Review Of The Gravity Method Of Geophysical Prospecting And It’s Applications


The gravity method is described as a nondestructive geophysical technique that measures differences in the earth’s gravitational field at specific locations.

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The gravity method is described as a nondestructive geophysical technique that measures differences in the earth’s gravitational field at specific locations. It has found numerous applications in engineering and environmental studies including locating voids and karst features, buried stream valleys, water table levels and the determination of soil layer thickness. The success of the gravity method depends on the different earth materials having different bulk densities (mass) that produce variations in the measured gravitational field. These variations can then be interpreted by a variety of analytical and computers methods to determine the depth, geometry and density the causes the gravity field variations.

Gravity method is mostly applied in environmental and engineering field which is the area of concentration of this study. In this study, gravity data in engineering and environmental applications is collected in a grid or along a profile with stations spacing 5 meters or less. In addition, gravity station elevations must be determined to within 0.2 meters. Using the highly precise locations and elevations plus all other quantifiable disturbing effects, the data are processed to remove all these predictable effects. The most commonly used processed data are known as Bouguer gravity anomalies, measured in mGal.

The interpretation of Bouguer gravity anomalies ranges from just manually inspecting the grid or profiles for variations in the gravitational field to more complex methods that involves separating the gravity anomaly due to an object of interest from some sort of regional gravity field. To perform the later, there are several manual and computer techniques including graphical smoothing and polynomial surface fitting. The interpretation of separated (residual) gravity anomalies commonly involves creating a model of the subsurface density variations to infer a geological cross-section. These models can be determined using a variety of methods ranging from analytical solutions due to simple geometries (e.g., sphere) to complex three- dimensional computer models.


1.0                                                INTRODUCTION

1.1                                  BACKGROUND OF THE STUDY

Gravity is a potential field, i.e., it is a force that acts at a distance. The gravity method is a non-destructive geophysical technique that measures differences in the earth’s gravitational field at specific locations. It has found numerous applications in engineering, environmental and geothermal studies including locating voids, faults, buried stream valleys, water table levels and geothermal heat sources. The gravity method works when buried objects have different masses, which are caused by the object having a greater or lesser density than the surrounding material. However, the earth’s gravitational field measured at the earth’s surface is affected also by topographic changes, the earth’s shape and rotation, and earth tides. These factors must be removed before interpreting gravity data for subsurface features. The final form of the processed gravity data can be used in many types of engineering and environmental problems, including determining the thickness of the surface or near-surface soil layer, changes in water table levels, and the detection of buried tunnels, caves, sinkholes and near-surface faults. Relatively new applications include four-dimensional (4-D) gravity, where temporal variations of the gravitational field can used to determine variations in the water table (Mokkapati, 1995; Hare et al., 1999) and changing of subsidence levels in sinkholes (Rybakov et al., 2001). Table 1 lists the main uses of the gravity method in engineering and environmental studies.

The gravity method can be a relatively easy geophysical technique to perform and interpret. It requires only simple but precise data processing, and for detailed studies the determination of a station’s elevation is the most difficult and time-consuming aspect. The technique has good depth penetration when compared to ground penetrating radar, high frequency electromagnetics and DC-resistivity techniques and is not affected by the high conductivity values of near-surface clay rich soils. Additionally, lateral boundaries of subsurface features can be easily obtained especially through the measurement of the derivatives of the gravitational field. The main drawback is the ambiguity of the interpretation of the anomalies. This means that a given gravity anomaly can be caused by numerous source bodies. An accurate determination of the source usually requires outside geophysical or geological information.

The use of the gravity data is relatively straightforward as can be seen in the following summary of the fundamentals of the gravity method as applied to engineering and environmental studies including overviews of the theory, data collection, processing, and interpretation. For more detailed information on the gravity technique, numerous papers covering all aspects of the gravity method are available in the following journals: Geophysics, Geophysical Prospecting, Exploration Geophysics, Journal of Environmental and Engineering Geophysics, and the Journal of Applied Geophysics (see the reference list for a partial list of papers related for environmental- and engineering-type gravity investigations). For more detailed investigation on the theoretical background of the gravity method, the reader is referred to books by Grant and West (1965) and Blakely (1995). For overviews of the gravity method, the reader is referred to the books by Telford et al. (1990) and Robinson and Caruh (1988). Books by Burger (1992), Sharma (1997) and Reynolds (1998) contain a chapter on the gravity method with an emphasis on shallow applications, while overview papers by Hinze (1990) and Debeglia and Dupont (2002) specifically focuses on shallow gravity applications.

1.2                                            PROBLEM STATEMENT

Construction engineering involves the designing, planning, construction, and management of physical infrastructure, which may include roads, bridges, buildings, dams and utilities. All of these types of facilities have a physical footprint that requires some type of modification of the ground, whether that involves the insertion of anchors, ground compaction or excavation. They are also subject to potential geohazards including, seismic, landslide and flooding hazards.

Most construction engineering projects generally require information on subsurface ground conditions to maximum depths of only 50 metres (m) or so below the ground surface. This zone can encompass soil, groundwater, unconsolidated sediments, weathered rock, and competent bedrock. It can also contain products of our own invention, including utilities, buried waste, and contaminants. Typical parameters of interest to designers and engineers are the thickness and extent of layers, the physical properties of layers and any variability within them, and any structures within the ground. Having an understanding of what lies beneath before breaking ground can save significant costs to a construction project. Measurements of gravity provide information about densities of rocks underground. There is a wide range in density among rock types, and therefore geologists can make inferences about the distribution of strata. Because faults commonly juxtapose rocks of differing densities, the gravity method is an excellent exploration choice.

1.3                           AIM AND OBJECTIVES OF THE STUDY

The main aim of this study is to study in details gravity method of subsurface measurement. The objectives are:

i.                    To provide a better understanding of the subsurface geology.

ii.                  To study the application of gravity method

iii.                To discussed gravity method of groundwater exploration with field data

1.4                                  SIGNIFICANCE OF THE STUDY

The study throws light on the importance and application of gravity method of groundwater exploration. This study also discussed why gravity method is chosen than other types of groundwater exploration. This study will be of great benefit to whoever that is working or doing research on ground water exploration. This study provides solutions for determining subsurface properties of the soil before foundation of the building is made.

1.5                                       BENEFIT OF THE STUDY

The gravity method is a relatively cheap, non-invasive, non-destructive remote sensing method that has already been tested on the lunar surface. It is also passive – that is, no energy need be put into the ground in order to acquire data. The small portable instrument also permits walking traverses.

1.6                                    LIMITATION OF STUDY

As we all know that no human effort to achieve a set of goals goes without difficulties, certain constraints were encountered in the course of carrying out this project and they are as follows:-

  1. Difficulty in information collection: I found it too difficult in laying hands of useful information regarding this work and this course me to visit different libraries and internet for solution.
  2. Financial Constraint: Insufficient fund tends to impede the efficiency of the researcher in sourcing for the relevant materials, literature or information and in the process of data collection (internet).
  • Time Constraint: The researcher will simultaneously engage in this study with other academic work. This consequently will cut down on the time devoted for the research work.

1.7                                            SCOPE OF THIS STUDY

This study describes some of the civil engineering applications discourse where the geophysical method is especially suitable in the preliminary stage of site assessment. In this work, the combination of geophysical data and geotechnical measurements may greatly improve the quality of buildings under construction in civil engineering.



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