On-time modelling using system response convolution for improved shallow resolution of the subsurface in airborne TEM

We describe a new approach for modelling airborne transient electromagnetic (TEM) data which combines the use of on- and off-time data for inversion. Specifically, the response is modelled using system response convolution both during and after transmitter ramp-down. High near-surface sensitivity can be achieved through a combination of fast transmitter ramp-down, broad receiver system bandwidth, efficient suppression or explanation of the primary field, and by combining the use of on-time gates with accurate knowledge of the system response. The system response can either be calculated based on the transfer function of the individual system components (i.e. receiver coil, amplifiers, low-pass filters and current waveform) or it can be measured at high altitude. The latter approach has the advantage of avoiding the specific modelling of individual system components. By comparing model parameter uncertainty when the on-time gates are included in the inversion versus when they are not, we show that a significant improvement in near-surface sensitivity is obtained. The method is used to invert both synthetic and field data. In the inversion of synthetic data, we see clear improvements in the determination of thin shallow layers, especially when they are resistive. This is confirmed by inversion of field data where we observe more pronounced structures with better definition of layer boundaries and layer resistivities.