Quick guide to MASW and P wave refraction field survey and post processing using SARA's DoReMi and Geopsy's software

What you will need for your MASW and P wave Refraction surveys (figure 1).1. Vertical geophones (red boxes connected to yellow clamps on the bottom right)2. Laptop computer with SARA software 3. DoReMi digitizers (yellow ovals rolled up in the orange wheel)4. Seismic source (hammer and shot plate, explosives...)5. Trigger sensor (not shown here)6. DoReMi interface (yellow and black box in the suitcase)7. Field area of interest (I recommend somewhere beautiful. Possibly Volcan, Panama)8. Measuring tape (at least 50 meters long)

Fig 1. This photo shows the different elements of the DoReMi system needed to preform geophysical surveys.

Both MASW and refraction surveys are performed with a linear array set up. The number of geophones used and spacing between them depends on size of you field area and max depth of desired penetration. The recommended minimum number of geophones used for MASW is 12. For this example survey we will use 12 geophones spaced 4 meters apart.

Fig 2. For refraction and MASW surveys, geophones should be set up in a straight line. Geophones in this picture measure the vertical component of seismic waves. Geophones that are not positioned in a vertical

position will yield less accurate data. Note the picture on the bottom right showing digitizer-geophone connection.

Geophones should be placed in the ground with as much of the spike covered as possible. Place the geophones in a straight vertical position to increase accuracy of your results (figure 2). Digitizers should be laid out in a line and next to the geophones. One oval digitizer for every geophone. Clamps coming off of the geophone should be connected to the digitizer. One side of the clamp is bigger than the other and a correct connection between the clamp and digitizer matches the big side with the side of the digitizer of equal size.

Fig 3. The DoReMi interface links you computer to the survey line. When recharging the interface make sure that it is turned on.

Complete channels are then plugged into the DoReMi interface which can be connected to your computer via a usb port. The interface connects the geophones, computer and trigger sensor together (figure 3). At this point your system should be complete. By opening SARA DoReMi software (figure 4), you can check you computer and sensor communication. To check communication: Setup>>Instrument>>Connect or Setup>>Instrument>>Setup>>Communication Test. The program will then update you on the status of the connection.

To set up your test in SARA software start at the 'Work Folder' section. By clicking Change Folder you are able to change the the location of where the data will be saved.

The Sampling setup section allows you to customize the configuration to your survey. This can be done manually or you can use the default configurations by clicking on the Microtremor, MASW, Refraction, or Reflection buttons. After customization you can name and save you configuration by clicking the like named buttons.

Fig 4. DoReMi software graphical user interface.

The 'Sampling' section allows you to set gains and filters for you survey by clicking the Recording Options button. In this pop up window you are able to set the number of sequential shots that will be recorded.

The Channel Setup section allows you to add and remove channels. Number of channels should match the amount of geophones you have set up in the field.

A more detailed explanation of SARA software can be found at http://www.sara.pg.it/SOFTWARE/DOREMI/DoReMi_ENG_SoftwareSection.pdf.

After SARA software is set up we can start collecting data. In this example survey our seismic source is a 10 lb hammer hit against a metal shock plate. The depth of investigation is directly related to your seismic source and as a general rule, the more energetic the impact source (bigger hammer, explosion...) the greater the depth of penetration. To start data collection click F1 on the keyboard or Start on the desktop.

This will bring up a window showing communication of between each geophone and the computer. By clicking Ok we can begin. The software will often prompt for a test shot before it starts to record the real data. Shots can be started by clicking the Shot button and the interface will tell you it is ready by beeping. At this point it is your job to create some waves (figure 5).

After a shot is initiated, SARA software will display the waveforms recorded by the geophones (figure 6). You can review the data by zooming in and out and applying phase shifts to individual channels. The software will then prompt you to make new shots depending on what you set for the number of sequential recording. The shot record will also indicate how good your connections are between the digitizers and geophones. Notice the records below, the second from the left looks wrong. This type of record indicates a bad connection which can be easily fixed by cleaning and reconnecting the leads.

