Geophysical characteristics of salinisation at Cape Portland, NE Tasmania

Abstract

An integrated geophysical study of salinisation at Cape Portland, NE Tasmania, has mapped the distribution of saline areas, identified constrictions in the hydrogeologic basement and a possible source, store and transport mechanism for the salt. EM-31 data not only clearly delineates the extent of salt scalds, but also highlight areas of elevated conductivity not visibly affected by salt. EM-31 data maps the distribution of the salt at a much higher resolution than was previously possible using shallow drilling. Results from time-domain electromagnetic surveys confirm the responses seen in the EM-31 data and provide additional information about the subsurface distribution of conductive material. Conductivity depth pseudosections and layered earth inversions indicate depressions of up to 100 m in the resistive basement that are infilled with more conductive material. Ground magnetic data show the distribution of shallow Jurassic dolerite basement features and define a major negative anomaly that transects the study area. This feature is coincident with depressions identified from the time-domain electromagnetics and with a negative Bouguer gravity anomaly. The electromagnetic and potential field interpretations are consistent and indicate the presence of a major palaeochannel infilled with conductive material that cross-cuts the study area. Seismic refraction surveys and shallow auger holes in this zone provide additional support for the presence of a palaeochannel. This feature is inferred to be a major control on salinisation at Cape Portland.