Video: Geomagnetic test measurements with an 8-channel fluxgate array mounted on a self-developed boat driven by an electric motor (near Potsdam).

Geomagnetic prospection 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.

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).

Fluxgate or Caesium-Sensors?

Archaeologically important magnetic anomalies usually range between 1 and 10 nT. When high resolution data loggers (24 bit ADC) are applied, both fluxgate-magnetometers and caesium-probes are well suited to archaeological prospection. The advantage of fluxgate-gradiometers is that they are less affected by interfering magnetic fields (e.g. modern features or natural fluctuations of the earth´s magnetic field). Furthermore their sensitivity to near surface structures is higher. The advantage of caesium magnetometers is a higher data resolution when measuring the total field. It is of benefit in areas with little magnetic variances as the precise data is very susceptible to noise. Large multi-sensor arrays are mostly set up with fluxgate-gradiometers which are less expensive.

Fluxgate vs. Caesium magnetometer. Mapping of a circular earthwork in Saxony-Anhalt (plough marks trending NS). © IKARE Martin-Luther-Universität Halle-Wittenberg, www.ikare.uni-halle.de
		Fluxgate Gradient, ±6 nT/m		Caesium Total field, ±6 nT/m
Systems and configurations
Three-wheeled fluxgate gradiometer array with 10-channel data logger, here: an 8-sensors array
Mobile fluxgate gradiometer array with 6-channel data logger, applicable in plantations and oliven groves.
Cart system setup with 4 caesium-sensoren and two 2-channel data logger (Geometrics) measuring the magnetic total field respectively magnetic gradient.
Portable fluxgate gradiometer array, in hilly terrain with swivel-mounted sensors
Prospection in shallow water: Multi-channel fluxgate gradiometer array fitted to lightweight boat - combinable with GPR and sonar investigations