Application of the semi-airborne transient electromagnetic method over the VMS deposit and data interpretation incorporating induced polarization effects

The semi-airborne transient electromagnetic (SATEM) method has attracted increasing attention for its efficiency in various exploration scenarios. A recent geophysical survey in northern Gansu, China, employed the SATEM system to investigate the potential distribution of Volcanogenic Massive Sulfide (VMS) deposits. Several observed data profiles showcase significant late-time negative values, which were attributed to induced polarization (IP) effects associated with VMS minerals, as prior time-domain IP (TDIP) measurements revealed their high polarizability characteristic in such regions. More recently, interest in interpreting TEM data with IP effects has notably increased in the geophysical community as these effects can significantly disturb the data, leading to misinterpretation using the conventional resistivity-only (RO) inversion approach. Guided by the Cole-Cole model, which quantitatively describes the IP effect of materials using DC resistivity and other three IP parameters, numerous previous inversion studies have been successfully conducted to extract multiparametric information.

In this work, one field data profile is demonstrated in Figure. 1a and was inverted using a quasi-2D hybrid constrained inversion algorithm including three terms: (1). The classical data misfit functional; (2). Laterally smoothing regularization; (3). Fuzzy c-means (FCM) clustering regularization, which can facilitate the integration of the prior geophysical information. Local geological investigations suggest that VMS targets are primarily deposited in intact fracture spaces, which offer favorable conditions for mineralization and storage. The inversion results, shown in Figure. 1b, display clearly high-to-moderate resistivity interfaces surrounded by distinct IP value distributions. Moreover, the extending high IP distribution toward the deep is supposed to result from mineral dissemination, resulting in high resistive polarization anomalies and deeper conductive polarization anomalies caused by mineral enrichment. The above characteristics are considered the indicators of VMS minerals in such area.

To sum up, the IP-incorporated inversion facilitates the interpretation of TEM data collected over high polarization areas. However, the serious ill-pondness issue of multiparametric inversion brings a great challenge to result reliability, which largely depends on the selection of the starting model and inversion scheme. Integrating the geological and geophysical information in the inversion offers a promising way to avoid misinterpretation