INVERSION OF TIME-DOMAIN ELECTROMAGNETIC DATA FOR A HORIZONTALLY LAYERED EARTH

Time-domain electromagnetic (TEM) data are inverted to produce a conductivity model composed of horizontal layers of constant conductivity. The data can be values of the time decay of the vertical component of the magnetic field, or of its time derivative, measured at points either inside or outside a rectangular transmitter loop. Our inversion allows many more layers than there are data. This means that the constructed conductivity model not only fits the data to the required level, but also possesses particular characteristics. By suitable choice of the objective function to be minimized, our constructed model may have minimum structure in some well-defined sense and/or it may be close to some known background model. Our inversion algorithm works directly in the time domain. This requires fractionally more computing time than the alternative approach of transforming the data to the frequency domain before inversion. However, working in the time domain prevents distortion of the data and their associated measurement errors which may arise during the transformation. Also, the effects of the full transmitter current waveform can easily be incorporated by convolution in the time domain. Our inversion is applied to data from an environmental survey and the results are shown to compare favourably with a nearby well-log.