ANNALES 10/'^ 7 original scientific paper UDC 902.3:550.38 MAGNETIC SUSCEPTIBILITY MEASUREMENTS IN DOHNAS Blanko MUSIC University of Ljubljana, Faculty of Arts, Department of Archaeology, Si-1001 Ljubljana, Zavetiika 5 ABSTRACT On the basis of measurement of apparent magnetic susceptibility directly on the sections in dolinas, we can conclude that in the favourable conditions this method can be efficiently used as an additional method in the archaeological prospection in the karst, where other geophysical methods are less successful due to the specific natural conditions. Key words: archaeology, arhaeological projection, geophysics, karst, magnetic susceptibility, dolinas GENERAL I INTRODUCTION The aim of the magnetic susceptibility measurements in dolinas was to detect, in a fast and reliable way, the layers containing burnt fragments (use of fire in the past) which, associated with the archaeological survey, would provide us with a better archaeological interpretation of the dolinas. The use of magnetic measurements in archaeological prospecting Is based on two principles (Clark, A. 1990, 99); in the first place, top soil usually has a higher magnetic susceptibility than the underlying layers and geological bedrock (this tact was explained in the early works of Le Bojgne, E. 1955, 1960), and secondly, human activity increases the magnetic susceptibility of the soil. Magnetic properties of the soil are to a large extent determined by the concentration and the type of iron minerals it contains During accumulation these tend to concentrate in the upper layers of the soil. In temperate and humid climates, goethite Is the most common iron mineral found in soil, while in arid climates haematite prevails. An iron hydioxide, lepidocrocite is less common than goethite, and can usually be found in water saturated soil. During dehydration it can be converted into a much more magnetic maghaetnilfe (Clark, A. 1990, 100). Maghaemite b particularly important in archaeological prospecting, as its presence is considered to be an indication of human occupation. Its crystal structure is the same as that of magnetite and is similarly strongly magnetic. Numerous processes, some of them clue to human activity, can increase the magnetic susceptibility of soil. Le Borgrie (5955 and 1960) suggested that the transformation of haematite to maghaemite, with magnetite as an intermediate stage, was linked to the alteration of anaerobic (reducing) conditions in soil to aerobic (oxidizing) ones Tire same transformation can occui through the burning of organic materials; in the reducing environment of a fireplace, haematite is first converted into magnetite, which subsequently, during the cooling of ion air, is deoxidized to maghaemite. According to Tite and Mullins (1971), variations of the magnetic susceptibility on an archeological site are determined by the concentration of magnetic iron oxides in the soil, the duration of the use of fire at the site, and the reducing conditions created below the fire place. Consequently, a high magnetic susceptibility over a large area is typical for the sites that have been intensely inhabited for extended periods of time. Where the intensity and duration of settlement have not been extensive, only some archaeologicatly interesting structures, e.g. fireplaces, ceramic kilns, etc., can be located by means of the magnetic susceptibility measurements (Tite, M. S. and Mullins, C. E. 1971, 219). For archaeological prospecting on prehistoric sites magnetometry is frequently used as an efficient mapping tool, if no recognizable structures are present, or it due to erosion (or other causes) the site has "changed" its location, or if magnetic responses of archaeological 37 ANNALES 10/' 97 Branim MUŠlC: MACWTIC SUSCEPTIBILITY MEASUREMENTS iN DOL IN AS, V/ 4.' ¥ 20- "Q m8atv-2.6xi0 SI fid- 1.6x10""Si Cnhanccd •W 1s magnetic susceptibility Anomaltc-i - Ifflkj inrchiicology'») I—T1"1 i 9 lu II 12 apparent magnetic susceptibility {xl(T SI) T I i 6 7 Fig. 1: Distribution of apparent magnetic susceptibility values for all surveyed dolinas. SI. 1: Razporeditev vrednosti navidezne magnetne sus-ceptibilnosti za vse preučevane vrtače. features are very weak, the application of such methods is limited. In these cases direct measurements of magnetic susceplibiiity can be useful. For example, detailed measurements of the magnetic susceptibility of small samples of clay can be used for characterizing such samples. The response of a sample to an applied magnetic field depends on its mineral constitution, as well as its thermal history; measurements of its magnetic susceptibility (magnetization per unit volume and unit magnetic, field) at several magnetic field strengths are necessary to fully identify it (Dime, F. etal. 1994, 226). Such a series of measurements are analysed on a sample by the standard Honda-Owen method (Chern, M. Y. et al. 1992, 418), This method was applied lor detecting changes in the magnetic susceptibility in areas with very weak magnetisation that appear on carbonatic flysch bedrock. Soil samples were first heated in an oxidizing atmosphere to the temperature of 650° and later cooled at room temperature in a reducing atmosphere. The average final magnetic susceptibility value was three times as great as the initial one (Dime et al. 1994, 229). Using the same method, a high level of correlation between the magne-tometry, geochemistry and surface survey (distribution of slag and pottery) was achieved in the Late Roman settlement AjdovSiina above Rodik in SW Slovenia (MuSiC et al. 1995, 14), The method has proved to be exceptionally suitable also for detecting areas of burning in prehistoric sites in the Kras (MuSic et al 1994, 42). Fig. 2: Location map of dolina 14 near Avber. $/. 2: Položaj vrtače 14 pri Avberju. 38 ANNALES 10/97 BrjiikoMUSlC MAGNETIC SUSCCPTdJUITV MCAîuRLML.VIS In' DOI.tNAS. 37-42 APPLIED FIELD METHODOLOGY Apparent magnetic susceptibility in dolinas was measured with Kappameter KT-5 field instrument (Geofyzika, Brno.) (with a resolution of 1x1SI). The fundamental part of the instrument is an LC oscillator of 10 kHz, the inductivity of which is embodied by a flat measuring coil situated at the active face of the instrument. The frequency of the oscillator is measured with the coil at some distance from the rock/soil ("free space" measurement) and then with the coil applied to the surface. From the frequency difference the apparent susceptibility is computed by the microcomputer and displayed (The instruction manual for Kappameter KT-5, Geofyzika, Brno) This instrument allowed us to measure apparent magnetic susceptibility directly on the surface of the sections in dolinas. As magnetization in dolinas proved to be very high (2,6x10'3 Si [= Sieved] on average) and a:, it was possible to detect the relative differences in magnetic susceptibility among the samples, we avoided the more precise but time consuming measurements on Manics DSM8 and interpretation according to the Honda-Owen method The position and number of tine sections were selected for each dolina separately, according to their size, preservation of the sections, number of sherds discovered, etc. We measured the magnetic susceptibility of soils in parallel sections extending from the top to the bottom of each dolina. The distance between the measuring points was 20 cm. n—>—i—'—r : enhanced matiietizaliot! Lin boriiOHtal Tajer anomalies profite enhanced magnetic RoreplibiHyfJ-ts) background values T-'— —.— A ----8 -C ._._._ D ...... H ........f ........2s) background valuês (<2s) j 6 s i« ¡2 m hi is 20 sampling [mints (on 0 2m distance) Fig. 8: Measured values of apparent magnetic susceptibility in four profiles in eastern section of dolina 76 near Krepljc. Statisticaly significant anomalies of enhanced magnetic susceptibility were detected in thin irregular layer. Si. 8: ¡zmerjene vrednosti navidezne magnvtrie suscep-iibilnosti v Štirih profilih v vzhodnem delu vrtače /6 pri K repi j ah. Statistično značilno povišane vrednosti magnetne susceptibilnosti so bile ugotovljene v tanki poviti plasti. 41 ANNALES 10/'97 Br«*