Gravity and Magnetics Workbooks
The workbook contains the example of basic principles in using Gravity and Magnetics methods for exploration purpose....
ASSIGNMENT COVER SHEET SHEET
Sutthisrisaarng Pholpark __________________________________________
Gravity and Magnetics for Exploration 301 __________________________________________
Paul W. __________________________________________
DECLARATION I have read and understood Curtin’s policy on plagiarism, and, except where indicated, this assignment is my own work and has not been submitted for assessment in another unit or course. I have given appropriate references where ideas have been taken from the published or unpublished work of others, and clearly acknowledge where blocks of text have been taken from other sources. I have retained a copy of the assignment for my own records.
______________ _____________________ ______________ _____________ ____________ ______ [Signature of student]
For Lecturer’s Use Only:
Overall Mark: ______ ________ __ out of a tot al of ______ _________ ___
Lectu rer’s Name: Name:
Date Date Retur Retur ned:
CURTIN EXPLORATION GEOPHYSICS GEOPHYSICS 301 WORKSHOP 1 – 4 March 2014 OBJECTIVE
This workshop is an i ntroduction to the software package Geosoft Oasis Montaj which we will be using extensively in Geophysics 301 and in other units of the Ge ophysics undergraduate undergraduate course. In this workshop we will be b e using aeromagnetic data from the Ravensthorpe area of WA. This is an interesting and varied dataset from an area with known mineral deposits and potential to find more. The dataset is contained within within three files : rav3.xyz, rav4.xyz, rav5.xyz. These files files are in the Blackboard folder workshop 1 - march 4 2014 Each of these these files contains contains eastings, northings and aeromagnetic values in that order. Keep good detailed notes IN YOUR LAB BOOK – handwritten and electronic versions on what you do to help you in the learning process and for assessment during this course.
1. Copy files rav3.xyz, rav4.xyz, rav5.xyz, and magmap.con to a working directory within your geophysics computing account. 2. Log in to Oasis Montaj Start > All Programs >geosoft > oasis montaj > oasis montaj 3. Create new project. Use file > project > new Enter project name 4. Create new database – allow for up to 500 flight lines. Key in 500 in place of default value of 200. Database > New Database Enter new database name 5. Import rav3.xyz to the new database using Database> import>geosoft xyz This file is in geosoft xyz format 6. Import rav4.xyz into the database using append option 7. Import rav5.xyz into the database using append option
8. Split the three lines of of data in the database database into individual flight lines, using Database tools >line tools > split on x,y breaks. b reaks. Select all lines 9. Set coordinate projection parameters for database. Steps: Go to “Coordinates”. Select current x,y,z coordinates and select x,y and z channel names, followed by OK Select coordinate system and go to Coordinate system at bottom of this window Select Projected (x,y). Select projection method as Australian map grid zone 51. Select datum AGD84 – this is Australian geodetic datum for 1984 Check length units as metres Check table of parameters and if ok select ok, otherwise modify. The appearance of a magnetic map changes with location on the Earth. According to this reason, the user has to select appropriate projection method for the area of survey. A local datum is also important because it varies by a location of surveying. 10. Create map showing all the flight lines. Map tools> new map, based on x.y, paper size A3 landscape, scale ?, line path This will show dense pattern of lines – can zoom in and see more detail. What is line direction ? line spacing/s spacing/s ? (using right mouse click with cursor on map and select ruler function function ). Save map changes.
Figure 1 Flight Lines
This procedure allows user to display flight lines o f the survey. Scale refers to the relationship (ratio) between length that appears on a map and actual length of
data. A good scale allows user to display a map wi thout losing its details. In scale function, the automatically calculated scale is 1:194271.8. However, I use scale 1:20000 instead because it easier to calculate the actual data from the map. Line direction is a direction of surveying in each flight. The line direction of given data is North-South. Line spacing is a space between each flight. It can be determine by a space between each line or spaces between N line and divide by N for a better accuracy. The line spacing in giv en data is 100 metres. 10. Create base map with legend, scale bar, north arrow, title block, colour legend bar. Use margins around map of 2 cm left, to p and bottom and 14 cm on right of map. Use Map tools > Base Map
Figure 2 Flight Lines with Base Map
Only flight lines alone are unable to tel l the information about the map. So that base map is an essential tool to describe a ma p details e.g. scale, directions, map name. 12 Grid the magnetic data with appropriate grid interval. What is your choice and why ? Use grid and Image > gridding > minimum curvature etc. My grid interval is 25 which is ¼ of line spacing due to it is appropriate interval to display data without losing its detail(discontinuous from line to line). My gridding
method is minimum curvature because it displays all sufficient in my grid and doesn’t use much time in calculation. 13. Display the magnetic grid using a. single grid option b. Colour shaded option What sun illumination options are most suitable and why? Save map changes.
