Geoelectric

ABSTRACT This paper describes 2 case studies of long-term geoelectrical monitoring of aquifer contamination: a marine intrusion generated by excessive exploitation of drinking water in the Costinesti resort (located on the Romanian Black Sea shore), and contamination resulting from natural meteoric water infiltrations along a fault system in the vicinity of Brazi oil refinery (Ploiesti region, Romania). Both the long-term monitoring of the marine intrusion and the determination of meteoric water infiltrations along faults were confirmed by data obtained from hydrogeological wells. A potential method for simulating the apparent resistivity anomalies within this context (media with a continuous variation of the resistivity) is represented by the application of harmonic alpha media theory.

Forschungen von Hobby-Archäologen können mit amtlicher und akademischer Unterstützung eine Bereicherung für den Wissenschaftsbetrieb darstellen. An dem konkreten Beispiel einer villa rustica im unteren Selztal wird diese Arbeitsweise vorgestellt: Feldbegehungen und elektromagnetische Prospektion.

Les cubetes de sobreexcavació glacial dels Pirineus meridionals es caracteritzen per haver estat formades per erosió glacial i posteriorment han estat reblertes per una important successió estratigràfica. El reconeixement de la tipologia dels sediments que conformen el rebliment de la cubeta permet la seva reconstrucció paleogeogràfica. Donat que en aquestes cubetes no hi ha abundància d'afloraments de superfície, hom ha de recórrer a mètodes de prospecció directes i/o indirectes per obtenir dades del subsòl. Els mètodes directes (sondatges mecànics, cates, etc...) poden donar una informació amb molta resolució però molt puntual, mentre que els mètodes indirectes (sondatges elèctrics verticals, sísmica de refracció, etc.) poden donar una informació més extensa però amb menys resolució.

[The sub-terrain of hat-shaped hills]

The article is written in Danish and published in Geologisk Tidsskrift 2000:1 (Copenhagen: Dansk Geologisk Forening), pp. 1-2. Here follows a full translation in English:

The socalled ‘hat-shaped hills’ in Denmark have been the subject of many discussions and theories among Danish geologists throughout the 20th century, ever since the unusual internal structure of the hills was discovered by Vilhelm Milthers and Kristian Rørdam in 1900. The hills in question seem rather normal on the outside. They are found in plain moraine clay areas as singular round hills of various heights, usually until 20-30 m above ground level and about 200 m long. They are often located in series or rows, in places where glacial border lines are expected to have existed during the end of the Weichsel glacial period. The hills consist of sand and gravel sedimented in layers just like normal kames of stagnant glacial lake deposits, but contrary to these, the layers in the hat-shaped hills seem to be tilted and are often found standing almost vertical, but still with very well-stratified layers. On the top of the hills there can sometimes be found a thin layer of moraine clay.

The first hat-shaped hill, which subsequently was to be considered the ‘type location’ of the term, was Gedebjerg near Kundby in north-western Sjælland (Zealand). A large part of the hill has been removed and turned into a gravel pit, which provides a fine profile through the hill of about 100 m. Two smaller hills are situated right to the south-east of Gedebjerg and they also contain dislocated layers of gravel and sand. In all, the existence of about a dozen of such hat-shaped hills have been established in the Kundby area. Later on, other geologists have found hills of very similar appearance in other parts of Denmark. The largest hills of this specific type is the three Kulsbjerge near Vordingborg in south-eastern Sjælland, while the island of Langeland is practically covered by hat-shaped hills; more than one thousand hills are concentrated on the small, oblong island. Our inquiries directed to geologists and geological institutes in Nothern Europe and America indicates that phenomena — more or less similar to the Danish hat-shaped hills — are known in the USA and Canada, and especially in Poland and the Baltic states.

The somewhat peculiar name for the Danish hills was given to them by Milthers back at the turn of the century, because their shape reminded him of a certain type of ladies hats, which were in fashion at the time. A different and perhaps more ‘scientific’ name for the hills is ‘dislocated kames’, but the problem with this name is that it rules out all but one theory of the construction of the hills — and it is precisely the question of their construction which has given cause for all the theories and investigations of the hills.

The reason for the lively debate about the hat-shaped hills for all these years is that it has been quite a geological challenge to present a complete explanation for their shape, the vertical strata and the complicated tectonic structure, why the uppermost part of the strata is overtilted, the possible cover and ‘train’ of till, together with the location and grouping of the hills.

Sedimentological facies analysis and tectonical analysis have been performed especially in Gedebjerg up to our time. Based on these investigations, explanations have been offered as to how the sedimentations originally took place and how the layers later on were dislocated. However, the studies have not been able to determine the initial making of the hat-shaped hills with certainty, but they have produced enough new knowledge to modify the old theories and create new ideas as well. Both the analysis results and the validity of the different theories have continuously been questioned by fellow geologists.

