K. Mallick

On “New standards for reducing gravity data: The North American gravity database” ()

The paper “New standards for reducing gravity data: The North American gravity database,” which summarizes revisions to the North American gravity database, is testimony to the importance of gravity data reduction. Although the methods described by the authors establish the North American gravity database, geophysicists all over the world will take these methods as models to establish such databases for their respective regions or countries. As a result, it is important for the geophysical community to consider the benefits and drawbacks of these methods.

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.

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.

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.

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.

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.

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.

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.

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 anybodys 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).

Effectiveness of a new numerical technique in predicting isostatic gravity anomaly

Summary: New standards for reducing gravity data by Hinze et al. (2005) describe gamut of new approaches and concepts to bridge the gap between geophysics and geodesy. However, when the computation of the all important isostatic anomaly is considered, the determination of the parameters like ground elevation, crustal depths, density of crustal rocks and the density contrast at crust-mantle boundary by independent methods may come on the way of achieving high accuracy. With the backdrop of this scenario we present an effective new numerical approach that is not site-specific and does not require 0these parameters explicitly, yet gives residual gravity anomalies that compare well with the isostatic anomaly. We provide two examples from California, USA.