Gravity and Magnetic Modeling
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MAGIX PLUS v3
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MAGIX XL v3
Interpex offers two software tools that interpret magnetic and gravity profile data. The MAGIX family of software models data in 2, 2.5, and 2.75 dimensional earth models consisting of bodies and layers whose forward response matches the input data along a single profile line.
Horizontal Line
MATHEMATICAL BASIS
All the software packages in the MAGIX family use a generalized "2.75 dimensional" model to compute the theoretical gravity and magnetic response. The 2.75 D model represents the geology as polygonal prisms with horizontal axes and either finite or infinite extent in the strike direction. The strike direction need not be perpendicular to the plane of the profile. The forward model is based on the Rasmussen and Pedersen method.
The MAGIX family uses Inman-style ridge regression for the inversion calculations. This is a nonlinear least squares approach for obtaining the model which best fits the observed data. Parameters that may vary during an inversion include density, susceptibility, or the vertex of a body in X, Z or X any Z position.
The magnetic calculations take into account both induced magnetization and possible remanent magnetization. The remanent magnetization is entered as the inclination, declination and the Koenigsberger ratio.
MAGIX Family
Press here to download MAGIX XL v3 demo program
Data are read into the program via ASCII files of various formats. Options are available for data input including profile selection from maps and resampling of long profiles.
Data parameters are set using interactive dialog boxes. The program can calculate Magnetic or Magnetic Gradient data for Total Field, x, y or z components. Magnetic susceptibility units supported now includes cgs, micro-cgs and SI units
Magix XL XYZ import screen
Interactive profile selection by defining a polygon on a map of data points read from a XYZ file
Gravity density units include both gm/c3, and kg/m3 and calculations can be set to micro- or milligals.
Survey geometry is added by using a interactive graphics screen or text dialog boxes. The graphical geometry editing allows for animation of selected parameters. This helps the user understand the parameters required to specify the profiles in the 2.75D space.
Options for geometry include a profile or XY mode of coordinates and strike directions that is not perpendicular with the profile direction.
Magix XL geometry editing screen
Interactive graphical geometry editing screen
Body entry is done using the mouse in an interactive graphics screen. Keyboard entry into spreadsheet style editors is also available. Bodies are entered as a series of vertices. A variety of calculation modes can be chosen, one of which is Real-Time update of the synthetic response with changes in the model.
Body editing allows for features like vertex locking and homing, multiple vertex locking, susceptibility and density editing and strike extent editing. Bodies can be rotated, deleted, copied and split. All body editing can be done in the interactive graphics screen by mouse click and drag operations.
Keyboard entry and editing of bodies permits the user to enter model parameters from external sources and define bodies based on know values.
Model parameters can be saved to files for import with new data sets or comparisons of different models for the same data set. Models used for gravity data sets can be imported with magnetic data sets to function as starting models.
A variety of Output functions enables the user to save field data, synthetic data and model parameters to files for import or processing in third party software.
Graphical viewing is an integral part of the program and the user has full control over colors, axes labels, scales and data symbols. Autoscaling is built in with full user control over items to be scaled.
The regional editing allows graphical selection of the regional. The regional may also be calculated by fitting a polynomial to selected data points or groups of data points.
Magix XL regional removal screen
The interactive regional removal screen showing a third order polynomial fitted to selected groups of data points at each end of the profile
Model and data sections can be annotated by using the label editing sections. Features include mouse controllable sizing, placement and arrow editing. All labels can be set to a specific size, multiplied by a factor, or constants added to increase or decrease size.
Plate plot from MAGIX XL v3
Plate plot from MAGIX XL v3 showing difference curve, field data with the synthetic curve, the model and the map display of the model.