By performing a new set of investigations, using geoelectric testings for the first time in the matter of hat-shaped hills, we (a group of geology students at the University of Roskilde) tried to establish, what is underneath the hills. This is of interest, since the theories can be divided into two groups; one that says the material came ‘from above’ and was — one way or the other — put on the moraine bed by a glacier at the end of the Weichsel period, and then subsequently tilted by the same or another glacier. The other group of theories suggests the idea that the material was originally sedimented by an older glacier and later pushed up ‘from beneath’ by a younger glacier.
The very first and classic theory on the origin of the hills was made by Milthers (1900), who suggested that the sediments were originally deposited in at least 200 m deep crevices in an ice-marginal environment. The tilting of the strata to almost vertical happened during the melting away of the supporting ice walls. Andersen (1966) argued for a nearshore beach environment of an interglacial sea, where later ice movements should have brought large pieces of frozen sea sediments with them to the Kundby area. The idea of dislocated kames was presented by Rasmussen (1967), where the vertical layers of sediments should origin from glacial lakes in ice which was still moving or the sediments had been pushed over by a later ice.
If one from the first group of theories is correct, one should expect to find moraine clay beneath the hills as well as in the terrain around them. This, however, is not the case. A number of geoelectric measurements, performed by us in the autumn of 1998 and the spring of 1999, clearly established, that while moraine clay is found around and between the hills, the ground right beneath Gedebjerg and a small neighboring hill consist of sand or gravel until a depth of at least 50 m. Due to the conditions in the hill and the capacity of our equipment, is wat not possibly to perform valid measuring any deeper than this.

The results will come as a surprise for most geologists with an interest in the matter, as the most popular theory until today have been that the hills were sedimented right on the thick moraine till bed by one of the very last glaciers in the area. Our geoelectric findings are supported by other observations, however. Local farmers have informed us that subsoil water leak out of Gedebjerg in the early spring quite a few meters above ground level, while lower parts of the plain around the hill are still dry. Also the gravel pit was given up years ago, when they reached permanent wet sand at a height of about four meters above the surrounding terrain. These significant signs of sand/gravel beneath Gedebjerg, is confirmed by our tests.

Both Vilhelm Milthers and his son Keld Milthers performed countings of indicator boulders all over the country back in the first half of the century. Such countings were also performed in two of the hat-shaped hills of Kundby, Gedebjerg and Kirkebjerg. Their findings did not go well with the Milthers’ own idea about the making of the hills, as the stones in Gedebjerg and especially Kirkebjerg appeared to origine from the earlier glaciers in the Weichsel period (with home base in Norway and Sweden), while the rest of the region had a larger percentage of stones from the Baltic Sea, which is considered to be the base of the last ice covering Sjælland. Two theories support the fact of gravel and sand beneath the hills. One was presented by Asger Berthelsen in the 1970’s and the other by Niels Schrøder in 1997. They both build on the idea that old sedimentations were pushed up in openings of the moraine plain by the pressure of a later ice, which caused the significant dislocation of the layers, but the models differ in the exact mechanism of the event. Berthelsen speaks of a relative simple overturned anticlinal foldstructure, while Schrøder compares the situation with the reaction in salt diapires.

With our investigations, we do not claim to have found the correct theory for the origin of the hat-shaped hills. The geoelectric measurements do not give any indications to support one of the two ‘gravel-ground’-theories for the other. Actually, we cannot even totally reject the ‘moraine-ground’-theories, but these theories have to be modified to cope with the new knowledge. Also we must underline that our testings only include Gedebjerg and the small hill to the west of it. We cannot say anything about whether similar results would appear if geoelectrical measurements were performed in hat-shaped hills elsewhere in Denmark. But since Gedebjerg is the definition of a‘hat-shaped hill’, we do believe to have added a new perspective to the mystery of this Danish phenomenon, which will enter another century unsolved.

For the majority of the communities living in coastal areas, the main potable water resource is represented by the fresh underground water, which is in a hydrodynamic equilibrium with the salty sea water. The natural or anthropic contamination of fresh water with marine salt water produces significant changes in the aquifers resistivity, thus allowing the application of geoelectrical methods for the detection and monitoring of marine intrusions.
We present two case studies from Romania (Costineşti and Vama Veche areas) which are relevant for the application of Vertical Electrical Sounding (VES) method for mapping and monitoring marine intrusions. The results of these investigations are represented by means of apparent resistivity cross sections, which clearly evidence the spatial and temporal evolution of such intrusions. The processing and interpretation of VES curves were carried out using a set of newly developed computer programs for the evaluation of site effects and forward modeling of the media characterized by a continuous resistivity variation.

We present 3D electrical resistivity tomography (ERT) imaging of the archaeological deposits at Arma Veirana cave (northern Italy), to date only partially explored. The archaeological importance of the cave is due to the presence of a rich Mousterian layer, traces of Late Upper Palaeolithic (Epigravettian) temporary occupations and an Early Mesolithic burial of a female newborn. ERT is rarely employed in Palaeolithic cave contexts because Palaeolithic remains are typically disseminated in loose deposits and either do not possess high electrical resistivity contrasts or are too small to be detected. Furthermore, some issues can derive from the confined environment in caves. In this view, our study represents an opportunity to assess the capability of this geophysical method to retrieve subsurface information of Palaeolithic cave deposits and create a framework for the improvement of ERT applications in such a peculiar cave context. The aim of this study was to define the features of ...