Exploration in weathered terrains 1989 perspective

Abstract

The ancient land surface of much of Australia is arid and deeply weathered. This leads to both problems and advantages in the application of geophysical methods to mineral exploration. The deeply weathered overburden, usually porous and permeated with saline groundwater, often forms a barrier for electrical and electromagnetic methods. Induced polarisation effects are often observed in TEM surveys and they can frequently be explained by shallow sources in the weathered zone. The weathering process also forms maghemite, a magnetic iron oxide which can obscure the magnetic response from deeper targets or cause spurious anomalies. Preferentially deeper weathering over sulphide mineralisation or kimberlites often causes electromagnetic anomalies which can be used for indirect detection of these bodies, or for geological mapping. High sensitivity, low altitude airborne magnetic surveys are readily applied in weathered terrains for geological mapping. They reduce the masking effects of near surface maghemite and can often effectively map paleodrainage, by detecting maghemite concentrated in palaeochannels. Maghemite causes superparamagnetic effects in TEM surveys which reduce the depth of investigation of the technique. Field procedures have been developed to minimise this effect.