Figure 3 Single Grid Option
Figure 4 Color Shaded Option
For the colour shaded grid, the image illuminated from the north. I use illumination inclination 45 degree and illumination declination 45 degree because the magnetic map is display in North-South direction and the chosen angles are appropriate angle to enhance magnetic details of a magnetic value. Colour shaded bar helps a reader to define magnetics value on a map. Each colour represents measured magnetic value in nT. 14. Create a new map of a 10 x 10 km subset of the data. Use Map Tools >, new map from x and y, and key in selected rectangle of coordinates selected using maps created in steps 11 -1 2. We can select subset area in map by enter desire data in data range to map function.
Figure 5 10x10 Selected Subset Area
15. Create a stacked magnetic profile map for this sheet. Using Map Tools > Profile.. Start off with coarse vertical scale for profiles and base value of close to average value of z1 eg 58000 NT To restrict the profiles to just the selected subset area – go to view/group manager ( left hand icon on third row down of the icons), highlight profiles in data area and select “Masked to View region”
Figure 6 Profile Plot Setting
Figure 7 10x10 Subset Area Magnetic Profiles
Figure 8 10x10 Subset Area Magnetic Profiles with Shaded Grid
We can create magnetic profiles map from each flight line by this function. Profile scale that I use is 100nT/mm and profile base is 58000 NT which is the average magnetic value in the area. 16. Open database and create profiles of individual lines using split screen – data in top panel, profile in lower panel.
Figure 9 Magetic Profile of Line231028
We can view magnetic profiles of each flight line by this function to observe more detail of our selected flight line. We can also select any point in magnetic profile to read actual data in database eg. easting values, northing values. 17. Grid filtering using MAGMAP to create derivative in Z direction Using input of magnetic grid created in step 11 o f these notes. This derivative is often called the first vertical gradient of the magnetic field and shows how the magnetic field varies with height above the g round. It is very useful in interpretation. MAGMAP is where most frequency domain gird filtering is done. Steps: Gx > load menu> magmap.omn This brings up MAGMAP in top row of optio ns Select MAGMAP 1-Step-filtering, In SetConFile select Derivative in Z direction, orde r 1. In SetTrend select first order and edge points. In SetExpand select 10 % expansion and rectangular In SetFill select maximum entropy option To run select OK at bottom left of magmap processing window Display this new grid using sun illumination. How does this grid compare with previous magnetic grid ? What is average value of the vertical gradient ?
Figure 10 MAGMAP derivative 1st order in Z-direction
Figure 11 Magnetic map with MAGMAP derivative 1st order filter(Left) and Magnetic map no filter(Right)
Edges of the magnetic map after MAGMAP derivative first order filter is clearer compared to the magnetic map before filter. Gradient filter or derivative f ilter helps to enhance area where magnetic anomalies are sudden changed.
Figure 12 Magnetic map statics with MAGMAP derivative 1st order filter(Left) and Magnetic map statics no filter(Right)
The average value of the vertical gradient grid is relatively low (0.0060403944nT) compared to the normal grid without filter (58620.696nT). Due to the derivative in Z-axis reduces overall magnetic values of the gri d. 18. Save all map changes, database changes, save project and exit from Oasis Montaj to end session. TO BE CONTINUED IN WORKSHOP 2
PGW 4 March 2014
Curtin Exploration Geophysics GEOPHYSICS 301 WORKSHOP 2 – 11 MARCH 2014 and ASSIGNMENT 1 OBJECTIVE This workshop continues on from workshop 1 – with more processing of the Ravensthorpe aeromagnetic dataset. Remember if any steps are unclear you have help facility in main menu to help you. This pair of workshops provides good learning of basic steps in oasis montaj required for processing and interpretation of aeromagnetic data.
Keep good detailed notes IN YOUR ELECTRONIC LAB BOOK on what you do to help you in the learning process and for assessment during this course. They will also help in writing up Assignment 1. STEPS STEPS 1. Contouring. Produce contour map for the subset area which you created in workshop 1 Mapping, contours, specify grid to be contoured, contour intervals. What contour intervals did you decide on and why ? As before add base map and legend information.
This is contour interval that I decided because it gives clear details in my magnetic map and it also cover major changes of magnetic anomalies.
Figure 1 Grid Interval(Left) and Line Colour(Right)
Figure 2 Magnetic Contour Map Without Grid(Left) and with Grid(Right) 2. Upward continuation Starting with the magnetic grid that you created in workshop 1, compute two new grids to simulate what would have been recorded if we had flown much higher than the original 80 metres above ground level. Compute and make images of two new grids upward continued by 1 km and by 4 km. Do these using magmap, 1 step filtering etc ( like you did for vertical gradient computation in workshop 1 ). Add legends as usual. How do these maps compare with the original magnetic gridimage.
