An EM Perspective of Selected Topographic Highs in Transdanubia
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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
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Inversion Process. NW. NE. After the resistivity field survey, the resistance measurements are usually reduced to apparent - PowerPoint PPT Presentation
Transcript of An EM Perspective of Selected Topographic Highs in Transdanubia
An EM Perspective of Selected Topographic Highs in TransdanubiaAhmed Al-Yaqoobi 2 , Judit Petrovszki1, Joseph Michael Court2
1Eötvös Loránd University Faculty of Science2 University of Leeds, School of Earth and Environment
1st -14th 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.
Milsom 2003
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 Ba a a
1Terrameter2Switch Box3Laptop 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
NWNE
SE
SW
SSW
NNE
SW
NE
After the resistivity field survey, the resistance measurements are usually reduced to apparentresistivity 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 improved through iterations with the 2nd pseudo section until the two pseudo sections are matching within an error of 5%.
Inversion Process
References:
Mlison, J. (2003). Field Geophysics. The Geological Field Guide Series. John wiley and sons Ltd.
Parasnis, D. S. (1982). Principles of Applied Geophysics. Chapman and Hall.
A Balatonfelvidek foldtana, MAFI Budapest, 1999
Loke, M. H. (2004). Tutorial : 2-D and 3-D electrical imaging surveys
Location 1: Line 2 cross-section
Location 2 ResultsLocation 1: Line 1 Results Location 3 Results
NW SE
NESW
SSW NNE NESW
EM31 and VLF methods completed preliminary scans before deciding where to centre the more intensive Resistivity survey. Hence the Resistivity scans were focused around the anomalies in question. These anomalies have resistivities in the range of limestone, backed up by the outcrop of freshwater limestone found at location 3. Further work could be completed south of Öreg Hegy, north of location 3, in an attempt to correlate the anomaly across this area and determine if the limestone is discontinuous or was once a homogenous layer. The obtained profiles also provide a valuable contribution to a better understanding of the structure and formation of the Enying ridge.
Conclusion
Acknowledgements:
RES1 group: Al-Rajhi, Mohammed. Veasey, Martin. Pudleiner, E’va. Takacs Peter. For the Resistivity section at Location 2.
Gravity group: Hamzah, Hamizah. Ellwood, Helen. Erdős, Zoltan. For the topographic data at Location 1 and 3.
Szekely Balazs and Zamolyi Andras for the outcrop photo at Location 3.
Thankyou to everyone who helped in attaining the field data, processiong and interpretation.
Fresh water limestone found in the dirt track at location 3 line 2, 200m from the start.
Location 3: Outcrop
Pseudo section
Inversion model
