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Über dieses Buch

The process of regional-residual separation in potential field is age-old. Broadly, there are two techniques for regional-residual resolution, viz., graphical and analytical. Both the techniques have their own respective shortcomings. In this book, the authors have described the technique based on finite element method in which only eight (or twelve) nodal observed gravity values are used for the regional computation, thereby eliminating the possible contamination of anomalous fields and also the technique does not assume an explicit model and physical properties like density of rocks etc. in the regional computation. The book discusses the advantages of this technique viz., it is not site-specific; the computation is independent of any prior assumptions as to the form and depth of shallow or deeper structures; it can handle data distributed at random or on a regular grid on the map space; and the neighbouring surveys join smoothly.

The book focuses on application of this new technique which has been demonstrated in different fields, such as hydrocarbon, minerals and groundwater, structural studies, earthquake and engineering studies and impact structures.

Inhaltsverzeichnis

Frontmatter

Introduction

Of all the geophysical methods the gravity method appears simpler in theory and its applications. However, it was the gravity that came on the way of establishing the unified field theory of Einstein. It is always attractive, the repulsion is not yet discovered. There were attempts to discover the wave nature of gravity field, but it has remained without success.

K. Mallick, A. Vasanthi, K. K. Sharma

Regional and Residual Gravity Anomalies: The Existing Issues

The Bouguer gravity anomalies obtained after various corrections to the observed field represent the combined responses of various masses lying at depths below the ground surface. At this stage the interpreter separates the effects which are likely to be associated with the geological features of interest or the target body from the rest of the response. In case of problems dealing with mineral and hydrocarbon exploration or the delineation of structures for engineering and foundation purposes, the interpreter is concerned with depths ranging from a few tens of metres to a few kilometres. There are also problems dealing with deep-seated structures at crustal or even Moho depths. Therefore, the zones of interest are different depending on the problems at hand.

K. Mallick, A. Vasanthi, K. K. Sharma

New Computational Schemes

Precise separation of gravity regional and residual components is the key step for the modelling of geological structures. Over the past six decades a large number of techniques, both in frequency- and space-domain, have been developed to compute the gravity regional anomaly. As there are discrepancies in all the existing techniques, a new space-domain technique based on the concept and properties of the element shape functions used in finite element approach (FEA) is recently developed (Mallick and Sharma, 1997, 1999) and applied to a variety of research problems (Sharma et al., 1999; Mallick et al., 1999; Mallick and Vasanthi, 2001; Vasanthi and Mallick, 2001; Kannan and Mallick, 2003; Vasanthi and Mallick, 2005a; Vasanthi and Mallick, 2005b; Vasanthi and Mallick, 2006). The technique is robust, well-performing and not site-specific. This technique overcomes most of the drawbacks encountered in the existing analytical methods.

K. Mallick, A. Vasanthi, K. K. Sharma

Applications to Geological and Environmental Problems: Hydrocarbon

The potential field method, specially the Gravity method, is very fast and inexpensive too. In view of this, this technique has been used as a reconnaissance method to delineate the potential zone before seismic studies are taken up. However, gravity method has a bigger role to play. There are instances where the geology is complex, or sedimentary rocks are overlain by high velocity volcanics leading to poor reflections.

K. Mallick, A. Vasanthi, K. K. Sharma

Applications to Geological and Environmental Problems: Minerals

Successful campaigns of mineral exploration depend more on the understanding of the patterns of the occurrence of minerals than on the choice of geological and geophysical techniques. It will be amply clear from the following examples. The host rock for diamond is kimberlite which occurs in older cratons, or rejuvenated by younger orogenic events. Therefore, before applying gravity or magnetic methods to locate kimberlite pipes, it is profitable to delineate the rejuvenated margins.

K. Mallick, A. Vasanthi, K. K. Sharma

Gravity Method in Structural Studies

The geologists feel comfortable with gravity method, since the gravity data often faithfully reflects the subsurface structure. Unlike in seismic due to wave propagation, magnetic due to both attraction and repulsion, and electrical and electromagnetics due to induction, the attractive gravity field produces relatively simpler patterns of anomalies, like a series of highs and lows over regions with undulating basements and buried structures. A qualitative interpretation of gravity data reveals a good deal of information. Further data processing and quantitative interpretation have added more meaning and often supplemented the finding by other methods. In seismic, for example, where reflections are weak or absent like in Moray Firth, North Sea, gravity helps to establish the structure.

K. Mallick, A. Vasanthi, K. K. Sharma

Isostatic Studies and Vertical Crustal Movements

A bird when flies off a tree, it is said, can change the rotation pattern of the earth. At the very first thought, it appears absurd, but when one thinks of the formation of high mountain ranges such as the Himalayas by an uplift of 1.0 mm per year, then one will be convinced of the truth in the above statement. The earth processes are minute as well as slow, but over a geological time scale of millions of years, they build structures beyond anybody's comprehension. Equally amazing is the controversy over the cause-and-effect relationship between the tectonic uplift of the mountains and the climate changes. It is believed that the rise of the Tibetan Plateau has caused the cooling of the northern hemisphere (Press et al., 2003) and the formation of the river systems. The models of the evolution of the landscapes by cycles of uplift and erosion have been illustrated by Burbank and Anderson (2001).

K. Mallick, A. Vasanthi, K. K. Sharma

Earthquake Studies and Engineering Applications

The application of gravity method in various geological situations has been described in the preceeding chapters. Some other new aspects have been considered here. Earthquakes, though very destructive, have provided useful information with regard to the structure of the earth, the process of mountain building, presence of faults, thrusts and engineering properties of soil and rocks.

K. Mallick, A. Vasanthi, K. K. Sharma

Gravity Studies on Impact Structures

Over centuries it was believed that the economic mineral deposits and hydrocarbon accumulations are the outcome of the internal geological processes of the Earth. There was a paradigm shift in the perception when the space photographs of the surfaces of different planets and satellites were sent to the Earth in the middle of the last century. These photographs clearly showed circular structures on the planets and the satellites.

K. Mallick, A. Vasanthi, K. K. Sharma

Backmatter

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