Saturday, March 23, 2013

Prepare Near Surface Models using Grapher.

INTRODUCTION:
                               Near Surface Models are routinely prepared while doing onland data acquisition for determining the Optimum Depth (OD) of shot holes. Seismic exploration with explosive materials is carried out using several techniques. One of those methods is the use of shot holes that are drilled and loaded with explosive materials. When the explosive material is detonated, the resulting seismic waves are reflected or refracted by various geological structures located as deep as several miles below the earth’s surface. These waves are detected by sensitive geophones placed on the earth’s surface and are recorded on magnetic tapes or other media for subsequent data processing to determine subsurface geological structures. Certain kinds of structures are favourable for the accumulation of gas and oil  When charge is used as seismic source it is very important to place the charge in the proper layer for the desired energy transfer. One of the methods employed to do that is by generating near surface models (NSM) of that area so as uphole details can be properly interpolated.

Let me show you how to create a simple NSM model which incorporates the data information like the uphole position, lithology, velocity, optimum depth, elevation, etc using Grapher software.

The following is a part of uphole location map.


Preparation of Data:
                   Let us prepare an spreadsheet (excel) sheet which contains all the data of the uphole so that it will be easy to import to the Grapher.

SL - Source Line no. UH-Uphole;ELV-elevation;OD-Optimum Depth;D1-Depth 1 with velocity V1;COR_D1-Corrected Depth1(Elevation-Depth).

Importing into Grapher:
For this tutorial I am using Grapher 8.7 although the layout is pretty same for older and newer versions.
1. Start Grapher. Go to File >> Open and open the excel file you have saved as above. The excel sheet is displayed.
2. Now select two columns SL and ELV (while simultaneously pressing control key) and New Graph >> 2D XY Graphs >> Line/Scatter. Now the graph will be displayed.
3. The details of the axis, plot, etc will be displayed in Object Manager box. If any plot is clicked then its properties will be displayed in Properties box. (If Object Manager and Properties box are not visible then it can be turned by going to View >> Toolbars/Managers and check whether there is tick mark on Object Manager and Properties inspector).
4. The limits of axis can be changed by clicking on the particular axis in Object Manager and then in properties box select Axis tab  and in Axis properties in Axis limits the minimum and maximum can be changed.
5.  The default plot name will be like Line/Scatter Plot 1. This name can be confusing when more plot is added to the graph. So the best way is to change it to a more meaning full name like in this case Elevation. Right click on the Line/Scatter Plot 1 and select Rename object to change name.
6. Now the plot for elevation is ready. Let us add the plot for depth d1. Right click on Graph 1 in Object Manager. Go to Add plot >> Line/Scatter plot >> OK >> in choose axis click ok. A diaglog box will appear for  opening worksheet. Select the worksheet as created before. Now a new plot will be created with name Line/Scatter Plot 2. For the plot by default the first two columns A & B would be selected for X column and Y column. To change that click on the plot in object manager and the in properties box select plot tab. In plot properties, in X column let it be SL. For Y column select Cor_D1(m). If the auto update is ticked at the bottom of properties box then the plot will be updated automatically. Rename this plot to a meaning full name. Then add plots for Cor_D2, Cor_D3, Cor_OD, Datum.
7. Now the plots which we have created will be more of zigzag due to insufficient sampling. It would be better if the plot is smoothen. Let us smoothen the first plot Elevation. Click on Elevation and then in properties box go to plot tab. In fits click to add/edit fits. Fits dialogue box will appear. In available fits select spline smoothing and Add and click apply and ok. A plot called Fit 1:Spline smoothing will appear. It can be renamed as Fit1:Elevation for better understanding. Now remove the tick mark near the plot Elevation which is created in step3. So now the original plot disappears and new smoothed curve appears. Do this for all the plots. So now they will be smoothed.
8. Now let us add colours for each layer. Go to Object manager and select the plot Fit1:Elevation. In properties box, go to 'Fill' tab.  Under fill properties for pattern by default it will be none. click on none and select some pattern. In the foreground change the colour of the pattern. Let the 'Direction' be 'Down'. Cutoff by default will be the origin of y axis. let us change it to a value so the fill is little above the origin. Suppose if the cutoff value by default is -8 then make it as -6.
9. Now do this for other two layers too except the datum!. For the lower most layer select solid pattern and white colour, so other layers will be blocked.
10. Now let us add the lithology information. Right click on Graph 1 and select add plot. In the select plot type select floating bar and click ok. In choose axes box click ok. Then a dialogue will appear for selecting the excel sheet. Choose the sheet created at first.
11.A floating bar plot with name 'Floating Bar 1' will appear. let us rename it to clay as we are going to represent the lithology of clay in the bar.
12. In object manager select Clay and in properties box go to plot tab and then in X column select the default SL (similar to other plots). Then for Y1 column select column V:Cor_C and for Y2 select column W. Here Y1 represents the upper limit and Y2 represents the lower limit. Width of the bar can be changed by changing the percentage value in Bar width.To change the colour and pattern of the bar go to Fill tab and change the desired properties. Now similarly add the floating bar for Clay+Sand and Sand.
13. To add legend right click graph1 in object manager and select add legend. The legend will appear in graph. Each entry can be edited by selecting the 'legend 1' in the object manager and in properties box go to Legend tab and then click on 'Click here to edit entries'.
14. Now let us how to add labels. Let us add the label for Uphole number. To add the labels we cannot do that with the plot created by fitting. So we have go to the original plot. Now in this case let us select the plot 'Elevation' in the object manager and then put a tick mark so that it will appear. Now we dont want the actual plot since we have a smoothed plot for that data. So after select elevation plot go to properties, line tab and in style select the invisible.
Then go to labels tab put a tick for display labels, uncheck labels in y column, then in column select 'Column B:UH No.'. To add prefix to the numbers click on 'click here to set label format' go to 'format' tab and then select prefix or suffix needed, background colour etc. Alignment of the label can be done in plot tab by changing alignment and x offset, y offset, etc.
15. To add text boxes and other details go to 'draw' in the menu and select required options.

Export options:
1.Tiff for display
2.PNG for web
3.If only lines are used then vector pdf for printing and display.
Don't save as jpeg as compression artefacts are more in jpeg compared to png.                     

Download the excel sheet used in this tutorial for practice.

Feel free to record your suggestion, doubts, etc.

Disclaimer: All the data sets used in this tutorial is fictitious. 

2 comments:

  1. but u did not explain how to add average velocity

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  2. What I have done is that I have just added the weathering and sub weathering layers and included the average velocity as text box. Then inserted the arrow through shapes.

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