Figure 3 Upward Continuation 1 km(Left) and Upward Continuation 4 km(Right)
Upward continuation filter use to match data at different levels. From upward continuation simulations, magnetic grid image resolution and magnetic amplitudes of both upward continuation 1km and 4km are obviously lower than original magnetic grid image. In addition, the resolution of grid image and magnetic amplitudes of upward continuation 4km simulation is relatively lower than the grid image of upward continuation 1km. So we can conclude that magnetic amplitudes and a resolution of magnetic grid image decrease as a distance to the source increases. 3. IGRF – international geomagnetic reference field This global model enables you to calculate total field intensity, declination and inclination for any lat, long, height above sea level and date. Calculate appropriate figures for the Ravensthorpe survey using lat = -33.5 deg, long =120.5, height above sea level of 100 metres and date of 2000/01/01 Write down the figures you obtain. Field Strength = 59570.6292nT Inclination = -67.78539047degrees and Declination = -0.29965751 Magnetic anomalies in any area change over a time period. IGRF helps us to calculate a reference magnetic field which use in an interpretation in an area of survey. We can use these values in a reduction to pole filter in the next question.
Figure 4 IGRF result 4. Reduction to pole From the original magnetic grid, create a reduction to pole grid. This simulates vertical inclination of the Earth’s field and often moves magnetic anomalies to be more centred over their geological sources . Process is : magmap, 1 step filtering, set control file etc. Add legend as usual. How does this image compare to original magnetic image ? Can you detect horizontal offsets in features and if so by how many metres ?
Reduction to pole filter feature is to centre anomaly over a source(shift magnetics anomalies in to the pole). Hence, the image from reduction to pole filter is shifted to
the actual pole. The image that I created is shifted to the left of x-axis and shifted down in y-axis compared to the original image.
Figure 5 Magnetics Grid with RTP filter (Left) and Original Magnetics grid(Right)
The red line represents magnetics value from normal grid(no fi lter) and the green line represents magnetics value from reduction to pole grid. We can fi nd horizontal offset by compare the x,y value between nearby peaks of magnetic of these lines. Magnetic value without filter : 59117.3nT Easting(x) : 238089.1m, Northing(y) : 6274546.9m
Figure 6 Magnetics Profile(no filter)
Magnetic value RTP : 59973.0nT, Easting(x) : 238034.0m, Northing(y) : 6274496m
Figure 7 Magnetics Profile(with Reduction to Pole filter)
So that horizontal offset between the normal magnetic grid and the reduction to pole magnetic grid in easting(x) is 238089.1-238034.0= 55.1m and in northing(y) is 6274546.9-6274496=50.9m 5. Grid profiles For interpretation purpose it is often useful to produce profiles at right angles to the anomaly trends. This can be done using the gridprof gx facility in oasis montaj steps : - create new database ready for grid profiles - gx, select gridprof.gx - line name gp1 - manually digitise position of line end points from the image for two new lines - specify data spacing required in the computed profiles (I choose 25m.) - open database to display data values along the new profiles and create stacked profile display on top of the original image. Probably useful to draw the profiles in white to contrast with the image colours.
I created straight line which cut through anomaly trends as seen in the pictures below. After that magnetics profile is auto generated in the database. From the data base the red line represents magnetics value from normal grid(no filter) and the green line represents magnetics value from reduction to pole grid.
Figure 8 Magnetics Profile Along the line
After that I generated a magnetics profile along the line of the reduction to pole grid and the original grid to compare their profiles. These profiles are look familiar but not exactly the same.
Figure 9 Magnetics Profile Along The L ine With RTP filter(Left) and Original Magnetics Profile Along The L ine(Right)
6. Horizontal gradient computation on magnetic profile data In interpretation it is often useful to be able to recognise inflection points – positions of mazimum ( +ve and –ve ) horizontal gradient., and also positions where gradients are half those at the adjacent inflection points. eg use in Peter’s half slope method of determining depths. These operations are easily done using convolution filtering on line data.
Do this on the gridded profiles computed in step 5 above. Steps : Database Tools > Filters > Convolution Filters, select input magnetic channel, specify new gradient channel ( hgrad ) to be computed and input the coefficients 0.0833, -0.6667, 0, 0.6667, -0.0833. These are as given in lecture 2 . The new hgrad channel is added to the database of grid profiles. They are not immediately displayed. You will need to select display to view all channels. Use profile display to show grid profiles and gradient profiles. Do you see how to recognise where the inflection points are ? In the picture below, the RED LINE is magnetic values and the Green LINE is horizontal gradient value. We are able to recognise local inflection points by detecting local maximum values or local minimum values of horizontal gradients. HGRAD filter is used in finding an actual depth from magnetic anomalies(workshop4). For example The square point in GREEN LINE is a local maximum horizontal gradient value(238.3). So that the local inflection point is a point in RED LINE which the magnetic value is 59669.0nT and the position is(239132.4,6273248.8).
