Geo-elektrisch onderzoek bij de geologische overzichtskartering van West-Vlaanderen
De Moor, G.; De Breuck, W. (1965). Geo-elektrisch onderzoek bij de geologische overzichtskartering van West-Vlaanderen. Natuurwet. Tijdschr. 46(8): 215-240
In: Natuurwetenschappelijk Tijdschrift. L. Walschot/Natuur- en Geneeskundige Vennootschap: Gent. ISSN 0770-1748, meer
The existing geological maps show many lithological as well as stratigraphical inaccuracies. A new reconnaissance survey has been made. This survey included a study of the thickness and the lithology of the Quaternary sediments and a lithological and stratigraphical study of the upper part of the Tertiary substratum. The study has been realized mostly by geo-electric probings with direct current and the electrodes in Wenner arrangement. About 500 probings were executed over the whole territory of the West-Flanders. They are grouped together in areas where terrain conditions allowed the application of this method. The data comprise at first the values of the apparent resistivity, which have been measured in each point by electrode separations varying between 1 and 100 m. The values of the apparent resistivity, measured by the electrode separations of 10, 20 and 40 metres are represented by resistivity maps. The specific resistivity Qo for the upper layer and Qb for the substratum are reported on two other maps. On the other hand by interpretation of the data, an isopach map of the Quaternary sediments and an isohyps map of the geo-electric contact level have been established. The resistivities measured for sediments saturated with water are mentioned. The resistivity maps made it possible to distinguish different electrically homogeneous regions, which correspond also to hydrologic unities. The Dune Belt forms a region of high resistivity, where the depth of the Ypresian clay has been determined between De Panne and Nieuwpoort. In the vicinity of Knokke the depth of the Bartonian clay has been measured. Between Nieuwpoort and Heist the geo-electric method could not be used since either the Tertiary sediments have the same composition as the Quaternary overburden, or salt water is present in the subsoil. In the Polders the rapid horizontal variations in resistivity are connected with the degree of the salinity of the phreatic water. In the Transition Region between the Polders and the Inner Flanders two zones can be distinguished. To the West between Bruges and Gistel Quaternary drift sands an a Tertiary underground dipping very strongly under the coastal plain, form an elevation of high resistivity. Near Gistel the substratum is formed by Ypresian clay. Near Roksem the resistivity of the substratum corresponds to that of Ypresian sand. Between Sint-Andries (near Bruges) and Varsenare, the resistivity of 20 Ωm indicates in the subsoil the sandy clay of the Lower Paniselian. East of Bruges high resistivities are encountered in the Quaternary. The substratum between Sijsele and Oedelem is formed by Bartonian clay, which gives very low resistivities. Between Oedelem and Oostkamp very high values down to great depths are measured. The Tertiary sands (Wemmelian, Ledian and Paniselian) have here the same composition as the Quaternary sediments. The contact between Pld and Plc indicates the existence of north-south oriented gullies, which may have a hydrologic interest. In the Inner Flanders one can distinguish between a central and southern region with an Ypresian clay substratum, and a northern region where more recent Eocene sediments are making out the subsoil. The electric prospection made it possible to understand that the clayey facies of the Ypresian occurs on a higher level than previously assumed, sothat its outcrop has a much larger extension than mentioned on the previous map. The top of the clay indicates two anomalies. North of Ypres it shows a steeper slope than south of this town. In the vicinity of Ardooie, north of the Mandel valley, occurs a west-east oriented basin which is filled up with Ypresian sand. Sudden discrepancies in the top of the clay have also been measured. These have been confirmed by auger boring and mechanic drilling. The resistivity values of the clayey substratum vary from zone to zone, indicating the presence of different lithological horizons. Three large Pleistocene valleys have been excavated in the clay substratum: the valleys of the Lys, the Scheldt and the Mandel. Their dimensions and the thickness of their Quaternary filling sediments are well visible on the isopach and isohyps maps. The values of the specific resistivity for the Quaternary sediments give an idea of their lithological composition. From sOuth to north in the northern region one comes first in a zone where the resistivity values at small depth indicate Ypresian clay. This zone is bordered by a region with higher resistivities corresponding to a thin sandy Quaternary cover on Ypresian sand. Further north the contact 1evel occurs on higher altitude on the Aarsele-Hooglede cuesta, formed by Paniselian clay (Plc) .Then depths of the contact level become greater again and the resistivity values indicate a clayey substratum.
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