Bosm, W., EBERLE, D. and REHLI, H.-J. 1988. A gyro-oriented 3-component borehole magnetometer for mineral prospecting, with examples of its application. Geophysical Prospecting 36, A triple axis borehole magnetometer is described that consists of a Forster-probe (fluxgate) triplet (sensitivity 1 nT), a Forster-probe gradiometer (sensitivity 2 nT/40 cm), a gyro unit (mean angular drift approx. 0.5"/h) which is equipped with accelerometers (sensitivity 1/100"), and a data transmission unit (with multiplexer and 16-bit AD converter). The sensitive fluxgate-magnetometer can detect weakly magnetic or small source bodies. Data from the gyro and the accelerometers allow the 3-component magnetic field values to be transformed to north, east and vertical components. Since they do not rely on magneticallydetermined directional data, the results are not disturbed by local anomalies of the magnetic declination. Furthermore, the magnetometer can also be used in vertical boreholes. 3component measurements are carried out at discrete points in the neighbourhood of a source body to locate its position, and within the source body to determine the direction of magnetization. The strength of magnetization and information on magnetic classification are obtained by continuous measurement of one or more components within the source body. Calculation algorithms and computer programs are available to simplify data processing and interpretation. Survey examples are discussed. 933-961.Paper read at the 45th EAEG meeting

A study of four airborne electromagnetic data sets collected during the past decade by the BGR helicopter‐borne geophysical system over various geological settings shows that such data sets indicate not only layered horizontal geological features but also vertically and laterally confined structures, such as buried valleys. Indicative geological markers comprise conductive cover layers, conductive fills in resistive bedrock, and incisions in a conductive host filled with resistive materials. In cases where the valleys are incised into a magnetic basement and refilled with non‐magnetic material, the magnetic data records can also be used for identification and thickness estimates of the infill. From a hydrogeological point of view, buried valleys are becoming increasingly important as they host groundwater reserves, which are in many cases big enough to satisfy the future demand for fresh water. This paper describes two examples from the North‐West German lowlands and another two from the Namib coastal plains, where the Kuiseb and the Omaruru ephemeral rivers reach the shore of the South Atlantic Ocean.

The earthquake and the resulting tsunami on December 26, 2004, damaged the freshwater supply system in the coastal areas of northern Sumatra. In response to this natural disaster, the German-Indonesian HELicopter Project ACEH was initiated to assist the Indonesian government in its effort to plan and realize a sustainable reconstruction of community infrastructure by providing geophysical and hydrogeological data to serve as a basis for spatial planning. After two successful airborne geophysical surveys funded by the Federal Institute for Geosciences and Natural Resources of Germany, conducted on the north coast near the city of Banda Aceh and along the west coast between the towns of Calang and Meulaboh, Coca-Cola Foundation Indonesia funded an additional survey on the northeast coast around the town of Sigli. Helicopter-borne electromagnetics (HEM) revealed shallow freshwater resources up to several kilometers inland. Along the coast, however, the investigation depth of the HEM system was constrained due to near-surface saltwater. Here, ground-based transient electromagnetics was utilized on several profiles close to the town of Sigli, revealing deep coastal freshwater resources. The combination of airborne and ground-based electromagnetic techniques has proven to be highly effective to estimate the freshwater potential of the Sigli marshlands. Numerous sites for planned water wells could be identified as promising freshwater sources and approximate drilling depths provided.

A study of four airborne electromagnetic data sets collected during the past decade by the BGR helicopter-borne geophysical system over various geological settings shows that such data sets indicate not only layered horizontal geological features but also vertically and laterally confined structures, such as buried valleys. Indicative geological markers comprise conductive cover layers, conductive fills in resistive bedrock, and incisions in a conductive host filled with resistive materials. In cases where the valleys are incised into a magnetic basement and refilled with non-magnetic material, the magnetic data records can also be used for identification and thickness estimates of the infill.

The Federal Institute for Geosciences and Natural Resources of Germany (BGR) completed a research and development project aiming at optimizing its helicopter-borne geophysical system for high resolution site characterization. The overall objective was to adapt the existing helicopter-borne geophysical system used for groundwater and mineral exploration to survey conditions where the anomalies to be recorded are much smaller. The BGR helicopter-borne system permits simultaneous electromagnetic (AEM), magnetic (AMAG), and gamma-ray surveying. At the suggestion of the BGR, the AEM system manufactured by Geoterrex-Dighem, Toronto, Canada, was improved compared with the Dighem III system formerly used. The new system operates at five frequencies and the transmitter and receiver dipole moments are increased up to 25%. In addition, the system is now calibrated during flight. The sensitivity to waste objects was augmented by reducing the sensor heights from more than [Formula: see text] (AEM) and [Formula: see text] (AMAG), respectively, to less than [Formula: see text] about ground level by means of installing a magnetic sensor and a laser altimeter in the AEM bird. Enhanced spatial resolution was achieved by decreasing the sampling distance along line from about [Formula: see text] and by reducing the line separation from about a hundred meters to less than [Formula: see text] due to better navigational and positioning instruments. The modified system was tested over two former military training areas south of Berlin, Germany. Special surveys to locate steel drums, scrap metal, steel pipes, petrol tanks, ordnance, buried at depths from [Formula: see text] were carried out with nominal bird heights of [Formula: see text] and flight-line spacings of [Formula: see text]. Due to the extremely weak AEM and magnetic anomalies produced by these materials, suitable detection algorithms were developed to recognize and to identify these weak anomalies. They were tested using an airborne data set collected over an area where thousands of anomalies had been found. Many of them were subsequently verified on the ground. More than 90% of the anomalies selected for verification could be confirmed either by visual inspection of the ground surface or ground geophysical surveying or excavation. The modified AEM system not only allows better detection of waste but also better investigation of the environment. The AEM data could be reliably inverted to resistivity vs. depth sections using multi-layer inversion procedures. These resistivity data provide information about the hydrogeology and lithology, e.g., the depth of the groundwater table or the distribution of clay and silt. Thus, AEM can be successfully used for the hydrogeological and geological mapping of the near surface.

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