US 7439977 B2
A method for generating, displaying and manipulating the orientation of three-dimensional vectors on a two-dimensional surface for visualization, statistical analysis and correlation. The method involves collecting vector data, transforming each vector into spherical co-ordinates, then assigning, a unique color related to a position on a spherical color model. A two-dimensional raster image filled with this unique color is centered at the point of measurement for each vector orientation as obtained from calculations or instrumentation capable of measuring this orientation. This methodology offers the user the ability to discriminate the location of specific orientations as well as the ability to define an enhanced full color gamut gradation for a specific range of orientation. This rendering of color-coded vector orientation enables easier understanding by the viewer of large data sets.
1. A method for displaying the three-dimensional vector orientations of magnetic fields on a two-dimensional surface comprising the following steps:
a. establishing a sampling grid over an area of geological interest having magnetic fields;
b. locating equally-spaced measuring stations for measuring said magnetic fields on said sampling grid, wherein said measuring stations are designated by letters;
c. creating a two-dimensional map of the sampling grid;
d. establishing a calibration station using a stationary tri-axial magnetometer for calibrating said magnetic fields, wherein said calibration station is located proximate to said sampling grid, and further wherein the calibration station is located in a magnetically quiet area;
e. conducting a calibration step at the calibration station, wherein said calibration step comprises the steps of:
i. obtaining a first measurement of the magnetic field in X, Y and Z directions using said stationary tri-axial magnetometer;
ii. obtaining a second measurement of the magnetic field in X, Y and Z directions using an operator held portable tri-axial magnetometer;
iii. determining the effect of said operator holding the portable tri-axial magnetometer on said second measurement; and
iv. calibrating said effect to the portable tri-axial magnetometer so that the effect of the operator is nullified;
f. conducting a survey step of obtaining magnetic field measurements at each of said measuring stations using said portable tri-axial magnetometer and recording the time at which said magnetic field measurements were taken, wherein the magnetic field measurements are represented in three dimensions as Cartesian coordinates Xa, Ya and Za where “a” indicates the measuring station designation, and wherein said survey step occurs over a period of time;
g. measuring the magnetic field at the calibration station over said period of time and determining an average magnetic field measurement over the period of time in order to obtain a calibration value corresponding to the time that the magnetic field measurements are made;
h. correcting the magnetic field measurements by subtracting said calibration value in order to obtain a calibrated value for each of the magnetic field measurements Xcalibrated, Ycalibrated and Zcalibrated;
i. correcting said calibrated value for each of the magnetic field measurements by subtracting the value of the magnetic field of the earth at each measuring station to obtain a residual value for each magnetic field measurement Xresidual, Yresidual and Zresidual, wherein said value of the magnetic field of the earth is determined by applying the International Geomagnetic Reference Field; and
j. correcting said residual value by subtracting the values of induced magnetic fields to obtain a remanent value for each magnetic field measurement Xremanent, Yremanent and Zremanent.
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The present invention relates, in general, to data processing and color-coded raster rendering of information, and more specifically, to a method for displaying the orientation of three-dimensional vector data on a two dimensional display or surface. This methodology has specific applications in the field of geophysical surveys.
The term “ferromagnetic” used hereinafter shall constitute the inclusion of antiferromagnetic and ferromagnetic materials.
The term “Currie temperature” or “Currie Point” used hereinafter shall also encompass the Ne'el temperature as would be used in the context of antiferromagnetic and ferromagnetic materials.
In the study of vectors within vector fields it is often useful to correlate information that relates to the magnitude, orientation and spatial relationship of the individual vectors within said field.
While numerous effective methodologies exist for displaying the magnitude of individual vectors, the ability to observe commonality of individual vector orientation within vector fields containing thousands of individual vectors on a surface or within a volume can be difficult to discern.
The applicant is aware of several attempts in prior art, which relate particularly to the visualisation of three-dimensional vector fields. Reference may be had by turning to Form PTO/SB/08B where references are cited thereon.
Current methodologies for indicating vector orientation consist of streamlines, hedgehogs, stream-ribbons, stream-polygons and textures maps3. In all these methods there are difficulties in observing the vector orientation orthogonal to the observation surface or in selection of unique desired orientations.
Many applications exist for the use of an effective display of vector orientation including studies in fluidics, electromagnetic fields, finite element analysis, magneto hydrodynamics, plasma physics, astrophysics magnetic and digital elevation models (DEMs). We will present an embodiment in which the application of the present method is illustrated in the field of geophysical geomagnetic mineral exploration. It is to be recognized that this embodiment is but only one example of the usefulness of this Vector Orientation Visualization Method.
Geomagnetic surveys have many uses and are of particular interest in mineral exploration, as they may identify areas likely to contain some types of valuable deposits. They can also identify various lithologies, and provide some types of structural information.