Figure 10 Local Inflection Point(Example1) The square point in GREEN LINE is a local minimum horizontal gradient value (-185.2). So that the local inflection point is a point in RED LINE which the magnetic value is 59778.7nT and the position is(239308.8,6273426.0).
Figure 11 Local Inflection Point(Example2)
GEOPHYSICS 301 – WORKSHOP 3 – 17 and 24 March 2014 OBJECTIVE This workshop is about gravity surveying in diamond exploration. The dataset is from the Kimberleys, WA. The workshop also provides an example of regional / residual separation and terrain corrections. STEPS 1. Read into your working directory the oasis montaj database Bollinger gravity.gdb contained in Blackboard. 2. Display station locations using mapping and symbols. What is the data spacing? This is the coordination that I use in this workshop
Figure 1 Coordination System
Figure 2 Station Locations
Figure 3 Data Spacing
The data spacing in this map is (160/4)=40m 3. Grid the channel boug_anom2.3 What is appropriate grid spacing ?
Figure 4 Bouguer Anomaly for Density 2.3g/cc
Figure 5 Bouguer Anomaly profile for Density 2.3g/cc
The appropriate grid spacing is (160/4)/0.25=10m Bouguer anomaly is a corrected anomaly after the survey. Bouguer anomaly=observed value of g+ free-air correction-Bouguer Correction+terrain correctionlatitude correction- Eotvos correction
4. Compute regional /residual nd rd Use grid, filter, trend (use all points). Use 2 and 3 order trend surfaces. •
Figure 6 Residual Grid 2nd Order Derivative(Left) and The Grid Statistics(Right)
Figure 7 Residual Grid 3rd Order Derivative and The Grid Statistics(Right)
The second/third order derivative is used to enhance the local features and remove the regional from bouguer anomaly. The second/third order derivative filter is applied to bouguer anomaly in order to obtain residual grid. •
What does the local regional look like? Does 2 order do a better job than 3 order regional? The local regional has difference levels of g ravity anomalies. The range of gravity anomalies of 3rd order regional is between -0.124 t o 0.194 mGal (0.318mGal) and the 2nd is between rd nd -0.142 to 0.186 (3.28mGal). The 3 order regional do the better job than the 2 in enhancing the differences in gravity levels. Use : gridmaths, subtract grids where grid 1 = bouguer grid, grid 2 = residual grid, grid 3 = grid 1 – grid 2 = regional grid
Figure 8 Regional Grid
Figure 9 Regional Grid Profile
Figure 10 Bouguer Anomaly Grid(Blue Line), Regional Grid Profile(Red Line), Residual Grid Profile(Green Line)
5. What does the topography look like – display topography grid? What is the range o f topography data?
Figure 11 Topography Grid(Left) and The Grid Statistics(Right)
Topography can be obtained from DEM or Digital Elevation Model which represent the bare ground surface without any object. In the topography, there is a low topography area on the left of the displayed topography and a high topography area on the right of the displayed topography. The levels of topography are gradually decreasing from the left to the right. 6. Do gravity terrain corrections Use gx, load menu, gravity.omn . Compute gravity terrain correction grid.
Figure 12 Gravity terrain correction panel
Figure 13 Terrain correction Grid(Left) and The Grid Statistics(Right)
What is range of terrain corrections? The range of terrain correction between 0.0006486 to 0.1137043 mGal(0.11 mGal).
Figure 14 Topography grid(Left) and Terrain correction Grid(Right)
How do the terrain corrections relate to the topography? Terrain corrections grid are computed from topography(DEMs). Terrain corrections help correct the effects of topography eg. hills, valleys, cliffs in the survey area in order to clarify the underlying target.
7. Where are there potential kimberlite targets – given that we are looking for small negative bouguer gravity anomalies due to the weathered clay tops over the pipes? List coordinates of 6 potential targets.
Figure 15 Potential targets
Were terrain corrections useful in helping to identify the targets? Terrain corrections are useful in removing the effects of topography in the observed gravity. In this survey, we are interested in the anomalies of the subsurface body, hence the effect of topography need to be removed in order to point out the potential targets more accurately. To be written up in your workbooks by March 31 PGW 17 March 2014
Workshop 4 – March 25 and April 1, 2014 AEROMAGNETIC DEPTH ESTIMATES OBJECTIVE
This workshop illustrates the material covered in lecture 4 of March 24. Papers for steps 5 and 6 are included in Blackboard under Workshop 4 Remember that the flying height for the Ravensthorpe aeromagnetic survey is 80 metres above ground. STEPS
1. Compute analytic signal of the aeromagnetic data for Ravensthorpe. Use Grid and Image > Filters > Analytic Signal 2. Select two suitable grid profiles across the Ravensthorpe aeromagnetic grid and the anal ytic signal grid. Suitable means a single magnetic anomaly undistorted by adjacent anomalies and where the profile is at right angles to the source. Sampling interval 25 m was used.
