Using 1dcsemnda on Unix at SOC
1dcsemnda is a forward modelling program which
computes a Horizontal Electric Dipole (HED) response, i.e.
the signal amplitude at given locations on the sea floor for a
layered model.
In order to calculate the HED response from a model of the seafloor resistivity structure
that you specify you need to:
1. construct a model input file to be calculated - *.inp;
2. run the program 1dcsemnda to compute the HED response;
3. plot the result.
LOGGING ON TO SOC UNIX MACHINES:
Log on to ISS PC
Go to Programs-> Internet and run "X - Telnet -ftp"
Once this has done its stuff (takes a couple of minutes every time) you must do two
things.
First, run Programs -- Internet -- X-Telnet-ftp -- Exceed -- Xconfig. Double click on
the keyboard icon in the window which pops up, and select the 102 key UK
keyboard from the pulldown menu. Then run Programs -- Internet -- X-Telnet-ftp -Exceed -- Xstart. This will popup a new window. Set up the menus as follows:
Telnet for start method
One of pacific.soc, atlantic.soc, reykjanes.soc, indian.soc for the
host
Xwindow for the program type
xterm for the command to run
Then hit the "Run!" button at the top of the window. You should get a box for
username and password (these will be the SOC Unix/mail ones)
The window which pops up gives a direct connection to oriana, and is your
"Terminal" window. You will find it useful to get a scroll bar -Ctrl-middle mouse button within the window will pop up a menu, and highlight
"Enable scrollbar". NB that in the terminal you should use the "Del" key, not the
backspace.
Before you can run the program, you will need to type “setup geophys” in the xterm
window. You will find some useful input files to start from in
~then/classes/soes6002/em/ex1
1
HOW TO …
(1).
Construct the input file
If you wish, for example, to calculate the response from the following model:
Layer
Thickness, km
Water
1
2
Bottom Halfspace
3.0
0.5
1.5
Infinite
Resistivity,
Ohm*m
0.3
3.0
100.0
2000.0
To create a new file model*.inp you need to use an editor such as nedit. For
example, type :
nedit model1a.inp &
To create the file, enter (or edit) line by line the following information and then
save the file in a suitable directory. Structure of the input file (*.inp) for
1dcsemnda looks like the following
Line & Value
Meaning
1.
model
2.
3.0
3.
2
3.0 0.5
100.0 1.5
5.
2000.0
6.
30.0
7.
0.5
8.
30
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.7
0.9
1.2
1.7
1.9
2.9
91.0
91.0
91.0
91.0
91.0
91.0
91.0
…
0.5
0.5
0.5
0.5
0.5
8.7
8.9
9.2
9.7
10.0
91.0
91.0
91.0
91.0
91.0
- name of the input file
- thickness of sea water layer (i.e. water depth) (km)
- number of layers in the model (excluding sea-water layer and bottom halfspace)
- resistivity (Ohm-m) and thickness (km) of first layer of seabed
- followed by subsequent layers – e.g. in this case, a total of 2 layers
- resistivity of the bottom layer (which is a halfspace, so does not have a thickness) (Ohm-m)
- source elevation above the sea floor (meters)
- receiver elevation above the sea floor (meters)
- number of points (up to 200)
- frequency (Hz)
range (km)
azimuth (degrees)
(for each point; in this case there are 30 rows)
…
2
model.out
- name of data output file
You can find such models for some of the problems (already created) in your
working directory. You will have to do some editing to create all the input files you
will need.
Note that in the case of q1 and q2, the sea floor is a simple halfspace, and so
the value for line 3 is 0, and this line is followed immediately by line 5.
(2).
Run the 1dcsemnda program
Run the program with the command:
1dcsemnda
<
model*.inp
This computes the resulting amplitude as a function of range,
and writes the results to an output file model*.out .
(3).
Plot the data (output file)
The output file *.OUT is an ASCII file of 5 columns – range, frequency, azimuth,
amplitudes of both radial and azimuthal components (dimensionless).
The output written to screen also contains the model description.
Example
----------------------------------------------------------------------------------------------HED Forward Modelling Program
Computes HED response for up to 16 layer Earth.
Input TITLE for this run :
Input Source Layer thickness (km) :
Input the number of layers in the model
(EXCLUDING source (sea-water) layer and bottom halfspace) :
Input the bottom halfspace resistivity (Ohm-m) :
Input source elevation (meters) :
Input receiver elevation (meters) :
Input number of points (up to 200) :
Input for each point: frequency(Hz), range(km),
azimuth (degrees):
Enter name of data output file :
model 1
Model Parameters :
T/X height = 1.0 m, R/X height = 1.0 m,
3
Layer
Depth to top (km) Thickness (km) Conductivity (S/m)
Water
Halfspace
0.00
6.00
6.00
Infinite
3.2000
.50000
H.E.D. RESPONSE
Range(km) Freq(Hz) Azimuth(deg) Srad Sazim
0.50 1.00 91.00 0.003115 0.230669
1.00 1.00 91.00 0.001746 0.213883
1.50 1.00 91.00 0.001623 0.134796
2.00 1.00 91.00 0.001283 0.077472
2.50 1.00 91.00 0.000884 0.044308
3.00 1.00 91.00 0.000569 0.025154
3.50 1.00 91.00 0.000353 0.014046
4.00 1.00 91.00 0.000212 0.007724
4.50 1.00 91.00 0.000124 0.004198
5.00 1.00 91.00 0.000072 0.002262
5.50 1.00 91.00 0.000041 0.001210
6.00 1.00 91.00 0.000023 0.000644
6.50 1.00 91.00 0.000013 0.000341
7.00 1.00 91.00 0.000007 0.000179
7.50 1.00 91.00 0.000004 0.000094
8.00 1.00 91.00 0.000002 0.000049
8.50 1.00 91.00 0.000001 0.000026
9.00 1.00 91.00 0.000001 0.000013
9.50 1.00 91.00 0.000000 0.000007
10.00 1.00 91.00 0.000000 0.000004
-----------------------------------------------------------------------------------------------
Plot amplitude Sazim vs range (X axis - range, km & Y axis – dimensionless
amplitude Sazim or Srad ) with the command:
plotxy < plot_inp1
This will create a post-script file called mypost.
You may look at the plot with the command:
gs mypost
For a manual see PLOTXY_notes.pdf and commands_catalog.pdf.
Alternatively, you can grab the output file and import it into Excel to plot, or use
the GMT programs if you prefer!
4