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

Jianhua Yang,Shuang Liu,Xiangyun Hu

doi:10.1038/s41598-020-68494-1

Abstract

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.

Highlights

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
We define the error between projection anomaly and observed modulus difference anomaly firstly and carefully discuss the error caused by amplitude, inclination and declination individually and together between the projection anomaly results with those of the modulus difference anomaly for strong magnetic anomalies
The error from using the projection anomaly as an approximation comes from two elements: the value of the magnetic anomaly and the magnetization direction

Summary

Comparison of the projection and modulus difference anomalies

We first test the method through the cube model, which has a length of 100 m and a central burial depth of 100 m. The error is so obvious that projection anomaly is no longer suitable for interpretation of high-precision measurements This indicates that ΔTpro cannot be regarded as equal to ΔTtrue in strong magnetic bodies because the error between them will be further enlarged during data processing and interpretation. When the error is calculated for data with the magnetic remanence (Fig. 5b; I0 = 45°, D0 = 0°), the amplitude of the magnetic anomaly is 24,721 nT and the maximum error is 6,935 nT, which is 28% of the total field magnetic anomaly. Take the error seriously and use modulus difference anomaly rather than projection approximation for forward modelling to meet the needs of high-precision and quantitative process and interpretation in mineral exploration

Inversion of the projection and modulus difference anomalies

Field example

Conclusions

Findings

Additional information