SEISMIC DATA ACQUISITION

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.

Thursday, January 3, 2013

Prepare SPS for onland seismic data acquition

INTRODUCTION

More often the seismic data is processed by people who are not involved in the acquisition of data. So there is need to exchange data about the position coordinates of source, receiver and their relation. Not only the position coordinates but also other details like drilled depth and uphole time to mention a few have also be to exchanged. Different systems are used for acquisition and processing having their own file formats. So there was a need for industry standard to exchange these data. So in 1993 Society of Exploration Geophysicist (SEG) adopted the SPS standards used by Shell as Industry standard. The original specifications can be downloaded from the link given in references.

SPS standard consist of four simple text files as follows:

Receiver file - R file
Source file - S file
Relation file - X file
Comment file - Optional

which are formatted to avoid confusion. These standard is used to 3D seismic data acquisition and can be used for other geometries also.

Preparation of SPS

Though there are many ways of preparing SPS an easy way to prepare is by using spreadsheet like excel or calc. Let us have a simple geometry and prepare SPS for it manually.

Geometry:

To keep the manual work less let us consider a simple END ON geometry with following details

Receiver lines - 5

Number of receiver per line - 20

Receiver line interval - 280 m

Interval between two receivers - 80m

Interval between two shots - 80m

Number of Shots per line - 5

Number of shot lines - 2

Shot line interval in 320 m

The unit template is as in the figure below

Fig 1. Unit Template

Let us do a in-line swath rollover towards the right side. Since the Shot line interval is 280 m we have to place one more Shot line parallel to existing shot line which is 280 m away and we have to add 4 new receivers in the each line (Hint: 320/80).

So now let us prepare the SPS for a total number of 10 shots.

R file

Receiver file contains the information about the receivers (or stations) type of reciever (geophone, hydrophone).

Fig 2. R and S file format

The figure shows the information put in each field. Each field has particular number of columns. By default the columns are right justified. Some columns are left justified which are mentioned and we have to modify it.

Now let us start preparing R file using excel sheet. Open an excel sheet (any spreadsheet) and follow the steps below.

1. Column width adjustment - The column width for the first column as given in the table in 1. . So we have to adjust the column width of each column in the excel sheet upto O column. Now go to the first column. Right click on A and select column width. A dialogue box appears enter 1. Now right click on B and select the column width. Enter 16 in the dialogue box. Repeat it for all the columns till O. Select the column B and align text left. This is the only column which is left aligned in R file. Select the columns K L M and right click format cells. Select number and decimal place 1. Now the excel sheet is ready as below.

Fig 3. Formatted excel sheet

2. The bottom leftmost geophone is the first geophone. Let the first receiver line be 101 with starting picket 101. Let the next receiver be 103, 105 and so on. The next receiver line be 108, 115, etc. Let the easting and northing of geophone (101,101) be for example (100000, 2000000)

3. Now let us enter the information of first geophone in the excel sheet. In the cell A1 type R as per the format given in Fig 2. In the B1 we have to enter the line number of first geophone (101) then picket number in C1. In D1 enter the point code as 1. In E1 enter the reciever code for E1 for geophone. In K L M enter easting, northing, elevation. Other cells can be filled with relevant information. If they are not know they can be left.

Fig 4. R excel

4. Repeat step three for all the geophones, i.e., 120 geophones (5 x 24). So you will be having 120 rows.

5. Now save it as Formatted text (space delimited) with extension (*.prn). Click ok and ok. Now the saved file is a simple text file which can be opened with any text editor. So your R file is ready

S file

Source file like receiver contains information about the source points like its position coordinates, type of source, etc as given in Fig 2. Another excel sheet can be prepared in the way as prepared for R file. All relevant information shall be entered for each shot point and saved as *.prn file format. For this geometry with two shot lines there will be 10 rows because of 10 shots. The prepared S file is as follows.

Fig 5. S excel

X file

Relation file contains data about how source and receiver are related to each other. In simple terms it shows what receivers are active while a particular source point is activated.

Fig 6. X file format

The figure shows the information put in each field. Prepare another excel sheet as we prepared for R and S file. The fields are different from R and S file. Prepare and format (right and left alignment) the excel sheet and given in the figure above.

Let me explain the important columns to be entered for the X file.

A - enter X

B - Enter the number of the tape to be used for recording (optional)

F - Shot line number

G - Shot point number

I - starting channel number ( Channel number is a continuous number from 1 to 100 in this survey. Because there are 100 channels active for each shot)

J - Ending channel number

K - Channel increment(How many channels increased in each step)

L - Receiver line number

M - Starting receiver number

N - Ending receiver number.

The final layout appears as below

Since there is Receiver line number in column L We have to prepare the row information for every line in a shot. In this example. There are 5 receiver lines in a shot. So we have to prepare 50 rows ( 10shots x 5 receiver lines)

The x file is prepared as follows.

Fig 7 X excel file

Save the excel sheet as prn file.

Now the sps is prepared and final geometry looks like as below.

Fig 8. Geometry

Comment record (optional)

Revision history

1.Rev1.0

2.Rev2.1

The procedure explained above is for the SPS Rev1.0 . The columns are slightly different in SPS Rev 2.1. The difference between the two formats can seen from the following figures.