gravity and magnetics workbooks

8/19/2019 Gravity and Magnetics Workbooks

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ASSIGNMENT COVER SHEET

Student Name: __________________________________________

Student ID: __________________________________________

Unit Name: __________________________________________

Lecturer’s Name: __________________________________________

Due Date: __________________________________________

Date Submitted: __________________________________________

DECLARATION

I have read and understood Curtin’s policy on plagiarism, and, except where indicated, thisassignment is my own work and has not been submitted for assessment in another unit orcourse. I have given appropriate references where ideas have been taken from the published orunpublished work of others, and clearly acknowledge where blocks of text have been taken fromother 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 total of _________ Percentage:

Lecturer’s Comments:

Lecturer’s Name: Date Returned:

Sutthisrisaarng Pholpark

17682974

Gravity and Magnetics for Exploration 301

Paul W.

20/06/2014

20/06/2014

Sutthisrisaarng Pholpark


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CURTIN EXPLORATION GEOPHYSICS

GEOPHYSICS 301

WORKSHOP 1 – 4 March 2014

OBJECTIVE

This workshop is an introduction to the software package Geosoft Oasis Montaj which

we will be using extensively in Geophysics 301 and in other units of the Geophysics

undergraduate course. In this workshop we will be 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 three files : rav3.xyz, rav4.xyz, rav5.xyz. These files arein the Blackboard folder workshop 1 - march 4 2014 Each of these files 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.

STEPS

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 DatabaseEnter 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


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8. Split the three lines of data in the database into individual flight lines, using

Database tools >line tools > split on x,y breaks. 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 ? (using right mouse click with cursor on

map and select ruler function ).

Save map changes.

Figure 1 Flight Lines

This procedure allows user to display flight lines of the survey. Scale refers to the

relationship (ratio) between length that appears on a map and actual length of


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data. A good scale allows user to display a map without 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 byN for a better accuracy. The line spacing in given 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, top 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 tell the information about the map. So that

base map is an essential tool to describe a map 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


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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


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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.


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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


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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 theaverage 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.


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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 of 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 ground. 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 options

Select MAGMAP 1-Step-filtering,

In SetConFile select Derivative in Z direction, order 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 ?


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Figure 10 MAGMAP derivative 1st order in Z-direction


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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 filter

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 grid.

18. Save all map changes, database changes, save project and exit from Oasis

Montaj to end session.

TO BE CONTINUED IN WORKSHOP 2

PGW4 March 2014


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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)


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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)


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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 = -0.29965751Magnetic 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


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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 filter) and the green line represents

magnetics value from reduction to pole grid. We can find horizontal offset by compare the x,y value

between nearby peaks of magnetic of these lines.

Magnetic value without filter : 59117.3nTEasting(x) : 238089.1m, Northing(y) : 6274546.9m

Figure 6 Magnetics Profile(no filter)


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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.


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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 Line With RTP filter(Left) and Original Magnetics Profile Along The Line(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.


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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 notimmediately 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)


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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


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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 correction-

latitude correction- Eotvos correction

4. Compute regional /residual

• Use grid, filter, trend (use all points). Use 2nd

and 3rd

order trend surfaces.


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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 2nd

order do a better job than 3rd

order

regional?

The local regional has difference levels of gravity anomalies. The range of gravity anomalies

of 3rd order regional is between -0.124 to 0.194 mGal (0.318mGal) and the 2nd is between

-0.142 to 0.186 (3.28mGal). The 3rd

order regional do the better job than the 2nd

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


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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 of

topography data?

Figure 11 Topography Grid(Left) and The Grid Statistics(Right)


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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)


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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


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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 analytic

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 Excel file.

Figure 3 Export file


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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 magnetometer.

Figure 4 Magnetic profile

Figure 5 Analytic signal

Figure 6 Half height Half width method

Depth below the ground of Dyke 1 is 57.5 m, Dyke 2 is 95 m


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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 accuracy, the

sampling interval may need to minimize.


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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 the two nomograms in the paper to calculate width and depth of the

dyke source.