Fig 5. Seismic wave production using a hammer. For greater survey depths, a more energetic input source is needed such as a heavier hammer or explosives.

Fig 6. Multichannel shot recorded displaying refraction data. Notice the second channel from the left. Results such as this indicates a bad connection between the geophone and digitizer often due to dirt or a reversed lead.

The field portion of the survey is now over. The DoReMi is designed for its ease in set up and clean up. Just unplug the geophones from the digitizers and roll up the digitizers into the wheel (figure 7). Now you are ready to process your data.

When all your data is acquired, you can review it by clicking F3 or the Seismograms button. This button will bring up a 'File Management' window which shows all the data you recorded. You can re-review shot records here by simply clicking on desired file. For more information on SARA software and processing data please visit http://www.sara.pg.it/scat.asp?idscat=25.

Fig 7. To clean up your survey simply unplug the geophones from the digitizers and roll the digitizers up in the wheel.

All data records saved from your survey will be in .drm format. By clicking on the 'convert' down down menu you can convert your files to .cvs, .SEG-2 and .SEG-Y. Geopsy software accepts both .SEG file types so .drm files will have to be converted. You can interlace two separate records by going: Arrange>>interlace (figure 8). This combines two records which allows you to make a 12 geophone survey into a 24 geophone survey. For example, for the MASW survey shown, I initiated seismic waves at 2 and 4 meters from the first geophone. By interlacing these two records I obtain a 24 channel record with 2 meter spacing between each geophone.

After converting your files to .SEG format they can be uploaded into Geopsy software. To process Refraction and MASW data we will use the Geopsy and Dinver plug-ins. All Geopsy software can be downloaded from geopsy.org. Geopsy.org is also a great reference for software use, methodology and parameter settings.

Upon opening Geopsy, a preference menu will pop up. Make sure under “Signal names” that the Set “Rxxx” as name where xxx is the receiver number is checked.(figure 9) To upload your file data go File>>import signals>>files.

Fig 8. By clicking on the seismograph button this file management interface is brought up. Old records can be

viewed, converted to new files or interlaced with other records.

Fig 9. Geopsy graphical user interface. This preference window will pop up every time you open Geopsy.

Refraction Processing

With your Refraction data uploaded, Geopsy will display two windows labeled 'Table' and 'Graphic.' The 'Graphic' window will display the wave forms recored by your geophones. By right clicking on this window and scrolling down to 'New hodochrone' you can start drawing a hodochrone directly on the graph. By clicking at the point of first arrivals at separate channels, a line is drawn between the point displaying its slope. The slope is the velocity of the seismic wave with one unit of separation between each geophone (figure 10). Multiplying the slope by the spacing between the geophones you used in the field yields observed velocity of the seismic wave. You can improve first arrival picks on your hodochrone by right click>>Edit hodochrone. Too view records better you can use the 'Ampl. Slider' at the bottom of the 'Graphic' window to increase wave amplitudes. Also by right click>> Zoom you can draw a box around the area you wish to enlarge.

Fig 10. Uploaded refraction multichannel shot recored with hodochrone drawn connecting first arrivals. Multiplying the slope of the hodochrone by the spacing between you geophone will yield the velocity of the

seismic wave.

MASW Processing

For MASW processing we will use both the Geopsy and Dinver plug-ins. After uploading your .SEG MASW data into Geopsy go to the drop down menu Tools>>Linear f-k for active experiments. This will cause two screens to pup up, the 'Linear FK toolbox' and the 'Linear fk results' windows (figure 11). The 'Linear FK toolbox' has four tabs and will be used to set the filters and parameters for a velocity spectral graph which will be displayed in the 'Linear fk results' window. Geopsy.org is a good reference for a more detailed understanding of what parameters to input into the 'Linear FK toolbox.'

Fig 11. Geopsy plug-in displaying MASW multichannel shot record. By using the linear FK toolbox, geopsy will convert data into a FK power spectral density graph (left).

