Electrical resistivity investigations of hydrological problems in Western Samoa

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Risk, G.F 1979 Electrical resistivity investigations of hydrological problems in Western Samoa. : . Report / Geophysics Division 152 63 p

Abstract: Electrical resistivity soundings, probing to about 100 m depth at Mulinu'u, Leulumoega, Faleolo and Tausagi, demonstrate that salt water aquifers with resistivities from 1 to 50 ohm-m can readily be detected under land and distinguished from overlying fresh water lenses which have resistivities from 50 to 100 ohm-m, and from dry rocks which mostly have resistivities greater than 500 ohm-m. The depth of the water table and thickness of the fresh water lens could not always be accurately measured, but places where the thickness of the fresh water lens is tens of metres could usually be distinguished from places where it is only one or two metres. A slightly modified portable conductivity meter proved capable of measuring chlorinity variations of sea water along the shore, which might pinpoint places where fresh water enters the lagoon. The sequence of old, weathered, (and probably impermeable) Fagaloa volcanics have resistivities between 35 and 105 ohm-m, which is less than that of the younger overlying rocks. Near Tanumalala, a deep conductive layer with an uneven surface was found and interpreted as being the Fagaloa rocks. Insufficient work was done to establish whether the Fagaloa rocks form basin-like traps containing fresh water