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Higher precision at higher speed: Geomagnetic prospection near the threshold of sensitivity with the digitiser LEA D2
Last modified: 2011-12-21
Abstract
A new multi-channel digitizer for fluxgate gradiometer arrays is presented. It is characterised by a very high measuring resolution, broadband ADCs of 24 Bit bandwith, sampling rates up to 500 Hz and flexible GPS interfaces. Extensive field tests with several sensor types have been realized since 2010. The ruggedized and waterproof digitizer was succesfully applied in several large-scale archaeological prospection projects in Germany, Spain, Portugal, Italy, Jordan and Turkey. The advantages of the new geomagnetic system are shown by large-scale prospection examples of archaeological structures of very low magnetisation. Especially sites situated in sandy environments are often characterised by unfavorable conservation conditions for organic remains due to an increased acidity of the soil. But compared to other fluxgate arrays even very small magnetic anomalies in the range of ±1 nT can be detected in these soil types. Case studies from several sites in Germany and Turkey are presented.
The economic advantages of fluxgate magnetometers especially in large-scale prospections of archaeological sites and landscapes provided the starting point of the efforts to improve multi-channel fluxgate systems. In contrast to alternative magnetometer types applied in archaeological research like caesium (Cs) or SQUID probes, fluxgate magnetometers can be assembled to large arrays (6 to 16 probes) with comparatively low costs. Only such arrays allow the fast and efficient prospection of large areas.
The most important precondition for the successful application of fluxgate arrays in archaeological research is a high-quality data logging exploiting the dynamic range and the maximal resolution of the probes to a maximum extend. Using a high-resolution broadband data logger with high sampling rates (up to 1000 Hz) the lower measuring accuracy of fluxgate sensors compared to Cs or SQUID sensors can be fully compensated.
Fluxgate sensors can provide equivalent results to Cs sensors in archaeological prospection. The approx. twentyfold better accuracy of Cs sensors (ca. 0.01 nT) compared to fluxgate sensors (ca. 0.1 to 0.2 nT) take only in few cases effect in archaeological applications. The reason for this is, that the crucial factor for the successful detection of archaeological structures and objects is the the ratio of their proper and the ambient magnetization. The ambient magnetization (caused by geological and anthropogenic structures) superimposed by the magnetic anomalies of the archaeological targets determines the practical range of the required measuring resolution. Due to this limitations the dynamic range of archaeological magnetograms rarely drops under ±1nT. Below this range of ±1nT the magnetograms are usually oversaturated i. e. geological and anthropogenic influences dominate the results.
From 2009 to the beginning of 2011 the new multi-channel digitiser LEA D2 for geophysical measuring systems was developed. The project was funded by the ZIM program of the German Federal Ministry of Economy. By the end of 2011 the digitiser was prepared for a broader commercial exploitation.
The economic advantages of fluxgate magnetometers especially in large-scale prospections of archaeological sites and landscapes provided the starting point of the efforts to improve multi-channel fluxgate systems. In contrast to alternative magnetometer types applied in archaeological research like caesium (Cs) or SQUID probes, fluxgate magnetometers can be assembled to large arrays (6 to 16 probes) with comparatively low costs. Only such arrays allow the fast and efficient prospection of large areas.
The most important precondition for the successful application of fluxgate arrays in archaeological research is a high-quality data logging exploiting the dynamic range and the maximal resolution of the probes to a maximum extend. Using a high-resolution broadband data logger with high sampling rates (up to 1000 Hz) the lower measuring accuracy of fluxgate sensors compared to Cs or SQUID sensors can be fully compensated.
Fluxgate sensors can provide equivalent results to Cs sensors in archaeological prospection. The approx. twentyfold better accuracy of Cs sensors (ca. 0.01 nT) compared to fluxgate sensors (ca. 0.1 to 0.2 nT) take only in few cases effect in archaeological applications. The reason for this is, that the crucial factor for the successful detection of archaeological structures and objects is the the ratio of their proper and the ambient magnetization. The ambient magnetization (caused by geological and anthropogenic structures) superimposed by the magnetic anomalies of the archaeological targets determines the practical range of the required measuring resolution. Due to this limitations the dynamic range of archaeological magnetograms rarely drops under ±1nT. Below this range of ±1nT the magnetograms are usually oversaturated i. e. geological and anthropogenic influences dominate the results.
From 2009 to the beginning of 2011 the new multi-channel digitiser LEA D2 for geophysical measuring systems was developed. The project was funded by the ZIM program of the German Federal Ministry of Economy. By the end of 2011 the digitiser was prepared for a broader commercial exploitation.
Keywords
magnetics, low magnetisation, data logging, high-resolution, multi-channel systemslandscape