An EM Perspective of Selected Topographic Highs in Transdanubia Ahmed Al-Yaqoobi 2 , Judit Petrovszki 1 , Joseph Michael Court 2 1 Eötvös Loránd University Faculty of Science 2 University of Leeds, School of Earth and Environment 1 st -14 th September 2007 Using three parallel electro magnetic methods the project aims to explore the area to the East of Lake Balaton including Öreg Hegy and the Enying ridge in an attempt to explain the spatial distribution and origin of the topographical highs. The methods are quick to apply and provide instant results. Location map Location1 map Location2 map Location3 map Introduction A Scintrex ENVI VLF unit was used. VLF uses the high-power military communication transmission in the 15-30 kHz bandwidths which are continuously transmitted by many stations around the world. Eddy currents induced by a VLF magnetic field in conductive material produce secondary magnetic fields with the same frequency as the primary field but with different phase. VLF systems measure the contrast between the primary and secondary field, but do not measure the direct conductivity as does EM. However, VLF systems operate at high frequencies resulting in finer resolution than EM surveys. VLF method EM31 method The resisitivity survey was carried out with an ABEM unit. The system uses four electrodes in a Wenner array (current electrodes are A and B and potential electrodes M and N). They are separated by the electrode spacing a which varies from 1 to 24 for the 160m spread. The system injects a known current and measures the resulting potential between M and N and calculates the apparent resistance for that spacing. For each spacing the array rolls along the line repeating this process and plotting the results in a pseudo section. The depth of each spacing is approximately (3a)/2. 1 . 2 . 3 . A M N B a a a 1Terrameter 2Switch Box 3Laptop with ERIC.exe a = 4m Resistivity method The most important formation in the survey area is the Nagyvazsony Limestone Formation, a freshwater limestone. It does not form a continous layer within the formation, but occurs in the form of significant lenses in the white, greenish- white calcareous mud. According to Loczy, sand, loess sandstone, clay and marl can also been observed in the area. A Balatonfelvidek foldtana (1999) Geological Setting Line 1 Line 2 NW NE SE SW SSW NNE SW NE After the resistivity field survey, the resistance measurements are usually reduced to apparent resistivity values. To obtain a model for the shallow subsurface an inversion needs to be applied. A pseudo section is produced from the apparent resistances measured in the field (See Loc. 3). RES2DINV.exe produces a basic model based on this section using a least squares inversion. 1D profiles of the pseudo section are fed into the model and a modified pseudo section is created. This section shows the resistances you would measure in the field if the model was correct. The model is Inversion Process