ids - proyek akhir

8/10/2019 IDS - Proyek Akhir

1/11

1

Geodesi Fisis

A GUIDE TO HIGH PRECISION LAND GRAVIMETER

SURVEY

Program Pascasarjana Teknik Geomatika Fakultas Teknik

Universitas Gadjah Mada (UGM) Yogyakarta

Dany Laksono (13/353080/PTK/08949)


2/11

2

A Guide to High Precision Gravimeter Survey

H.O. Siegel

Reviewed by: Dany Laksono

Introduction

This book discuss about definition and the way to obtain high precision gravimeter survey. The

writer explains about steps that need to be taken by anyone who wants to conduct a gravimeter

survey, what to follow and what to avoid during the survey.The book is divided into twelve chapters

or sections, with each of them focus on different aspect of Gravity survey as follow:

1. The first and second chapters deal with basic understanding on gravity data: where does the

gravitational field come from, and what kinds of variation does it have in different part of the

earth;

2. The third chapter discussed about the many application of gravity data and the requirements

of each field to obtain gravity data;

3.

The fourth chapter talks about the gravimeter device itself;4. The fifth chapter discussed in depth about some important factors to achieve high precision

data from a gravity field survey

5. The sixth chapter deals with corrections that should be given to survey data in order to obtain

highly precise gravimeter survey measurement

6. Seventh chapter mention the standard procedur that needs to be followed by gravity data

surveyor

7. The eigth chapter discuss about data processing and presentation of gravity data

8. Chapter nine discuss about the interpretation of gravity data, including determination of depth

from gravity data

9. Chapter 10 mentions some historical case studies on the implementation of high precision

gravity data survey.

10.Chapter 11 is the list of references, while chapter 12 is dedicated to one of the available

gravimeter for high precision gravity survey, Scintrex CG-3M

This resume will give a brief review of each chapter, while underlining the most important aspect on

that chapter.


3/11

3

Earths Gravitational Fieldand Its Variation

The force of gravity is essentially the strength of attraction between two bodies with mass.According to Newtons Universal Law of Gravity, the attraction force between two bodies with mass

M1and M2, respectively, can be notated as follow:

Where r denotes the distance between center of both masses, and G = gravitational constant, i.e.

6.673 x 10-11m3kg-1s-2.

Figure 1: Newtonian Law of Universal Gravity

Since the earth is a non homogenous body of mass with highly variable density, the

gravitational attraction is also varied from point to point on the earths surface. The gravitational

acceleration force as a result of these variation can be expressed as an integral over the variation of

of earths body volume:

In this equation, d is the objects (or, in this case, the earths) density.


4/11

4

To give approximation of the shape of the earth, one could obtain a model by assuming that

the gravitational force in the earth is uniform in all points. Model of the earth which derived from

such assumption is called as spheroid. However, a more accurate model of the earth is to take

variation of gravitational force into account. The model which considers the variation of so-called

Mean-sea level surface is termed geoid. Geoid, as an equipotential surface of the earth, differs from

spheroid as a result of a number of variations. Below are some of the variations that caused geoid to

differ from spheroid:

1. Variation with latitude

This variation takes sentrifugal and gravitational force into account, and occurred with

change of observed latitude. This variation can be expressed as:

Where 1 Gal = 10-2 m/s2

2. Variation with elevation

a. Free-air effect

The effect resulted from vertical gradient change above the surface of the earth.

Denotes as:

b. Bouguer effect

The effect as a result of additional mass gained from change of elevation above the

surface of the earth. Denotes as:

c. Elevation effect

Combinations of Bouguer and Free-air effect. Expressed as:

for elevation increment above the surface, and

for increasing depth below the surface


5/11

5

3. Terrain effects

This variation is caused by local irregularities in the topography around gravity station.

Knowledge on local topography is needed to calculate this effect and its cause to gravity

measurement. The effect of Bouguer, Free-air and terrain effects can be expressed with

diagram below:

Figure 2: Bouguer, Free air and Terrain Effects

4. Variation with time

a. Earth tides

b.

