Improving conductivity models using on-time EM data

Abstract

Modelling of data from half-sine time-domain airborne EM systems is traditionally limited to the period following the transmitter turn-off, the off-time data. The joint use of on-time and off-time data widens the resolvable range of conductivities. Using a half-sine waveform with a base frequency of 25 Hz and a pulse width of 4 ms, conductivities and conductances as low as 0.5 mS/m and 0.02 S, respectively, become resolvable. Also, with the constraint of on-time data, automated modelling becomes more stable as noise at late time is less likely to be confused with signal.Conductivity-depth sections of QUESTEM-450 (Lane et ah, 1998) data from Forrestania, Western Australia were modelled using a layered-earth inversion (LEI) and a current imaging technique. The known mineralisation, including massive sulphides and a banded iron formation, is indicated by the off-time data. For both methods, additional use of on-time data led to a model improvement.