics and Lavas
Estimated Youngs Modulus (lbs. wt/sq.in.)
7500 - 8000
Unweathered Cambrian 11000 _ 18000 Pyroclastics and Lavas
1. 6-1.8 x 106
x 10 6to 11.6 x 10
The depth to bedrock between Pegs 30 and 36 could perhaps be greater than is indicated. This possible lower boundary is shown on Plate 2 by a dotted line. The lower boundary indicates what is consider~d to be the maximum possible value of the depth to bedrock in this region. In general the work indicates that there is no deep gully in the bedrock along the traverse line. To estimate the Young's Modulus values shown in Table 1, a density value of 2.7 (c.g.s. units) was taken as an average for the Cambrian pyroclastics and lavas and a Poisson's ratio of 0.3 for rock of/7?50G to 11,000 ft/sec. velocity and 0.1 for rock of 18,000 ft/sec. velocity (Birch, Schairer and Spicer, 1950). . Althou~h the thickness of overburden between pegs as much as 50% in error, elsewhere, it is considered the is not in excess of 20%. Along the traversa there is no could be used as control data for improving the accuracy calcula.tions.
30 and 36 may be maximum error drill hole which of the seismic
The geophysical survey provided information on the depth to bedrock at the dam site. The overburden consists of soil, scree, talus and wea.thered bedrock. The bedrock consists of Cambrian pyroclastics and lavas.
- 4 -
The seismic survey indioated that the overburden is fairly uniform along the traverse but is thickest between pegs 30 and 36. In this region it reaches the depth of 66 feet. The general uniformity of the overburden thickness is supported by the resistivity measurements.