MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW) TEST METHOD.
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MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW) TEST METHOD.

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MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW) TEST METHOD.

1.0. SCOPE AND APPLICATION: 

1.1. This Method Statement (MS) covers the multi-channel analysis of surface waves measurements using seismograph (e.g., Geode) for subsurface investigation and will be followed by laboratory staff during field geophysical surveys. This MS was prepared in accordance with the manufacturer’s instructions and related technical references.

MASW measurements are applicable in mapping subsurface conditions for various uses including 

geologic, geotechnical, hydrogeological, environmental, and archeological investigations. The MASW method is used to map anomalous geologic conditions including weak zone detection, void detection, fault/fracture detection, etc. Seismic shear-wave velocities are related to mechanical materials properties. Therefore, characterization of the material (type of rock, degree of weathering, and rippability) is made based on the modeled shear-wave velocities.  

2. EQUIPMENT: 

2.1. Seismograph (GEODE) 

The GEODE, see below photo, is a 24-channel digital recording seismograph system specially designed for collecting high-resolution seismic information. This type of seismographs allows the stacking and enhancement of repeated seismic waves that are initiated from the same source point. Ultra-portable 24 channel seismic recorder attaches with the laptop and the acquisitions controlled by special software. This system can be used for MASW, reflection, refraction, down hole, VSP and monitoring applications.   


2.2. Sensors/ Geophones  

The moving – coil geophone is the basic vibration sensor. The coil and its support spring make a pendulum with the natural frequency, and this is specified for all geophones. Frequency is measured in cycles per second, calls Hertz and abbreviated HZ. The output of the geophone is reasonably flat in responding to earth vibrations with a frequency higher than the natural frequency of the geophone; electrical signals from the geophones are amplified, digitized, and stored in the seismograph’s memory. Geophones are the sensors planted in the ground to detect the motion; Vertical geophones with natural frequency of 4.5 Hz are recommended for most shallow Seismic Surface Waves survey, the geophones are presented in photo below.  

2.3. Geophone Spread Cable  

It is a multi – conductor cable with connectors molded at intervals along the cable. The geophones will have connectors which mate with those on the cable. The mating connector used on the geophone is a clip with the colored insulator. The take-outs and clips are different widths and colors to encourage the use of the same polarity each time the geophone is connected. See below photo.

2.4. Trigger/ Hammer Switch  

The sledgehammer is synchronized by an impact sensitive hammer switch tab bed to the handle and 

connected directly to the trigger connection on the seismograph. An extension cable may be used to allow the sledgehammer to be located away from the seismograph. See below photo.  


2.5. Trigger Extension Cable  

Trigger extension cable is used to lengthen the cable that is built in to hammer switch. This allows the distance in between the sledgehammer and the seismograph to be increased, see below photo. 


2.6. Strike Plate  

Approximately the same weight as the hammer, is placed on the ground and used an impact point for the sledgehammer. This striker plate provides more efficient coupling and more precise triggering. Strike plat can be made of metal or plastic. Plastic plate should be used in high-risk areas to avoid generating sparks and reduce the vibration impact on the area at time of hummer strike. See below photo.  


2.7. Source  

For MASW, a sledgehammer of 8 to 20 pounds (4 to 9 kg) suits most surveys. The hammer is used along with striker plate, hammer switch and hammer switch extension cable.  

2.8. Power 

The Geode operates from a nominal 12 volts DC. This may be rechargeable battery pack, a standard  automotive battery, 12 – volt vehicle power or an AC – powered or an AC – powered 12-volt DC supply. 

2.9. Field Software 

The main program for operation the Geode knows as the Seismic Controller Software (SCS). The SCS for  Geode provides a flexible solution for all survey refraction, reflection, or specialized acquisition, like marine surveying. The operating software has functions necessary for collection, processing, and interpretation directly in the site, see below photo.  


