干旱区生态环境知识资源中心

As an effective and efficient geophysical tool, airborne EM (AEM) is specifically suitable for the exploration in areas of high mountains, desert, swamp, and forest. With the development of national economy of China, the demand for mineral resources increases sharply, geophysical explorations in areas with favorable geological conditions have been accomplished. The exploration target is switched to areas with unfavorable and complicated geological conditions, such as in Western China. Airborne EM works efficiently due to its moving platform of helicopter or fixed-wing aircraft, no human access is needed to the survey area. This technology is especially applicable for exploration in Western China with rugged mountains and deserts. In this paper, we review AEM technology systematically with the goal to make this technology quickly applicable in the mineral exploration in China.

To make a comprehensive review, we introduce the AEM technology in following sequences. We first present basic AEM theory of on/off time, apparent resistivity and depth, footprint, etc. Then, we introduce the developments of AEM technology in Western countries, including CGG/Fugro, Geotech and Aeroquest in Canada, SkyTEM survey in Denmark, and we address specifically the status of this technology in China. Since the modeling and inversions are fundamental for AEM, we present 1D/2D/3D forward theory based on semi-analytical solutions, finite element, finite difference and integral equation methods. For the inversion theory, we follow the rule from simple to complex models by presenting 1D Marquardt, LCI, and 2/3D inversions by GS, NLCG and QN. Finally, we introduce successful applications of AEM technology in mineral, oil & gas, E&E, ground water, and natural hazard forest. We give our suggestions on future development of AEM technology in China.

AEM technologies have been well developed in Western countries and are playing very important role in mineral, oil & gas, E&E, ground water explorations. However, AEM has not been well developed in China to the extent of practical use due to the fact that no breakthrough has been made on key technologies. Due to the fact that time-domain and frequency-domain AEM have different features and applications, we suggest to develop AEM technology in both frequency-and time-domain in China. The frequency-domain AEM is used in EKE, ground water exploration, while time-domain AEM is used in mineral, oil & gas in deep earth. Semi-airborne EM has serious problems of volume effect and thus is not recommended. Unmanned aerial vehicle (UAV) has a small load, making it difficult to transmit high-power signal. Besides, UAV doesn’t suit the complex climate and topography, so that it is not recommended, either.

To explore the vast area in Western China with rugged mountains, desert, etc., we need to develop geophysical technologies based on moving platforms, like airborne EM. Considering that different AEM systems suit for different exploration targets, both frequency-and time-domain systems need to be developed. Both semi-airborne and UAV are not recommended for airborne EM due to their inherent defects, a full airborne EM technology is the best solution.