A common method of performing such a survey is to measure the magnetic field at various points along the Earth's surface while recording the magnitude variations in the magnetic field strength. Although dip needle surveys have been used in the past, until recently, the orientation of the magnetic field was not generally measured. The magnitude of variations in the readings was primarily due to variations in the magnetic susceptibility of the constituent minerals within the rock, and to a much lesser degree, the variation of the orientation of the remanent field. Such surveys, while giving useful data, which related primarily to lithological effects, disregarded the information of the orientation of the magnetic field.
Recently, due to the introduction of tri-axial gradient magnetometer surveys, it has become possible to retrieve vector oriented gradient data. In addition, tri-axial fluxgate total magnetometer surveys can also be used to measure the orientation of the total magnetic field vector. Tri-axial fluxgate magnetometers have also been used in along a borehole to effectively delineate the partial outline of a dipole about a ferromagnetic ore body4.
The ability to effectively display the orientation of the magnetic vectors over a surface or within a block of rock as by borehole surveys can greatly add to the geologic understanding of the survey area and can lead to a better understanding of the location and extent of several varieties of ore deposits.
To understand the benefit of knowing the orientation of the vectors within the survey area it is important to understand the means by which variations in the orientation of the vectors can occur. After a high temperature thermal event, certain minerals in the earth will heat enough beyond their Currie Point Ne'el temperature to lose their magnetism. When the temperature lowers, these minerals regain their magnetism and will now have a thermal remanent magnetic field aligned with that of the earth at the time of going through their Currie temperature. Since the earth's magnetic field is known to change directions and flip at various times, the differently oriented remanent magnetic vectors of these remagnetized minerals can indicate spatially segregated intrusives of contemporaneous age, multiple pulsed intrusives within a similar rock type, the extent of their thermal aureoles and the presence of folds and faults. Similarly alteration due to chemical—known as chemical remanent magnetism, CRM—and pressure changes—known as piezo remanent magnetism—can result in the formation of new minerals, which will retain a remanent orientation of the earth's field orientation at that time—known as detrital remanent magnetism (DRM) caused by the quiescent sedimentation of fine magnetic material aligning with the earth's magnetic field at the time of sedimentation.—DRM is another remnance whose variation can be used to recognize later faults and folds.
For the purpose of this embodiment a magnetic survey can be performed using either a tri-axial fluxgate magnetometer or tri-axial gradient magnetometer. The vertical gradient can also be calculated if both horizontal gradients and the total magnetic field are known. These types of surveys produce the magnitude of the magnetic field for each of the three-orthogonal directions. The data is collected for process and analysis over the survey area by establishing a two-dimensional grid over the area of interest. Measurements of the magnetic field are taken at specific points on the grid either by aircraft or by an operator on the ground working along a cut or demarked grid. Each collection point is identified as a point on the two-dimensional grid, and will have an associated three-dimensional vector representing the magnetic field at that point. Alternatively, if the data should be collected along a borehole as a function of its distance from the collar, the location of the three-dimensional vector can be represented from the surveyed borehole information as a point in 3D space with x, y, z co-ordinates. The x,y co-ordinates can match the two dimensional grid of the surveyed area of interest mentioned earlier.
Peters Reference1: This reference describes a method of representing an electromagnetic vector field defined over a three-dimensional surface. A triangular grid represents the surface, and the method uses color shaded magnitude contours and directed vector lines to characterize the vector field. Referring to the FIGURES, it may be seen that in this reference, the direction of a vector field is represented using arrows, and the magnitude of the vector field is displayed using colors. Such visualization is suited to vector fields for which the direction changes gradually and continuously. The magnitude of the vector field can then be visually identified by the intensity of the colours used.
Simkin Reference2: This reference provides an overview of three-dimensional visualization, and only contemplates displaying vectors using arrows. The direction of the vector is shown using arrows while the length of the arrow visualizes the magnitude, In other words, the longer the arrow, and the higher the magnitude of the vector.
Yamrom Reference3: This reference describes a method of animating vector fields and discusses the subject matter of displaying a 3D-vector field by changing the texture of the viewed surface with respect to time.
According to an aspect of the present invention, there is provided a method of displaying three-dimensional vector field information on a two-dimensional display. After collecting the information from an area of interest, the Cartesian vectors are transformed into spherical co-ordinates. A colour model such as Munsell, HSV, HLS, or YIQ, is modified into a spherical colour model. In the embodiment of the modified HSV model, the spherical model is developed such that Hue is distributed evenly along the equator while the Value varies from white at the North Pole to black at the South Pole. Each of the vectors are plotted emanating from the origin of the coloured sphere using their spherical co-ordinates. An HSV colour is assigned to the area of the vector on the map based on the colour occurring at the corresponding piercing point of the vector with the designated HSV spherical colour model surface. This HSV colour is then used to fill the area or volume of influence of the vector. In the case of a monitor display the HSV values are converted to their corresponding RGB values. Ability exists using this model to define a particular orientation range of azimuth and dip such that the whole colour gamut may be displayed within a narrow range and hence further delineate variations in orientation.