= ℎ

ℎ

=

ℎ

Figure 7 A and C values

Dyke1 m

Left flank A C Width Height

Max HGRD(inflection point) 24.1079695 875

Half MAX 12.0539848

Half MAX left 10.6707248 775

Half MAX right 9.28143481 925

Distance between Half MAX LEFT AND RIGHT 150

inflection point to Half max right 50

Right flank A C Width HeightMax HGRD(inflection point) -18.253097 1025

Half MAX -9.1265487

Half MAX left -7.3486614 975

Half MAX right -10.092031 1100


inflection point to Half max left 50

Distance between inflection point 150

Dyke2 m

Left flank A C Width Height

Max HGRD(inflection point) 36.581791 2475

Half MAX 18.2908955

Half MAX left 17.9803739 2400

Half MAX right 21.25704 2550


inflection point to Half max right 75

Right flank A C Width Height

Max HGRD(inflection point) -42.797007 2675

Half MAX -21.398504

Half MAX left -24.162386 2625

Half MAX right -21.910362 2775

Distance between Half MAX LEFT AND RIGHT 150inflection point to Half max left 50

Distance between inflection point 200

80 38.57

2 2.666667

3 4

130 144.45

140 87.5

180 85.7

3 3

2.5 3


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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

peak 58693.03 2575

low 58408.27 3075

mid 58550.65 2450 58547.142700 58558.6

3. pick point F2' and F2''

F2' 2550 58679.66

F2'' 2475 58581.82

4. find point F1' and F1''

F1' 58421.64 2877.143

F1'' 58519.48 2726.91

5. Calculate L, M, N

L 251.9098

M 176.9098

N 150.2337

5. Calculate U, X0 and F0

U 118.001

X0 2608.909

F0 = intercept of the two magnetic values that cross the y axis when x = 0F0 58682.96


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Trimmed


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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

Max 1/2 max 3/4 max

Magnetics 270.7658 135.3829 203.0743

Station 0 123.7471 78.80152

Phi 1.570364751

width = 1/2max * sqrt(4-(phi^2 -1)^2)

depth = [1/2max * (phi^2 – 1)]/2

Width 184.176

Depth 90.70946

Compared the result with graphical method.

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


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7. How do estimates compare between the different methods?

The calculated depths below the magnetometer are in the table below.

MethodDyke1 Dyke1

Depth Width Depth Width

Half Height - Half Width 57.5 n/a 95 n/a

Peter's half slope 13.75 n/a 13.75 n/a

Bean’s method 64 135 5.7 180

Koulomzine n/a n/a 10.7 190

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 in

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


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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 depth 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 including 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 using 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 to 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


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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


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9. View image using view > map > grid images

10. Select model > line control > model 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 model 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


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12. Select model type (icon below modules on top row).

Select Polygon, density = 2.39, strike length = 50000 ( 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

LINE:S6550000N

LINE:S6600000N

15. Save session.

16. Further detail to be provided in class.

Paul Wilkes

31 March 2014


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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


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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 model 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

Strike length: 10000


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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


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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


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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

Figure 19 Depth estimation from E-W analytic signal

East-West Peak Half-Peak(Left) Half-Peak(Right) Width Depth

Station 155 114 196

Analytic signal 0.164631 0.082123041 0.0828091274100 2050


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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.


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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 meterand 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 coupleof 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 bouguergravity 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 underworkshop 7 – gravity processing – 13 and 20 may 2014

These files are :

• Tammin survey.xls containing the surveyed heights and positions for each oflines 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 andcommas 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 andE57.

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 usingExcel.

Estimated gravity functions were created from measured base station

gravity data in different times of the day and then constructed a linear equation tocalculate 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 on the day of survey


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Base station readings functions


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7. Average the drift corrected observed gravity data and transfers this data into the

spreadsheet containing the positional survey data ready 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 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

of the geology below the datum, without the effects of topography and latitude.

Where h = height above sea level in metres, density = 2.67 g/cc or other density asselected.

Density = 2.67 g/cc was used.These equations are for gravity data in mgals.

