Geophysical modelling of Strenči magnetic anomaly in Latvia using SimPEG open-source software

Abstract

Since the European Union is foreseeing a great increase in the demand of critical raw materials in the coming decades, new ore deposits will have to be explored (European Commision 2020). This has given an incentive to renew the mineral exploration also in Latvia, with a few of the magnetic anomalies known to be caused by various metal ore deposits, all of which lie in the Proterozoic crystalline basement. One of these anomalies lie in the northern part of Latvia – the Strenči magnetic anomaly. A geophysical exploration, involving magnetic, gravity and seismic exploration, as well as core drilling and geochemical analysis, was carried out in the late 20th century (Vetrennikov et al. 1986). Since then, no exploration has been done in the area. But the advent of powerful open-source modelling software which runs on consumer-grade computers has presented an opportunity to build new geophysical models based on old and new data.To examine the possibility of developing a modern geophysical model of the Stenči magnetic anomaly, an open-source software SimPEG (Simulation and Parameter Estimation in Geophysics) was used (Cockett et al. 2015). The input data was Total Magnetic Intensity (TMI) measurements - a combination of the data from the exploration in the last century and newly acquired data. Quality control measurements of the previous data revealed uncertainty of ±135 nT, giving a rather large uncertainty of approximately 6% for the combined input TMI data. Based on the previous geophysical core logging data it was determined that the magnetic ore maintains a large remanent magnetization which may severely impact the geophysical model (REF). Thus, two models were developed: the first one based on the inversion of the magnetic susceptibility, but the second one using Magnetic Vector Inversion which takes into account remanent magnetization.The developed models and data from previous research allowed to conclude that the magnetic anomaly is caused by metamorphosed granulite facies crystalline basement rocks of the Proterozoic to a depth of five kilometers. It was also discovered that the magnetization vectors coincide with the general direction of the dip of the rock strata. It was calculated that the predicted ore body contains a significant amount of critical minerals used in renewable energy technologies. A conclusion was made that the previous research has gathered a wealth of data, that can be used in regional crystalline basement research but not for detailed geophysical exploration of anomalies due to the uncertainty of the old data. The use of open-source software has enabled a very cost-effective development of sophisticated geophysical models which are impaired only by the quality of input data. Development of such models may be the first step into geophysical exploration which may attract interest for further investment.This research was funded by “MikroTik” and University of Latvia Foundation, project no. 2258, and by the University of Latvia grant No. AAp2016/B041//Zd2016/AZ03 project “Climate change and sustainable use of natural resources”.