Geo-elektrische profielen bij de geologische en hydrologische detailkartering
De Breuck, W.; De Moor, G.; Burvenich, T. (1970). Geo-elektrische profielen bij de geologische en hydrologische detailkartering. Natuurwet. Tijdschr. 52: 105-136
In: Natuurwetenschappelijk Tijdschrift. L. Walschot/Natuur- en Geneeskundige Vennootschap: Gent. ISSN 0770-1748, meer
Geoelectric traverses have been made in several regions of the F1anders, in order to test their usefulness for mapping purposes. The three regions selected had previously been covered by a grid of resistivity soundings mostly in Wenner arrangement.In the Beernem area different arrays have been used along the same line. These multiple traverses have provided data which allowed a comparison of the different arrays as well as a detailed subdivision in geoelectrical homogeneous zones coinciding with lithological units.Along traverse 136 GP/EE axial dipole measurements have been made over a distance of 2,7 km. For every point two values were determined : one for an array 2-20-2 and another for a 5-20-5 array. The first number stands for the length in meters of the current electrode separation, the third for the length of the voltage electrode separation, and the second for the distance between the two inner electrodes of bath dipoles.On the traverse 136-212 GP / AA Wenner arrangement perpendicular to the line has been used over a distance of 9,6 km to determine at each spot Q20' Q40' and Q80. Along the same line apparent resistivities have been measured with axial dipole arrays 2-20-2, 5-20-5, 5-40-5 and 10-40-10, parallel with the line.In the Westhoek area, a part of the coastal plain with salt-water encroachment in the unconfined aquifer, 21 traverses (194 GPW to 194 GPW21) have been run in the vicinity of Veurne (Furnes). The resistivities Q10 were measured by a longitudinal Wenner array. Since the traverses had been confined to a geoelectric homogeneous zone a quantitative interpretation could be performed. A relationship between Q10 and the depth of the saltwater - fresh water interface was computed from previous soundings. This relationship has been used to convert the apparent-resistivity data along the traverses into depth to interface values.The polder area of the F1emish Valley north of Ghent also has an unconfined aquifer with brackish water at variable depths. The situation is very similar to the one in the coastal plain although the variations here may not so readily find an explanation. One single traverse 142 GPW1 of 3550 m length has been run with a Wenner array for Q10. A relationship based upon former soundings has been used to convert resistivity data into interface depths.Resistivity traversing proves to be a very accurate and speedy tool for detailed mapping. Even a qualitative interpretation of a traverse, multiple if possible, can be sufficient to outline the boundaries between lithological or hydrogeological units. Quantitative interpretation is possible when in conjunction with a sounding grid, previously established, a relationship between the fixed electrode array and the unknown factor is found.Once a relationship is established, fixed-electrode separations allow a more detailed survey than resistivity soundings. These have to be long enough to obtain a reliable curve and hence average depth or resistivity data over longer a distance than fixed-electrode measurements which tend to be as short as possible. When traversing for qualitative purpose dipole arrangements have an advantage over Wenner arrangement in being speedier.
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