Figure 1 The Chosen Dyke
Figure 2 Magnetics profile
Analytic signal and Horizontal gradient were compute for using in further process. The database was exported and converted to and Ex cel file.
Figure 3 Export file
3. Calculate depth from the half height half width of the peaks in the Analytic signal profile. This distance is approximately equal to depth below m agnetometer.
Figure 4 Magnetic profile
Figure 5 Analytic signal
Figure 6 Half height Half width method
4. Use Peters half slope method with a factor of 1.6 to get depth to source. Use horizontal gradient calculation on grid profiles - as we did in workshop 2. Look for positions of maximum gradient (inflection point) on either flank of the chosen anomaly and then the positions of half maximum slope either side of the inflection points. The horizontal distance between these half maximum slope positions is divided by the Peter’s factor to get depth below magnetometer. Calculate for both flanks of the chosen anomalies.
Depth below the ground of Dyke 1 is 5.9375 m, Dyke 2 is 13.75 m Due to the sample interval is large, the selected half maximum slope points are far from the real half maxima. Hence, the depth estimations of this method may not unreliable. To improve the accura cy, the sampling interval may need to minimize.
5. Use Bean’s method on the same anomalies chosen in step 1. Get values for the Bean parameters A and C and use these with t he two nomograms in the paper to calculate width and depth of the dyke source.
Left flank Max HGRD(inflection point)
Half MAX Half MAX left
Half MAX right
Distance between Half MAX LEFT AND RIGHT
Right flank Max HGRD(inflection point)
Half MAX left
Half MAX right Distance between Half MAX LEFT AND RIGHT
inflection point to Half max left
Distance between inflection point
Left flank 36.581791
Half MAX left
Half MAX right Distance between Half MAX LEFT AND RIGHT
inflection point to Half max right
Max HGRD(inflection point)
inflection point to Half max right
Right flank Max HGRD(inflection point) Half MAX
Half MAX left
Half MAX right
Distance between Half MAX LEFT AND RIGHT
inflection point to Half max left Distance between inflection point
50 200 Figure 7 A and C values
6. Use Koulomzine et al method on ONE of the anomalies selected in step 2. Use conjugate points method to construct symmetric component of anomaly and then use the nomograms to calculate width and depth of the dyke source. STEP 1 : Choose anomaly
2. Find peak and low of anomaly
3. pick point F2' and F2''
4. find point F1' and F1''
5. Calculate L, M, N
150.2337 5. Calculate U, X0 and F0
F0 = intercept of the two magnetic values that cross the y axis when x = 0 F0
Then normalize the symmetric curve to datum. Datum = F_max+F_min-F0 Curve – datum = normalized This helps us choose max and ½ max values
width = 1/2max * sqrt(4-(phi^2 -1)^2) depth = [1/2max * (phi^2 – 1)]/2 Width
Compared the result with graphical m ethod.
Width = 2*1/2max*W =2*135.3829*0.75=203.07435 m Depth = 2*1/2max*D =2*135.3829*0.3=81.22974 m
7. How do estimates compare between the different methods? The calculated depths below the magnetometer are in the table below.
Half Height - Half Width Peter's half slope
Bean’s method Koulomzine
The calculated depths from the different methods are different. -
Half Height - Half Width method is the fastest way to estimate the depth.
In the graphical methods eg.Bean’s method may have error due to wrong estimation i n choosing points in the graph.
Peter's half slope may have high error due to the sampling interval.
Koulomzine’s method is time consuming but giving the calculated width of source body close to Bean’s method
Write up what you have done in your electronic workbooks and include plots of results. To be completed by April 11 . Paul Wilkes 24 March 2014
CURTIN EXPLORATION GEOPHYSICS Geophysics 301 semester 1, 2014 Workshop 5, April 1 and 8, 2014 Introduction to ModelVision
1. Read in from Blackboard : perth gravity in mga50 perth gravity in mga50.ers This is grid of isostatic residual gravity for the whole of the Perth Basin. This grid allows for variations in water de pth for the offshore part of the Basin and variations in depth to moho for both onshore and offshore. The challenge is to model the depth to basement for selected profiles i ncluding offshore and onshore data. 2. Display the grid in Oasis Montaj – no need to import - just use grid display and select .ers option rather than . grd option. 3. Create a new database ready to receive profiles from step 4. 4. Create a number of grid profiles across the grid us ing gridprof.gx In grid profiles select 500 metre data spacing. Create grid profiles along 6600000 N , 6550000 N, 6500000 N Use map input – map coords to select coordinates and from 250000 E t o 500000 E. Data is in GDA 94 MGA 50 coordinates. Close database in Oasis Montaj.
