Simultaneous interpretation of electromagnetic and magnetic data using linear filtering

Soil magnetic properties are clearly different from those of the parent rock. The correlation they exhibit with a wide range of pedogenic and anthropogenic processes constitutes a major source of information in both Soil Sciences and Archaeology. They can be measured either by the magnetic method which records the lateral changes of any types of magnetisation or by electromagnetic prospecting which measures magnetic susceptibility (in frequency domain) and magnetic viscosity (in frequency or in time domain). For Slingram E.M. frequency domain measurements, it was established that under the "Low Induction Number" assumption, the magnetic susceptibility response is in phase with the primary field. Furthermore, the susceptibility response of a given magnetic feature is linear: it corresponds to the convolution product of the response of a point magnetic source, the impulse response, by the susceptibility distribution. As magnetic anomalies generated by induced magnetisation contrasts are a convolution of the magnetic impulse responses by the same susceptibility distribution, it is theoretically possible to calculate directly from E.M. data the induced magnetisation anomaly and to establish the possible existence of other types of magnetisation. However, the impulse responses for the two methods correspond to different functions of the depth, z, of the sources; this parameter may also explain differences between magnetic and E.M. results.