The following table lists the differences between the programs in the MAGIX family. Except for these differences the programs work the same
Product Selection Table
MAGIX PLUS v3 MAGIX XL v3
Maximum number of bodies 25 200
Maximum number of vertices 240 2000
Maximum number of data points 200 1000
Graphical Geometry Animation N Y
Regional removal by fitting a polynomial curve to the data N Y
File Input/Output from within the Interactive Graphics Interpretation N Y
Invert for density or susceptibility for all or selected bodies only N Y
Support for the input and output of LCT data and model files N Y
Sunday, 7 December 2008
Gravity and Magnetic Modeling
MAGNETIC METHODS
INTRODUCTION
Magnetometer surveys measure small, localised variations in the Earth's magnetic field. The magnetic properties of naturally occurring materials such as magnetic ore bodies and basic igneous rocks, allows them to be identified and mapped by magnetic surveys. Strong local magnetic fields or anomalies are also produced by buried steel objects. Magnetometer surveys find underground storage tanks, drums, piles and reinforced concrete foundations by detecting the magnetic anomalies they produce.
EQUIPMENT
Magnetometers are highly accurate instruments, allowing local magnetic fields to be measured with a high degree of precision. Magnetometer systems used for commercial applications include proton precession, caesium vapour and gradiometer magnetometers. The systems operate on broadly similar principles utilising proton rich fluids surrounded by an electric coil. A current is applied through the coil, which generates a magnetic field that temporarily polarises the protons. When the current is removed, the protons realign or precess along the line of the Earth's magnetic field. The proton precession produces a small but measurable electric current in the coil, at a frequency proportional to the magnetic field intensity.
Gradiometers measure the gradient of a magnetic field rather than total field strength. Magnetic gradient anomalies generally give a better definition of shallow buried features such as buried tanks and drums, but are less useful for investigating large geological features. The depth penetration of magnetic surveys is unaffected by high electrical ground conductivities, which makes them useful on sites with saline groundwater, clay or high levels of contamination where the GPR and Electromagnetic methods struggle.
DATA ACQUISITION & PROCESSING
Data acquisition for magnetic surveys involves taking point readings at regular intervals on a survey grid. The spacing between grid lines and reading stations is dependant upon the application. Generally smaller targets require higher resolution surveys using denser survey grids. Modern caesium vapour magnetometers and gradiometers are more sophisticated allowing data to be collected either in continuous mode or as point readings.
Data is stored digitally on site, and later downloaded on to a PC for post-survey processing and interpretation. Various interpretation techniques are applied to the data using specialist interactive software to identify the targeted anomalies. A combination of contouring and colour shading is used to highlight anomaly patterns. Survey results are presented as plans tied to site co-ordinates, in an engineering compatible format readily understandable by the client.
APPLICATIONS
+ Finding buried steel tanks and waste drums
+ Detecting iron and steel obstructions
+ Locating unmarked mineshafts
+ Accurately mapping archaeological features
+ Mapping basic igneous intrusives & faults
+ Evaluating the size and shape of ore bodies
Friday, 14 March 2008
ABSTRACT
MODELING OF
AND ITS SURROUNDINGS,
USING GRAVITY DATA
By
KUSDINAR KARTASAPUTRA
H1C003009
Departement of Physics
Universitas Jenderal Soedirman
Geophysical survey was done with gravity method at Menggala and its surroundings,
Gravity Method...
Basic Concept. The gravity method measures small spatial differences in the gravitational pull of the earth. If the gravity field over an air-filled void is measured, and the void is close enough to the ground surface, then a decrease in the gravitational pull across the void will be measured. This concept is illustrated in figure 1, which shows an underground void and the gravity profile over this void.
The instrument that is used to measure the gravity pull is called a Gravimeter. This instrument does not measure the absolute value of the pull of gravity, but measures spatial differences in the gravity pull; essentially relative gravity.
Voids can produce reasonably sized anomalies when they are large and fairly near the ground surface, but only very small anomalies result from voids at depth. Clearly, the size of an anomaly depends on the size and depth of the void, and at some point it cannot be observed. Thus, it is important to be able to measure the gravity field with as much accuracy as possible. Generally, for a spherical void to be detected, the depth to its top surface should be no greater than its diameter.
