ELECTROMAGNETIC METAL DETECTOR Ramboll is making use of a highly sensitive electromagnetic metal detector to map metal objects below ground. Introduction The currents generate a The electromagnetic metal secondary magnetic field that detector (EM) can be used for a can be measured as an electrical variety of purposes: current in the coils. By measuring the electric current in • Locating buried tanks • Landfill delimitation the two coils after approx. 0.1 • Mapping of pipelines second, one can quantitatively • Munitions search measure the occurrence of • Archaeological studies metallic materials near the Figure 1: The operator carries a instrument. The measurement is backpack with batteries and a Equipment practically independent of the control unit together with a The instrument consists of 2 geological conditions. handheld computer. square coils of 1 m x 1 m with 0.4 meters between them. On Data Collection the lower coil there is a wheel, Data is collected by covering the so the metal detector can be survey area with parallel target pulled over terrain. The bottom lines. The spacing between coil acts as a source; an electric target lines depends on the Contact pulse is transmitted through the desired degree of detail. To Uffe T. Nielsen coil and generates a short-term locate single objects, data Project Director magnetic field. This magnetic should be collected with ½ - 1 +45 5161 6782 [email protected] field generates currents in all meter between the lines. By conductive materials near the delimiting groups of objects over Gert Andersen coil. After approx. 0.1 second, a larger area (for example, a Geophysicist there will only be electro- landfill), larger line spacing can +45 5161 6705 magnetic currents left in metallic be used. [email protected] materials. Rambøll Danmark Hannemanns Alle 53 DK- 2300 København S www.ramboll.dk degree of horizontal accuracy in Data is typically measured in the interpreted by a geophysicist form of the induced electrical with experience in this field. the designation of objects. By current for every 20 cm along combining data from the two the line. The position of the coils, it is also possible to Applications instrument is determined by estimate the depth of buried means of traditional land survey Buried tanks can be detected to objects. methods, or with GPS, approx. 3 meters deep. Because Archaeological studies can be depending on the scope of the there will often be other metal performed quickly over a large instrument and accuracy installations at the site, it is area with EM, providing an requirements. advantageous to use an instrument with two coils. By overview of buried metals with a combining and comparing data high degree of sensitivity, and from both coils, external noise including metals that a can be suppressed during data magnetometer would not detect, processing. This also enables such as bronze, silver, and gold. objects to be differentiated depending on their distance from Detection Level the surface. The strength of the measured Landfills, which are often response signal from metallic incompletely mapped, are found objects is affected by several often in Denmark. They pose a factors, including the distance latent threat to groundwater from the coil, the shape of the resources. Since landfills often object, and the material itself. contain large amounts of iron The detection level – that is, the scrap, they can be quickly and response sufficient for the safely delimited by EM. detection of objects – depends on the surroundings. Signal Pipelines and cables will appear noise from other metal objects as line structures when mapping occurs frequently, which with EM. Therefore, they can be increases the required detection interpreted with a high degree of level for a valid response. confidence, even with strong background noise from other Ramboll has performed several full-scale tests before solving metal objects. Weak signals from deep pipes and cables can major tasks; for example, Figur 2: EM data from a military performing a munitions search in testing field. The data shows the also be interpreted with position of unexploded grenades, confidence. a test field with buried detonation pipes, and shrapnel. A 5 munitions. meter grid has been drawn in. Munitions searches are a task Data processing and well-suited to EM mapping. The In connection with archaeo- interpretation EM sensitivity is comparable to logical research, Ramboll has the magnetometers traditionally measured the response from Ramboll has extensive well-preserved excavations to experience from solving many used for these tasks, but EM assess the level of detection tasks by electromagnetic metal data from is far easier to detector. EM data is always interpret and results in a higher prior to a similar search for buried artifacts. .