Once your parameters are set hit the Start button at the bottom of the 'Linear FK toolbox' and a velocity spectrum graph will be created in the 'Linear fk results' window (figure 12). To pick dispersion curves click on the velocity spectrum graph then go Tools>>Pick curves. Just like the hodochrone for refraction, a line will be drawn connecting each click of the mouse. Curves can be edited by selecting the velocity septum graph and right click>>edit.

After you are done picking a curve, you can review it in the 'Linear FK toolbox' Curves tab (figure 12). The Curves tab has a number of functions. Bad dispersion curves can be easily cleared by clicking the Clear button followed by the Start button. Once you have picked a curves you like it can be saved by using the 'Actions' drop menu. Action>>save prompts a save menu to pop up and files will be stored as a Mutli-column file in the location of your choosing.

Fig 12. Blow up of Linear FK interfaces. The Linear FK toolbox on the right displays the parameters of the points on your dispersion curve.

To invert the dispersion curve we will use the Dinver plug-in. Upon opening Dinver, a small window will pop up asking for available plug-ins. For MASW, the 'Surface Wave Inversion' box should be checked before hitting the Ok button (figure 13).

Fig 13. To use the dinver plug-in to run the inversion of MASW dispersion curve data, make sure the box next to 'Surface Wave Inversion' is checked.

Now Dinver should be open. In the upper left corner of the window is an area with three separate tabs. The tabs are named Targets, Log, and Runs (figure 14). To upload your dispersion curve click on the Targets tab. Next check the box next to 'Dispersion' and click on set. This will bring up a window on the right named 'Dispersion Target.' In this window click on the load button and locate then open the dispersion curve you made in the previous section. If this is done correctly your dispersion curve will appear in the 'Dispersion Target' window.

Fig 14. Dinver plug-in graphical user interface. The different tabs explained in the text are highlighted in red.

Now it is time to set the parameters. For a more information on parameter setting please visit geopsy.org. The Parameters tab can be found in the bottom right corner of the Dinver window. Parameters are broken up into 4 sections: Compression-wave velocities, Poisson's ration, Shear-wave Velocities, and Density (figure 15). You can add and remove layers in each section by clicking on the buttons of the same name. It is recommended that for the velocity section you add more layers than you think are present at your study area. Inputing real values into your parameters will help drive the inversion of the dispersion curve to more realistic results.

Fig 15. Dinver plug-in showing uploaded dispersion curve picked from the FK power spectral density graph. Notice the lower half is showing the parameter tab display.

To start the inversion process click on the Runs tab to the right of the Targets tab. Next go to 'Runs' drop down menu at the top of the screen. By clicking on Runs>>Add a new run will appear in the Runs log. To start the inversion, select your run and click Runs>>start. The status of this inversion process can be viewed by clicking on the Status tab at the bottom right corner of the screen (figure 16).

Fig 16. After running the inversion, velocity vs. depth graphs can be viewed by going to the View drop down menu and clicking on Ground Profiles. Notice the lower half is displaying the status tab.

To view velocity vs. depth ground profiles, select your run and go to the 'view' drop down menu and click 'Ground Profiles.' The window will ask you to set a Maximum misfit number. If you look at Status tab (figure 16), to the right of the graph you will see a list of specks relating to your run. The maximum misfit value should be set to a number just a little bit higher than the Min misfit value given by your run. By clicking OK a window will pop up showing your velocity ground profiles (figure 17).

Fig 17. Velocity vs. Depth graphs. The different colors of models represent different misfit value of the model.

Two graphs should appear, one showing the compression wave velocities and the other th shear wave velocities. Graphs can be customized by selecting the graph and right click>>properties. Similarly you can zoom in by right click>>zoom. This allows you to draw a box around the area you wish to enlarge. After selecting a model that is appropriate to your field area, the Vs30 parameter can be calculated. The Vs30 parameter is a waited average of the shear wave velocities up to 30 meters depth and can be calculated using:

Where hi is the depth and Vi is the velocity at that respected depth.