Figure 1 Grid Profile and Line Paths
Figure 2 Coordinate System
5. Open ModelVision 11.0 6. Create new project : use GDA94 coordinates, Transverse Mercator and zone MGA50. Set up model in mgals.
7. Import grid profiles as Geosoft .gdb. Select all lines and all channels
8. Import grid in ERMapper (ers) format
9. View image using view > map > grid images
10. Select model > line control > m odel gravity > use regional select input channel. Select lines Model > defaults > defaults > model parameters Use background density of 2.67
11. View > X.section , select line, tick m odel gravity and display input channel.. Lower panel is depth section where we can create polygonal model for the sedimentary section. Edit regional model > edit regional. Create regional at about 75 mgals for starting point
12. Select model type (icon below modules on top ro w). Select Polygon, density = 2.39, strike length = 5 0000 ( may be changed later )
13. Draw in initial model in depth panel. Select immediate computation from M / I icon. 14. Manually adjust vertices of model by selecting green polygon symbol in second row of icons. Adjust model to fit data. LINE:S6500000N
15. Save session. 16. Further detail to be provided in class. Paul Wilkes 31 March 2014
CURTIN EXPLORATION GEOPHYSICS G301
WORKSHOP 6 – 8 April 2014. Revised write up of April 8. Aim : Forward modelling with Modelvision 12.0 to create synthetic models and recovering model parameters using Analytic Signal in Oasis Montaj Outline of steps: 1. Start Encom program ModelVision 12.0 2. File > new project 3. Project properties – untick local grid, set datum to GDA94, Projection to Universal Transverse Mercator, proj/zone to SUTM50, Mag units SI, T=60000 nT, inc = -60.0, dec =0.0
Figure 1 Project properties
4. Set up synthetic survey (Utility > synthetic lines) with 101 north south lines, ref pt x=500 000, y = 6 000 000, line length = 20000 m, pt spacing = 20 m, survey width 20000 m, line spacing = 200 m, azimuth = 0 degrees, select “create survey”
Figure 2 Synthetic survey
5. Go to view to see plan of survey lines view > view map > stacked profiles
Figure 3 Stack profile of survey lines
Figure 4 Synthetic survey lines
6. Create tabular model: model > body operations > create body > tabular, susceptibility = 0.1 SI units, strike length = 10000 m, thickness = 1000 m Or create your own body or bodies.
Figure 5 Created body
7. Go back to open map (step 5) and click on map to position centre of m odel on centre of grid of lines. 8. To see body parameters: double LMB with cursor on model (either in plan or X-section) > body properties > set or change thickness and extent – record these for later use to see how well you can recover them by later processing.
Figure 6 Body parameters
Depth : 2000 m
Body thickness: 1000 m Depth extend: 5000 m Dip: 90
Figure 7 Select lines for magnetic regional
Figure 8 Line control
9. To create model grid: model > grid control specify grid dimensions 100 x 100 m.
Figure 9 Specify grid dimension
10. To see magnetic grid: view > map > grid image
Figure 10 Grid image
Figure 11 Select grid display
11. To see individual cross sections: view > X-section, select line eg 51 (central north south line)
Figure 12 X-section
Figure 13 Line51 profile
12. To export model grid: file > export > ermapper ers format, use 8 byte real
Figure 14 Export model grid
13. In oasis montaj – create new project and view model grid using display grid etc
Figure 15 Grid display
14. Run analytic signal grid filter to create analytic signal grid and then create central north south profile and east west profile using gridprof.gx
Figure 16 Analytic signal panel
Figure 17 Analytic signal grid
15. Look at grid profiles in database(show profile etc) to see how well they recover model parameters – location and depth to top etc. Line space = 200. Sample interval 50m was used.
Figure 18 Analytic signal
Analytic signal 0.164631
Figure 19 Depth estimation from E-W analytic signal
16. Record all steps and results in your electronic lab books.
How well has Analytic Signal worked in recovering model parameters?
The error from the depth estimation calculated by analytic signal is only 2.5%. Hence, analytic signal performance in recovering model depth is excellent.
