Chưa thể tính được trữ lượng của loại nước ngầm nguồn gốc nội sinh này, nhưng nó giữ vai trò to lớn trong việc cung cấp nước thường xuyên cho các sông suối từ các vùng núi cao và sẽ cung cấp nước sinh hoạt một cách bền vững cho cư dân ở vùng núi cao, vùng trung du, hải đảo và sa mạc . | 210 Anders Vest Christiansen Esben Auken Kurt Sorensen At early times the offset configuration is extremely sensitive to small variations in the resistivity in the near surface. Extensive 3D modelling of such variations shows a pronounced influence on the measured fields before the current system passes beneath the receiver coil. In many cases these data are not interpretable with a 1D model even if the section is predominantly 1D. At later times after the current system has passed the distorting influence has decayed. The central-loop configuration is much less affected by near-surface resistivity variations. The offset configuration is sensitive to small deviations in the array geometry. For a simple 60 m half-space model a 30 error in the response is apparent near the sign change if the receiver coil is located 71 m instead of 70 m from the transmitter. In a routine field situation it is next to impossible to work with such accuracy. After the sign change the offset configuration is essentially equivalent to a central-loop configuration the central-loop configuration is insensitive to the placement of the receiver inside the transmitter loop. A compromise is to use a high-power system where early times are measured in the central loop configuration with a small current of 1 - 3 A. Late times is in turn measured in the offset configuration with maximum output current. In this way the four issues are addressed and the field production can still be kept high. Airborne TEM In this chapter we will give an overview of the airborne TEM system and discuss the specific topics where the airborne and the ground based techniques differ. We will focus on the relatively new helicopter systems as they have the sufficient accuracy necessary for groundwater investigations. Historical background and present airborne TEM systems. Airborne electromagnetic systems AEM have been used for more than 50 years. The development was driven by the exploration for minerals with its
