Geologi og Vandboring. 2.udgave

Abstract

This book is intended as a simple geologic hand-book for well drillers and water supply engineers working in Denmark.

The principles of physical geology are briefly treated in chapter I as an introduction to the geology of waterbearing beds in the subsoil of Denmark. The normal hydrologic cycle at the surface of the earth has been emphasized, illustrating the action of water upon landscape and subsurface. Glacial erosion and sedimentation are discussed rather comprehensively in order to make the user of the book familiar with the variability of the Quaternary deposits of the country. The work of the sea has also been treated, and the modes of marine erosion and sedimentation are outlined. The comparison of fluviatile, marine, and glacial deposits should draw attention to significant features and differences in rock composition and texture, and thus enable the well driller or engineer to determine the common rock types met with in water well drilling in this country. Weathering, disintegration, and crustal processes are only touched upon.

In chapter II, which deals with the historical geology, the glacial drift that veils the bedrock formation over great parts of the country is described first, since a non-geologist should understand the nature of the Pleistocene glacial deposits before he is confronted with the less accessible formations of the subsurface. Regarding the pre-Pleistocene formation the description has been divided into two sections, the first one treating the areas W of Øresund, and the second one dealing with the island of Bornholm. The formations are described in their stratigraphic order of succession, and the sequence of sediments is described in rather great detail, with emphasis on the formations that provide aquifers of practical importance.

Chapter III deals with the selection of drilling sites in general on the basis of geologic and hydrologic viewpoints, while chapter IV is a case history of the search for new groundwater supply areas for the waterworks of the township of Randers in Jutland. It includes an outline of existing major pumping stations in the town area and its surroundings, and a discussion of the geologic and hydrologic setting of the region. The plans for the test wells, which were proposed on the basis of geo-hydrologic considerations, are presented and compared with the results obtained through subsequent drilling.

Waterbearing gravels and sands in the Pleistocene and waterbearing Upper Cretaceous limestone were tested. It appeared that rather complicated hydrologic situations are found in the surveyed area. One of the most cumbersome problems from a technical point of view is the occurrence of salt water in the aquifers. The salt water is assumed to be derived from deep-lying pre-Cretaceous formations from which it may ascend into the freshwater zones through structural dislocations due to a deficit in hydrostatic pressure in the more shallow aquifers. A natural drainage of the Gudenaa valley through highly permeable gravel beds near the surface may be responsible for the lower hydrostatic pressure in these aquifers. The last part of the book consists of a compilation of significant well logs, a list of geologic terms, stratigraphic tables, a geologic map of the pre-Pleistocene surface, and two cross sections through important wells.