GEOPHYSICS 301 – WORKSHOP 7 – 13 and 20 May 2014 OBJECTIVE
This workshop is an introduction to gravity processing. It will be followed by an introduction to interpretation of the processed data. The data comes from a March 2004 gravity survey using a Scintrex CG3 gravity meter and optical levelling over possible paleochannels in the Tammin area in the Wheatbelt of WA. Four lines of data were acquired with 25 metre spaced stations along lines a couple of km apart. The underlying idea is that if paleochannels exist in this area they will be filled with sediments of lower density than the bedrock and therefore show as bouguer gravity lows which we can then model to work out geometry of the paleochannels. You are required to select a suitable site to position a pumping bore to lower the watertable. This needs to be sited in a paleochannel. STEPS
1. Read into your working directory the files contained in Blackboard under workshop 7 – gravity processing – 13 and 20 may 2014 These files are : Tammin survey.xls containing the surveyed heights and positions for each of lines A, B, C and E, the observed gravity data in march21.dmp •
2. Read the gravity data into a blank excel spreadsheet using spaces, columns and commas as delimiters . 3. Note that some of the column headings may slip relative to the data during step 2 4. Convert times in hours, mins, seconds to decimal hours. This will facilitate drift corrections. 5. Identify the base station data for each line. These are stations A73, B15, C48 and E57. 6. Calculate linear fits to the base station data for each day. Note that some lines are split over two days. You will need to allow for this in applying drift corrections. Calculate drift corrections for all observed data. Linear fits can be done using Excel. Estimated gravity functions were created from measured base stat ion gravity data in different times of the day and then constructed a linear equation to calculate gravities in each time interval to predict base station readings at all times of gravity stations. Drift correction = Predict base station readings – initial base reading o n the day of survey
Base station readings functions
7. Average the drift corrected observed gravity dat a and transfers this data into the spreadsheet containing the posi po sitional tional survey data read ready y for further processing. 8. The main gravity processing uses the following equations : gn1967 = 978031.8 ( 1 + 0.0053024 sin2(lat) – 0.0000059 sin2( 2*lat) ) Note : the unit in sin function is radian(for radian(for calculation in Excel) Bouguer gravity = observed gravity – gn 1967 + 0.3086 * h – 0.04191 * density * h The purpose of bouguer anomaly is to give the anomaly due to density variations variations of the geology below the datum, without the effects of topography and latitude. Where h = height above sea level in metres, density d ensity = 2.67 g/cc or other density as selected. Density = 2.67 g/cc was used. These equations are for gravity grav ity data in mgals mgals.. 9. It is required to convert the observed o bserved gravity into absolute mgals using a tie in done after the Tammin fieldwork by b y going to Fremantle Port where there is an absolute station established by Geoscience Australia. The Sc intrex CG3 measured 3801.090 mgals here where the absolute value is 979403.111 mgals. The difference of 975602.021 mgal has to be added to the drift corrected observed gravity data. drift corrected observed gravity data(ABS) = drift corrected d ata + 979403.111 10. Process the data for all four lines and produce p roduce profile plots profile plots in Excel for each line. line.
11. Import the data into Oasis Montaj and create creat e images and stacked profile plots.
Stacked profile plots Line A
12. Where would be suitable site for groundwater pump ? From the hypothesis if paleochannels exist in the area, they will be filled with sediments of lower density than bed rocks and therefore show as bouguer gravity lows, since the marked area in each profile of the picture below show re latively low bouguer gravity, then the suitable site for groundwater pump are the marked areas.
Later workshop will interpret the profiles created by this workshop. As before record in your work books the steps followed in this workshop. Complete the spreadsheet, profiles and Oasis Montaj plots by end May 23. PGW 12 May 2014
GEOPHYSICS 301 – WORKSHOP 8 – 27 May and 3 June 2014
INTRODUCTION TO EULER DECONVOLUTION This workshop uses Oasis Montaj to run Euler Deconvolution on gravity from the Merlinleigh Basin east of Carnarvon.. Background papers are provided in the papers by Thompson and Reid et al. See also Geosoft Euler 3d tutorial manual. Data processing. 1.
Gravity data is provided in file xyz3tc.dat on Blackboard. Read this data into your working directory and load into Oasis Montaj. Data columns are easting, northing, station, height above sea level, bouguer gravity for density of 2.2 g/cc, terrain correction, bouguer gravity after terrain correction ( use this channel ).
What is the data spacing of this survey? Create images of bouguer gravity with terrain correction applied and height above sea level. What is s uitable grid spacing ? How does the bouguer gravity relate to the hei ght data ?
Figure 1 Station Locations
Figure 2 Data Spacing What is the data spacing of this survey? Station spacing is 2 km and line spacing is 3 km. What is suitable grid spacing ? ¼ of the station spacing which is 500 m. How does the bouguer gravity relate to the height data ? From the topography data (height data), in the middle of the map shows high level of height (pink area), however, this area has low bouguer gravity. Between longtitude 200000 to 2500000, this area has low height but high bouguer gravity.