The utility of this methodology includes but is not limited to geophysical surveys.
For purposes of clarification and to provide a fill disclosure, it was deemed absolute that color drawing be included in the present application in order to fully understand the scope of inventiveness.
These and other advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawings in which:
While the invention will be described in conjunction with illustrated embodiments, it will be understood that it is not intended to limit the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
In the following description, similar features in the drawings have been given similar reference numerals.
Initially, in the case of a surface ground based geomagnetic survey the area of interest is selected and defined by setting up a grid over the area form which the magnetic field data is to be collected 10, in the case of an aircraft survey, location of the flight lines would be obtained by utilizing rapid precision Global Positioning System (GPS) equipment.
At each of the measuring stations 116, the magnetic field data, the time of the measurement and the station location is recorded within the grid 100.
In order to distinguish measurements from the various measuring stations 116, the measuring stations are denoted as LN(orientation)+(distance)(section). This is a standard notation in the exploration industry in North America and is further explained below. The ‘LN’ values represents the measuring station's line location with respect to a selected origin of the baseline 102 with L0+00 representing the measuring station at the origin on L00+00. In our example, since the lines 112 are orthogonal to the baseline and 100-meters apart, n=BLdistance/100, ‘distance’ being the distance along the baseline 102 from the selected origin. As one progresses in a northward ‘orientation’, the lines 112 are sequentially labelled L00, L01N, L02N . . . LN with (N+1) being the number of lines 112. The ‘distance’ value represents the distance the measuring station 116 is located away from the baseline 102 and the ‘section’ value represents the section 104 or 106 in which the measuring station is located. Therefore, for a measuring station in the Eastern section 104 located 50-meters northward away from the baseline 102 on the perpendicular line corresponding to L04N, this measuring station may be denoted as L04N+50E. In the grid of
After the grid 100 has been defined, a magnetically quiet area, close to the grid, is selected so that a tri-axial magnetometer base station 118 may be located (step 12) and appropriately positioned to collect signals which are used to assist in the calibration of the signals collected at the measuring stations 116 The base station 118 instrument's clock is set to match the clock of the portable mobile tri-axial instrument. This will be described in more detail below.
Prior to collecting the data (step 14) the operator initially records, from a distance, x,y,z readings in a magnetically quiet area with the tri-axial magnetometer, this could be a tri-axial fluxgate instrument suspended at the proper height and orientation by a remote nonmagnetic device. A second set of readings is then taken in the same location with the operator holding the instrument in the normal manner to take a reading. These two readings are recorded so that any magnetic effect from the presence of the operator can be removed. This procedure may be undertaken to confirm a consistent operator null correction
Having undertaken the above operator nulling procedure, the operator walks with the instrument along the baseline and lines such that the individual bypasses each measuring station at least once. At each measuring station, the individual records and stores magnetic field data in the form of magnetic field vector information along with the station location and the time the measurement is recorded. Care is taken in the positioning and orientation of the instrument, aided by a sighting and levelling device to maintain a consistent x,y,z orientation of the probes. The magnetic field vector information collected includes information corresponding to the magnitude and direction of the magnetic field at the measuring station location. This may be achieved by simply writing down the magnetic field data on a notepad or by entering such information into a data collector for later dumping on a computer. The recorded signals X1,Y1 and Z1 may be represented as Xm, Ym and Zm with “m” representing the measuring station designation.
At the end of each days work after the individual has collected the magnetic field data from each of the measuring stations visited, the individual retrieves calibration information from the base station corresponding to the time period over which the magnetic field data was being recorded. After retrieving the calibration information from the base station, the calibration information and the recorded signals are stored in a computer and the recorded signals are calibrated. This is a standard procedure known to those skilled in the art.
The calibration process allows for the magnetic field data retrieved from the measuring stations to be a truer indication of their actual magnitude and direction. It will be understood that the magnetic field data may be affected by external conditions such as solar storms or diurnal variations.
