Magnetic survey Spatial deviations of the natural magnetic field can be registered with the help of geomagnetic measurements. These deviations occur due to magnetic embeddings in the top soil which affect the magnetic field characteristics locally. The natural magnetic field is a three dimensional vector field measured in Tesla [T ] (SI unit describing magnetic field strength). It is composed of the actual Earth's magnetic field (ca. 50.000 nT), distortions caused by solar storms and a local anomaly field (up to several 100 nT). When it comes to industrial and archaeological prospection the latter is the component of main interest. Geomagnetic prospection is often the most effective survey method for non-destructive investigation of archaeological sites. Herein two techniques can be used to obtain the data of interest. Either the field components are separated during the measurement using orientated gradiometers, or the total field is measured and the components are separated in subsequent data processing.
Magnetic anomalies of archaeological features result from induced or remanent magnetism. Induced magnetism is caused by the Earth's magnetic field and is very much dependent on the susceptibility of the material. The susceptibility describes the degree of magnetization of a material in response to an applied magnetic field and can be either positive or negative. Magnetic anomalies of pits, ditches and postholes for instance are often detectable through positive induced magnetisation of a few nT. Remanent magnetisations have various causes, a well known one being thermoremanent magnetisation. This occurs when ferromagnetic materials are heated above the Curie-Point and cool down again, causing the magnetic moments to partially align within magnetic domains. Thus, fireplaces, burnt clay, fired ceramics and bricks can easily be detected on account of their special magnetic characteristics.
|Roman city of Ammaia (Portugal), eastern suburbs: magnetic mapping (+/-30 nT) and GIS-based interpretation. Project RADIOPAST.|
Eastern atlas’ aim to produce high quality magnetic maps necessitates high resolution. Therefore dense measure grids are chosen in the field. Data points are collected every few cm in the direction of the profile, the sensor separation being 50 cm. With the help of a multi-sensor cart system up to 10 sensors can be carried at a time. The special design allows the cart system to be operated even over complex and rough terrain. In this way, non-destructive magnetic investigations of large areas can be completed in reasonable times. Between 2 and 10 ha per day can be surveyed depending on terrain conditions (steepness, soil, vegetation especially trees, modern obstacles etc.). Furthermore the system is set up with real-time GPS-positioning and special hardware and software that makes it possible to visualize and interpret the magnetic data whilst still in the field. Our team is continuously developing new hard- and software solutions with a view to offering the best magnetic maps technically possible. We also offer bespoke solutions for special or difficult investigation areas (eg. geomagnetic in olive groves with retractable probes).