Figure 3 Bouguer gravity profile(green) and Height (red)
Figure 4 height above sea level(left) and corrected bouguer gravity(right) (inc=45/delc=45) 3
Load menu Euler 3D gx > Euler 3D
Figure 5 Load menu Euler3D 3.
Run Standard Euler and located Euler deconvolution with structural indices of 0, 0.2 and 0.5. Run to produce solutions with an accuracy of 5% of better. window width of 20 (this is multiple of g rid mesh) depth tolerance of 5 % max depth of solutions 10000 metres supply suitable names for databases of solutions. According to Euler equation, the derivative grids of magnetic field in each coordinate(x,y,z) need to be generated before processing Euler3D.
Figure 6 Euler equation
Figure 7 Generating derivative grids(dT/dx,dT/dy,dT/dZ) Standard Euler
Figure 8 Standard Euler menu
Figure 9 Euler deconvolution panel
Figure 10 Solutions from different SI in standard Euler deconvolution
Located Euler In order to employ located Euler method, analytic signal grid need to be produce prior the process.
Figure 11 Calculating analytic signal
Figure 12 Analytic signal color shaded grid (inc=45/delc=45) After that, analytic signal grid is used to compute grid peak locations, and then three new databases are generated in order to use in located Euler method for different structural indices.
Figure 13 Get grid peak locations
Figure 14 Locate grid peak panels
Figure 15 Located Euler deconvolution
Figure 16 Located Euler deconvolution panels for different structural ind ices
Figure 17 Solutions from different SI in located Euler deconvolution
How many are produced? Method Standard Euler
Number of solutions
Number of solutions within 5% Depth Tolerance
From the results, standard Euler method gives higher number of solution than located Euler method. In standard Euler, number of solutions increases with SI. Located Euler method gives a constant number of solution according to number of grid peak location in analytic signal, however, the n umber of solutions within 5% of depth tolerance are different. The standard Euler deconvolution, moves a window of a fixed size(20*500=10,000) over a grid of data and calculates Euler Deconvolution solutions for each window. I t gives one solution at each window location, which approaches the number of cell size in the grid. The Located Euler deconvolution modifies this procedure by first locating only those windows which encompass peak-like structures in the data. A peak-finding routine is first run which locates peaks from analytic signal and estimates a window size using the locations of adjacent inflection points. These locations and window sizes are then used to define the windows for Euler Deconvolution. The Euler deconvolution typically produces less solution than standard Euler because only a smal l subset of the grid cells will be the centers of "peaks" in the data.[Oasis montaj tutorial] 5. Grid depth estimates using minimum curvature and appropriate mesh size Display as new maps. Add colour legend bar to show range of solutions. Note that there are gaps where spacing between solutions is large. Standard Euler depth estimate grids and solution locations plot
Figure 18 Standard Euler depth estimate grid with SI=0 (Left) and Located solutions(Right)
Figure 19 Standard Euler depth estimate grid with SI=0.2 (Left) and Located solutions(Right)
Figure 20 Standard Euler depth estimate grid with SI=0.5 (Left) and Located solutions(Right)
Located Euler depth estimate grids and solution locations plot For located Euler method, in solution locations plot, I used the function proportional size symbols (the size of symbols is proportional to depth) to e mphasize the depth of each solution.
Figure 21 Proportional size symbols panel(1000m of data/mm)
Figure 22 Located Euler depth estimate grid with SI=0.0 (Left) and Located solutions(Right)
Figure 23 Located Euler depth estimate grid with SI=0.2 (Left) and Located solutions(Right)
Figure 24 Located Euler depth estimate grid with SI=0.5 (Left) and Located solutions(Right)
Figure 25 Located Euler depth : Located solutions SI=0(Pink), SI=0.2(Blue) and SI=0.5(Black)
How do the results compare for the different structural indices. Method
Number of solutions
Number of solutions within 5% Depth Tolerance
Figure 26 Number of solution table For standard Euler, the higher structural index, the higher in number of solutions, hence, the lower grid spaces. However, the results show that with SI 0.2 and 0.5, t he numbers of solution created are very close, 26489 solutions from SI=0.2 and 26584 solutions from SI=0.5.
Figure 27 Grid statics from standard Euler In grid stats, different structural indices gives different mean values of each grids, the higher SI the higher mean value of the grid. In addition, the range of data in each grid is also varied by SI, the higher SI the lower depth range.
Figure 28 Grid statics from located Euler For located Euler, SI=0 gives the highest number of solution, as much as 43 solution, compared to SI =0.2 gives 10 solutions and SI=0.5 gives 9 so lutions. The grid statics of SI=0.2 and SI=0.5 are i dentical. SI=0 produces the highest data range of the grid.