A seismic reflection experiment near Runanga, Westland

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Broadbent, M.; Jenkins, R. 1982 A seismic reflection experiment near Runanga, Westland. [s.l.]: [s.n.]. Report / Geophysics Division 155 24 p.

Abstract: An experiment was performed between Runanga and Rapahoe, a few kilometres north of Greymouth, to determine whether seismic waves reflected from Tertiary and Mesozoic sediments associated with coal seams could be recorded and identified. Reflections were observed at both sites occupied during the experiment, which were centred on National Yard Grid coordinates S44/770 964 and S44/776 957. Reflections observed at the western site have been tentatively associated with boundaries in or at the base of the Island Sandstone (about 180 metres below the ground surface), and within or at the bounds of the Rewanui Coal Measures (at about 400 metres depth). At the eastern site, reflections have been attributed to a boundary at about 170 metres depth. Reflections were generated using explosions 10 metres below the ground surface, and also by using groups of shallower, smaller simultaneous explosions at distances of more than 100 metres from the geophones where the reflections were detected. Average velocity estimates derived from the seismic measurements alone were 2.6 and 3.0 km/s for Rapahoe and Mawheranui Group sediments respectively. Sonic log measurements yield an average velocity of 3.3 km/s for the Mawheranui Group. The areal coverage of the experiment was restricted by the difficulty of gaining access to locations off roads and the vulnerability of local access routes, services, and structures to vibration induced damage. The scope of the experimentation was further restricted by difficulties encountered in drilling shot holes and by recording equipment limitations