9.

It is required to convert the observed gravity into absolute mgals using a tie indone after the Tammin fieldwork by going to Fremantle Port where there is anabsolute station established by Geoscience Australia. The Scintrex CG3 measured

3801.090 mgals here where the absolute value is 979403.111 mgals. Thedifference of 975602.021 mgal has to be added to the drift corrected observed

gravity data.drift corrected observed gravity data(ABS) = drift corrected data + 979403.111

10. Process the data for all four lines and produce profile plots in Excel for each line.


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11. Import the data into Oasis Montaj and create images and stacked profile plots.


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Stacked profile plots

Line A

Line B

Line C


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Line D

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 gravitylows, since the marked area in each profile of the picture below show relatively

low bouguer gravity, then the suitable site for groundwater pump are the markedareas.

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.

PGW12 May 2014


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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 e

of Carnarvon.. Background papers are provided in the papers by Thompson and Reid et al. See a

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 ).

2. What is the data spacing of this survey? Create images of bouguer gravity with terrain correction

applied and height above sea level. What is suitable grid spacing ?

How does the bouguer gravity relate to the height data ?

Figure 1 Station Locations


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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 lowheight but high bouguer gravity.

Figure 3 Bouguer gravity profile(green) and Height (red)


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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 grid 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


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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


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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 database

are generated in order to use in located Euler method for different structural indices.

Figure 13 Get grid peak locations


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Figure 14 Locate grid peak panels

Figure 15 Located Euler deconvolution

Figure 16 Located Euler deconvolution panels for different structural indices

Figure 17 Solutions from different SI in located Euler deconvolution


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How many are produced?

Method Structural index(SI) Number of solutions Number of solutions within 5% Depth Tolerance

0 21603 21603

0.2 26489 26489

0.5 26584 26584

0 718 43

0.2 718 9

0.5 718 10

Standard Euler

Located Euler

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 constan

number of solution according to number of grid peak location in analytic signal, however, the number o

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 o

data and calculates Euler Deconvolution solutions for each window. It 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 peakfrom 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 sma

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)


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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)


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Located Euler depth estimate grids and solution locations plot

For located Euler method, in solution locations plot, I used the function proportional size symbo

(the size of symbols is proportional to depth) to emphasize 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)



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Figure 25 Located Euler depth : Located solutions

SI=0(Pink), SI=0.2(Blue) and SI=0.5(Black)


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How do the results compare for the different structural indices.

Method Structural index(SI) Number of solutions Number of solutions within 5% Depth Tolerance

0 21603 21603

0.2 26489 26489

0.5 26584 26584

0 718 430.2 718 9

0.5 718 10

Standard Euler

Located Euler

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, the numbers of solution created a

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 t

higher mean value of the grid. In addition, the range of data in each grid is also varied by SI, the higher S

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.

gives 10 solutions and SI=0.5 gives 9 solutions. The grid statics of SI=0.2 and SI=0.5 are identical. SI=0

produces the highest data range of the grid.


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Figure 29 Comparing the result with the real geological data

Comparing the result with the real geological data from Merlinleigh basin, the area highlight in green

area should be Cainozoic sendiment(Cz) and the blue lines should be fault.

Did located Euler method work better than standard method?

When we do the standard Euler, the window of calculate is moved over every data point in the

grid, hence gives many solutions. Large number of solutions, however, these cloud of poorly define

solutions obscure the better solutions. On the other hand, located Euler method pre-screen the locatio

of each window by adopting analytic signal to locate the possible window location, hence gives the low

number of solution. So that located Euler gives more precise solution than the standard Euler method.

According to Reid paper the different types of model, e.g. Sphere, dike have different SI. Hence,

specifying SI depends on the shape of the body that we want to detect.

Reference :

Hocking, R.M.,1990 Carnarvon Basin in Geology and Mineral Resources of Western Australia, Geologica

Survey of WA Memoir 3, p 457 – 495

Oasis montaj tutorial

Due date : 6 June 2014

PGW 19 May 2014

gravity and magnetics workbooks

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