Figure 1. Gravity field over a void.
The Lacoste and Rhomberg model EG Gravity meter has an advertised repeatability of 3 microgals (980,000,000 microgals is the Earth's gravitational field) and is one of the preferred instruments for conducting gravity surveys (figure 2). Variations in gravity anomalies found when searching for cavities are typically a few tens to hundreds of microgals. These surveys are often called Microgravity surveys.
Figure 2. The Lacoste and Rhomberg model EG gravity meter. (LaCoste & Romberg)
Data Acquisition: In order to detect a void, gravity readings must be taken along traverses that cross the location of the void. Its expected size will determine the distance between readings (station spacing), with larger station separations for large voids and small separations for small voids. It is usually advisable to model the expected anomaly mathematically before conducting fieldwork. From this, along with the expected instrument accuracy, an estimate can be made of the anomaly size and the required station spacing.
Surveys are conducted by taking gravity readings at regular intervals along a traverse that crosses the expected location of the void. However, in order to take into account the expected drift of the instrument, one station (probably the first) has to be reoccupied every half hour or so (depending on the instrument drift characteristics) to obtain the natural drift of the instrument. Since gravity decreases as elevation increases, the elevation of each station has to be measured with an error of no more than about 3 cm. A more detailed description of the corrections to be applied to gravity data is described under Data Processing. Readings are taken by placing the instrument on the ground and leveling it. This may be automatic with some instruments, as it is with the Lacoste and Rhomberg model EG.
Data Processing: Several corrections have to be applied to the field gravity readings. Each reading has to be corrected for elevation, the influence of tides, latitude and, if significant local topography exists, a topographic correction. To understand the corrections, a gravity reading is first considered on the surface of a flat ground surface. Corrections are then applied to account for deviations from this condition. As the distance from the center of the earth increases, the pull of gravity decreases. The correction to account for this is called the elevation correction. If the gravity reading is taken on top of a hill, then there is a deficit of mass on either side of the hill, compared to a horizontal ground surface, as is illustrated in figure 3. This is corrected with a topographic correction. It is important to realize that mass higher than the reading site will also influence the data and has to be accounted for. This may occur in areas of significant topography or when surveys are conducted near large buildings. In a modern instrument such as the Lacoste and Rhomberg model EG, the meter may automatically apply the tidal and drift corrections.
Figure 3. Correction for topography.
Data Interpretation: Gravity data are interpreted using software to calculate the gravity field of geologic features, input by the operator. The program will then change the parameters (shape, depth) of the body (in this case void) until the calculated gravity field matches the field gravity field. This process is called inversion and is commonly used in geophysical interpretation. If several lines of data, or data from a grid, have been acquired, then a map of the gravity field may be produced.
The interpretation involves searching for areas where the pull of gravity has decreased and then modeling these anomalies to find the depth to the source of the anomaly and the approximate size of the feature.
Advantages: The gravity method responds directly to the mass deficit created by a void. The method requires a relatively small amount of instrumentation and is an unobtrusive method able to be conducted in environmentally sensitive areas.
Limitations: Gravity surveying is a labor-intensive procedure requiring significant care by the instrument observer. Gravity instruments require careful leveling before a reading is taken. This may have to be done manually or it may be performed by the instrument itself. Once the reading has been taken, the gravity sensing mechanism must be clamped to stop excessive vibration that would influence the sensing mechanism and cause excessive drift, thereby affecting future readings. The instrument must be placed on solid ground (or a specially designed plate) so that it does not move (sink into the ground). All of the stations have to be accurately surveyed for elevation.
Because the anomaly from voids rapidly becomes smaller with void depth, void detectability rapidly decreases with void depth. Because of this, pre-survey modeling using as much geologic information as possible is important to establish the expected anomaly size, and to determine if a gravity survey is feasible.