Atmospheric Pressure

c. Precipitation

d. Ocean Tides (sea level changes)

5. Variation with Geology

This variation is caused by irregular distribution of rocks and minerals below the earths

surface. Different minerals have effects on different density distribution, which in turn caused

the variation in gravity measurement.

Gravimeter and Applications of Gravimetric Surveys

This chapter of the book briefly discuss about various application of Gravimetric surveys.

Below are some of the notable applications of gravimetric survey:

1. Regional Geological Mapping

2. Petroleum Exploration

3.

Mineral Exploration

4. Geotechnical and archeological studies


6/11

6

5. Groundwater and environmental studies

6. Tectonic Studies

7.

Volcanology and Geotermal Studies

The instrument to obtain the gravitational force in a point of the earth is called Gravimeter.

Gravimeter can be divided into two basic categories:

a.

Absolute Gravimeters, which measure gravitational force in a single point

b. Relative Gravimeters, which measure the difference between different gravity station.

This category can further be divided into two sub-categories according to its design:

i. Astatic/Unstable Gravimeter, relies on unstable mechanism to obtain precise

gravity measurement

ii.

Stabel Gravimeter, use a rather stable mechanism to obtain gravity data

Some principal factors need to be considered in order to achieve highly precise gravitational data.

Below are some factors that contribute to the accuracy of gravity measurement:

1. Instrumental factors

a.

Shock and vibrationThe effect of shock and vibration caused gravimeter to possess offset and changes in

drift rates, which usually takes place during instrument movement. In that case,

moving between one station to another should be very precautious to avoid the drift

error

b. Power down

Power down caused the gravimeter to lose its ability to maintain its temperature,

which caused error in instrument reading

c. Extreme temperature shock

Change in temperature also need to be taken into account when moving the

instrument between one station to another, as extreme temperature change could harm

the instrument and devise instrument reading

d. Elastic Relaxation

Occurred in spring-balanced gravitimeter, where the spring does not return to its

original position during measurement

e. Leveling


7/11


8/11

8

3. Variations with elevation (CE)

4. Terrain Effects (CTE)

Figure 3: Corrections on Topographic Effect

Standard Field Procedure

In order to obtain highly precise gravimeter measurement, the design of optimum survey

layout needs to be considered. The use of different stations during gravimeter survey affect the

accuracy of the result. The election of which layout that used during survey is affected by theintended accuracy.


9/11

9

Figure 4: An Example of Gravimeter Survey Layout

Another consideration is to assess elevation and positional accuracy of the station. This is

critical to obtain highly precise gravimeter reading, since different elevation and position yields

different results. In conclusion, below are the steps that need to be taken in order to obtain precise

gravity measurement:

1.

Set up gravimeter in a stable manner

2. Establish the grid of proposed gravity stations and taking local topograpic variation into

account

3. Select the ideal base stations according to the grid

4. Making the measurement and distribute the appropriate drift corrections for each loop and

stations

5.

Take barometric reading when deemed necessary

6. Take the mean of the two or more sets of base station gravity values, corrected for drift,

instrumental level, tides and barometric pressure changes, etc., as being the correct values

for each base station

7. Complete each section of gravimetric survey

8. Establish the elevation and coordinates of each gravimeter station on the grid, by means

appropriate to the accuracy required of the survey

9.

Tie to national gravity grid when available


10/11

10

Data Processing and Presentation

An obvious step that need to be taken to obtain highly precise gravimeter data is to apply

correction. After that, the data can be presented either as table, diagram or contour map of the area of

interest. In order to deliver the measured gravity data into an easy to understand contour map, the

map maker needs to consider relation between Station Spacing, contour interval, and map scale. The

ideal relationship are given as follow:

Figure 5: Relationship between map scale and contour interval

Below is an example of Density Contour Plan and Bouguer Profile obtained using gravimetry

survey:


11/11

11

Figure 6: Bouguer profile (above) and Contour (below)

ids - proyek akhir

Documents