Should archaeologists be concerned about naturally varying magnetic fields?

Magnetic surveys of archaeological sites can be influenced by the natural time variations of the Earth’s magnetic field, and to a lesser extent its spatial variations. The influence of the natural time variations could be especially problematic for single sensor surveys with limited base station coverage, and it is this aspect we address in this presentation. At any one location in the UK the magnetic field varies by 10s of nanoTeslas (nT) every single day, and by 100 to 1000s of nT during magnetic storms. We quantify the global characteristics of the time-varying field using hourly standard deviations from approximately 150 sites throughout the world and spanning over 40 years. We illustrate in detail how they vary with location, time of day, month and phase of the solar activity cycle. The most vulnerable magnetic surveys are those done at archaeological sites in or near the high latitude auroral zones, especially during the local night time (fortunately unlikely from a practical point of view) in March and October in the maximum and descending phases of the approximately 11-year solar activity cycle. Surveys done close to the dip equator are also vulnerable. We describe briefly the causes of these patterns. The existence of spatially incoherent signals in archaeological magnetic surveys may sometimes be difficult to deal with in the post-survey analysis and independent data, from a base station or from a nearby observatory or variometer station, could be helpful in this respect. It should be noted that the forthcoming maximum in solar activity is expected in 2013/14. Details are provided of the network of observatories and variometer stations that could help isolate and remove these time-varying signals from archaeological magnetic survey data by providing substitute base station data in near real time. The World Data Centre for Geomagnetism operated by the British Geological Survey in Edinburgh and available online at www.wdc.bgs.ac.uk, is a good first point of contact for magnetic data and metadata from observatories around the world. In the UK continuous magnetic data series are available from the observatories operated by the BGS at Lerwick in Shetland, Eskdalemuir in Dumfries and Galloway and Hartland in Devon