Results from a Combined Broadcast‐Low Induction Number System

Abstract

A system combining broadcast‐frequency plane wave impedance measurements and data from a ground conductivity meter has been developed to predict depth to shallow conductive layers. A theoretical analysis predicts that the resolution capabilities of such a system should be very useful for agricultural application. Preliminary field tests indicate the prototype EM+C system is able to distinguish relative resistivity measurements between sites. Measuring the amplitude ratio and phase difference between tangential plane wave electric or electric and magnetic fields allows calculation of the ground apparent resistivity. Plane wave penetration is skin+depth limited, and by measuring apparent resistivity at different frequencies, depth discrimination is possible. Measuring ground resistivity at AM and FM frequencies, together with the EM31 provides three different measurements of apparent resistivity, eliminating much of the ambiguity inherent in current EM31 surveys. A theoretical analysis using models with characteristics typical of Australian agricultural areas was performed to predict inversion capabilities of such as system. Gaussian noise with a standard deviation of 10% was added to data. It was found that the resolution is reliant on an adequate resistivity contrast between the upper layer and the basement. For resistivity contrasts of an order of magnitude, the inversion routine is able to resolve both upper layer resistivity and, importantly, layer thickness for layers thicker than about 50 centimeters. If layer thickness is less than 4 meters, an indication of basement resistivity is also provided. Preliminary field measurements show that a prototype system is able to predict relative resistivity between individual sites. Field data from a salinity‐affected region of Australia is also presented.