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In magnetic prospecting, the total field anomaly formula that represents the projection of the magnetic anomaly vector on the geomagnetic field is widely used because it simplifies the calculation of forward modelling and inversion of magnetic data. However, the projection anomaly yields errors relative to the true observed magnetic anomaly, especially for high-amplitude magnetic anomalies such as in iron orebody and unexploded ordnance prospecting. In this study, we analyse the difference between the projection anomaly and observed modulus difference anomaly with physical parameters, and propose to directly invert for the modulus difference anomaly by constructing a nonlinear matrix equation between the model corrections and data corrections. The inversion is then implemented using a preconditioned conjugate gradient algorithm. Synthetic and field magnetic data were used to test the inversion method. Comparison of the two types of total field anomalies shows that the error of the projection anomaly increased with increasing total-field magnetic anomaly. When the total-field magnetic anomaly was < 5,000 nT, the difference between the projection anomaly and modulus difference anomaly results can be ignored. For high-amplitude magnetic anomalies, the modulus difference anomaly inversion produced more accurate representations of both the shape and location of the magnetic sources.

Inversion of high-amplitude magnetic total field anomaly: an application to the Mengku iron-ore deposit, northwest China