After selecting a first-recorded signal Xm, Ym, Zm from the mobile station, the time at which the signal was recorded is retrieved. The calibration values for this time is then calculated as explained above. If the reading was taken at 6:15, the base-station calibration values Xbasestation6:15, Ybasestation6:15, and Zbasestation6:15 are retrieved and subtracted from the measuring station values Xm, Ym and Zm corresponding to the first recorded signal in order to perform a first step of calibrating the measuring station value. After the first reading is calibrated, the magnetic field data from a second reading is selected and calibrated by retrieving the time measurement from the magnetic field data and calculating and retrieving the calibration values corresponding to that time measurement. This step is repeated until all of the magnetic field data from each of the measuring stations is calibrated. Therefore Xcalibrated equals Xm−Xbasestation(time) which is equal to Xm−|Xbavg−Xb(time)|. Values for Ycalibrated and Zcalibrated are calculated in a similar manner.
After the values Xcalibrated, Ycalibrated and Zcalibrated for each measuring station have been calculated. A second step may be performed to remove the geomagnetic field vector from these values in order to obtain a residual magnetic value from the magnetic field data.
This is achieved by calculating the Earth's magnetic field using the standard International Geomagnetic Reference Field (IGRF) procedures to determine the theoretical values with respect to the area of interest at the time of the survey These theoretical values obtained may be represented as Xigrf, Yigrf and Zigrf.
The residual magnetic data values Xresidual, Yresidual and Zresidual may then be calculated from the equations Xcalibrated−Xigrf, Ycalibrated−Yigrf, and Zcalibrated−Zigrf, respectively.
After calculating the residual magnetic values, further methods known to the trade5 may be performed which causes the induced magnetic field value to be removed from the residual magnetic data values to obtain remanent field values at the measuring stations.
In order to calculate the remanent values Xremanent, Yremanent and Zremanent, the equations Xresidual−Xinduced, Yresidual−Yinduced, and Zresidual−Zinduced, are respectively determined. The remanent value represent the magnetic field data values were frequently acquired at the time the ferromagnetic rock forming minerals were formed or went through a Currie temperature episode of its ferromagnetic minerals. This remanent vector data can allow for the discrimination. After calculating either the residual or remanent data these x, y, z Cartesian values are transformed to mathematical spherical co-ordinates using the following equations:
These mathematical spherical co-ordinates are illustrated clearly in
The mathematical spherical co-ordinates of the magnetic vector at each measuring station may then be represented by (rmath, Θmath, Φmath).
A conversion of the mathematical spherical co-ordinates (rmath, Θmath, Φmath) is then translated into the geological co-ordinates (rg, Θg, Φg) by the following equation:
After calculating the geological spherical co-ordinates for each measuring station, a unique colour based legend may be determined by assigning colour or patterns to the spherical co-ordinates. Several colour models or notations exist which have direct symmetry with respect to spherical co-ordinates such as red, green, blue (RGB); Commission Internationale de l'Eclairage (CIE), hue, saturation, value (HSV); hue, saturation, luminance (HSL), CIE XYZ, YIQ, Munsell, TekHVC, CIE LUV etc . . . The last three of these models allow for a more continuous perception of colour to the human eye. Although any of these colour models may be used in the preparation of a suitable colour model via the proper mathematical translation. In this embodiment a modified spherical HSV colour model is used, where hue variation is assigned the azimuth angle. Axial symmetry about dipole axis may also be illustrated by having hue vary as dip.
One example of assigning spherical co-ordinates to an HSV model may be realized by the following two equations, the use of each being dependent on the value of phi:
The above assignment relates a unique HSV colour value to the direction of a three dimensional vector corresponding to measured and calculated values from the measuring stations which can be applied to the area of interest.
After each of the spherical co-ordinate vectors have been associated with an HSV value, see the table of
These coloured areas located about each of the measuring stations may also be input into any of the many standard contour algorithms that exist for plotting of these coloured pixelated areas to produce a colour map with gradational interstitial colours applied between the lines and covering the whole area. These interstitial colours could represent, along observed trends, the shortest distance between the two colours as illustrated on the spherical colour model.
With the advent of computer processing capabilities, the unique display of this vector orientation data lends itself to a number of useful statistical presentations. In
Additionally the magnitude of the vectors may also be shown on the coloured illustration either as dashed black and white line contours or as a Digital Elevation Model.
Alternatively to the ground acquisition method described earlier, an aircraft flying over the grid may retrieve the above information. After the grid has been determined, the aircraft flies over a prescribed calculated grid lines to record the magnetic vector information at regular time intervals corresponding to a specified distance given the airspeed flown. In order to ensure that the aircraft follows the calculated grid lines, a GPS is to be used to guide the aircraft. In this aerial data collection no baseline or ground demarcation of lines is required.
The magnetic field vector information is collected via instruments on the aircraft as they fly over the measuring station and stored on record keeping equipment, such as a data logger, located on the aircraft. Calibration and corrections to the data are carried out later after each day's flying.
An example of the 3D versatility of this colour coded vector orientation methodology is also given in