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, and has identified constrictions in the hydrogeologic basement and a possible source and transport mechanism for the salt. EM-31 data collected in the area not only clearly delineate the extent of salt scalds, but also highlight areas of elevated conductivity not visibly affected by salt. EM-31 data has enabled the distribution of the salt to be mapped 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 saline material. Conductivity depth pseudosections and layered earth inversions indicate depressions of up to 140 m in the resistive basement are infilled with more conductive material. Ground magnetic data show the distribution of shallow Jurassic dolerite basement features and define a major negative amplitude anomaly which 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 more conductive material. This feature is inferred to be the major control on salinisation in the study area.