Seismic refraction surveys for coal exploration in the Homehills, Hawkdun and Roxburgh areas of Central Otago

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Broadbent, M. 1982 Seismic refraction surveys for coal exploration in the Homehills, Hawkdun and Roxburgh areas of Central Otago. : . Report / Geophysics Division 157 48 p.

Abstract: Seismic refraction surveys were made during 1979 and 1980 in the Homehills, Hawkdun, and Roxburgh basins formed in the Mesozoic and Paleozoic rocks of Central Otago. They were made to help define the extent of Tertiary coal deposits by determining thicknesses of Cenozoic rocks. The extent of the surveys in the three basins were 12.1, 8.8, and 8.2 line kilometres distributed over 9, 6, and 7 lines respectively. The seismic observations and sonic logs from drill holes were used to derive vertical sections showing distributions of compressional wave velocity with depth. Velocities detected in the Homehills and Hawkdun basins of below 0.5 km/s were usually associated with soil layers, those from 1.0 to 1.5 km/s with Quaternary sediments which are not water saturated, those from 1.6 to 1.9 km/s with Tertiary lacustrine or littoral sediments, those from 2.1 to 2.6 km/s with late Tertiary or Quaternary gravels, and those exceeding 3.2 km/s with Mesozoic basement rocks. No overall one-to-one relationship between velocities detected in the range 2.6 to 3.2 km/s and rock types was evident. Velocities encountered at Roxburgh included those between 0.5 and 1.5 km/s which were associated with non-saturated Cenozoic sediments, those between 1.5 and 2.3 km/s representing saturated Cenozoic sediments, those over 3.0 km/s representing Paleozoic schist, and those between 2.3 and 3.0 km/s for which a clear relationship with rock type was not established. Survey results indicate that most of the western boundary of the Homehills basin is unlikely to be associated with a major fault, but the north-east boundary of the basin is associated with a reverse fault having a throw of between 160 and 170 metres. A fault of throw exceeding 300 m has been detected, separating the Cenozoic filled Homehills basin, with its base at about 500 m above sea level, from the much deeper Ida Valley basin. The maximum thickness of Cenozoic sediments in the Homehills basin is about 250 m