Abstract
In most of the 20 th century the geo-electrical methods were primarily used for groundwater exploration and the application of the methods were normally followed by a borehole, and a moment of truth. In this process the use of DC (direct current) soundings have been developed to a high grade of excellence. In the last 25 years the geo-electrical methods are more used in connection with groundwater protection and planning, and new methods, as transient electromagnetic (TEM) soundings, have been developed that provide more measurements per hour. In Denmark this change is very explicit, and a paper has - compared results of TEM based mapping with results from wells in a test area north of Aarhus - and stated that: is time to do away with the old way of using geophysics.The present paper tests this statement and concludes that critical boreholes have been overlooked in the analysis. The test area was earlier mapped by DC-soundings, so it is possible to test the methods against each other. It is concluded that well performed DC-soundings with a Schlumberger configuration still provide the best base for hydrogeological mapping.
Highlights
In most of the 20th century Direct current (DC) resistivity soundings have been the most used method for hydrogeological mapping
From Denmark a review of hydrogeological mapping by continuous profiling and transient electro-magnetic (TEM) soundings is used as a base for establishing groundwater protection zones is reported by Thomsen et al [1].The review concludes, based on results from a test area, that “dense mapping with newly developed geophysical measurement methods -- accords geophysics a highly central role in the forthcoming hydrogeological mapping” and that “surface mapping with the new geophysical methods, combined with better interpretation programmes, has shown that it is time to do away with the old way of using geophysics”
The authors conclude that this aim was reached as the survey “demonstrates why dense mapping with newly developed geophysical measurement methods -- accords geophysics a highly central role in the forthcoming hydrogeological mapping”
Summary
In most of the 20th century DC (direct current) resistivity soundings have been the most used method for hydrogeological mapping. From Denmark a review of hydrogeological mapping by continuous profiling and TEM soundings is used as a base for establishing groundwater protection zones is reported by Thomsen et al [1].The review concludes, based on results from a test area, that “dense mapping with newly developed geophysical measurement methods -- accords geophysics a highly central role in the forthcoming hydrogeological mapping” and that “surface mapping with the new geophysical methods, combined with better interpretation programmes, has shown that it is time to do away with the old way of using geophysics” As this conclusion apparently is based quantitatively by a lot of borehole data, it is increasingly being cited by other authors who advocate the new methods for planning purposes. The present paper will investigate this question by reviewing the classical works on the accuracy of the interpretations depending on the accuracy of the measurements, and will test the results of the new methods against the results obtained by the classical Schlumberger soundings measured in the test area
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