oa Overly steep decays in airborne TEM data and their link to chargeability: example from the Howards East District, NT, Australia

The induced polarization (IP) response in airborne electromagnetic data has recently received attention due to its potential significance for mineral exploration and environmental applications, while also holding the prospect of improved geophysical models. A typical IP response produces negative transients in the late time gates. However, under certain circumstances, the presence of overly steep decays in themselves indicate the presence of chargeable material in the subsurface, i.e. where the chargeability is not strong enough to reverse the sign above the noise level, although it still decreases the signal.

In this contribution, we analyse a survey in the Howards East District in Northern Territory, Australia, and synthetic data. After a standard processing and removal of all noisy and negative data, we proceeded with a standard inversion that resulted in poor fit and unrealistic high resistivity in deeper layers in parts of the survey. Next, we inverted the data with a dispersive earth model, i.e. including the IP parameters. In the parts of the survey where the data could be fitted well with the “resistivity only” inversion the models changed minimally. In the remaining part, mostly with overly steep decays, the inverted models show chargeable, relatively shallow layers, the unrealistic high resistivity was not present anymore and in general, the data residuals dropped to be within the uncertainty of the data.

We further analysed the data, specifically: 1) the soundings linked to chargeable areas; 2) the loss of information due to neglecting the negative or late time data with synthetic models; and 3) we explored the ability of the IP inversion models to recover the moderately chargeable layers. In conclusion, a large-scale airborne IP inversion was executed without any specific processing. Until now a time-consuming specific processing seemed to be a necessary step towards a successful IP inversion with airborne data.