2.10. Processing Software 

SurfSeis 4.0 is seismic data processing software for subsurface shear-wave velocity profiling. It can perform data processing for both active and passive MASW tests. SurfSeis 4.0 reads the seismic data obtained from active or passive tests in SEG-2 format, extracts Rayleigh surface waves from the data, and then uses the surface wave information as an input to produce 1-D or 2-D shear velocity profiles.  

2.11. Pin Flags 

Surveyor using the pin flags to mark the start and end of each MASW profile as per the site work plan. 

2.12. Vehicle 

4x4 pickup is required during the site work to pull the Geophone Spread Cable and carry the equipment.  



3.0. GENERAL DESCRIPTION OF   MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW)

3.1. OVERVIEW 

3.1.1. MASW (Multi-channel Analysis of Surface Waves) is a geophysical method that uses the dispersive  characteristics of surface waves to determine the variation of shear wave (S-wave) velocity with depth. MASW is a non-intrusive method that is performed on the ground surface. Data are acquired by analyzing seismic surface waves generated by an impulsive source and received by an array of geophones. A dispersion curve that shows the velocity of the surface waves as a function of frequency is calculated from the data. A shear wave velocity profile (1-D profile of velocity as a function of depth) is then modeled from the dispersion curve. The resulting shear wave profiles from multiple locations along a survey line are combined and contoured into a 2D cross-section of shear wave velocity. Shear wave velocity is a function of the elastic properties of the soil and rock and is directly related to the hardness (N-values) and stiffness of the materials. 

3.1.2. MASW has advantages over the more traditional spectral analysis of surface waves (SASW), since  data are recorded at 24 or more geophone locations with MASW compared with only 2 geophones with  SASW. The additional data allow the surface waves to be extracted from other seismic waves more easily in the processing and increase the signal to noise ratio. MASW also allows a higher production rate than SASW, providing a greater data density. 

3.2. DEPTH OF INVESTIGATION 

The depth of investigation can range from 25-30 m or more using the MASW method. The depth of  investigation is governed primarily by the frequency of the surface waves that are recorded. Deeper  investigations may require large sources of energy and longer spread of geophones. 

3.3. LIMITATIONS 

The depth of investigation is limited by the seismic source, the frequency of the geophones, and the  geophone spread length. In addition, the presence of some underground objects such as large utility  conduits can interfere with the propagation of surface waves. The MASW model of S-wave velocities is a  gradational model and may not represent true S-wave velocities in areas with sharp geologic contacts or within hard rock.    

3.4. TYPES OF SPREAD CONFIGURATION 

3.4.1. Fixed Receiver Spread Configuration. 

3.4.1.1. The fixed receiver spread is useful if the survey line length of interest is not very long, if  the survey area is confined. The simple configuration for 2D MASW surveys is the fixed receiver  spread as shown in Figure 8 below, the geophones are set up in a line at fixed locations and the shot is moved through the spread. The first shot is located off-end at a near offset of one half the geophone intervals. The shot is then advanced at an increment equal to geophone interval so subsequent shots are located midway between the geophones. As shot number increases, the shot location advanced by one interval across the survey distance. The last shot is located off the opposite end by the same near offset of one half the geophone intervals. See below photo.


3.4.1.2. It is recommended, a few more off-end shots can be taken up to 20 % of the spread length with  incrementing by the shot interval (with symmetry on either side of the spread). The survey depth by this  configuration is approximately (a/4 to a/2). 

3.4.2. Roll along Spread Configuration 

3.4.2.1. Geophones are affixed to a streamer (Land streamer geophone) that makes the gravity contact with the ground and is towed typically by vehicle. The streamer connected to a seismograph positioned on the vehicle, the number of geophones equals to the number of recording channels, and all channels are kept active for each shot. The source is usually located between vehicle and the streamer or on the down–line end of the streamer and its location is incremented together with the streamer after each shot. The procedure of roll along array is shown in see photo below.   


3.5. DESIGNING A FIELD SURVEY. 

The following parameters shall be considered before starting MASW survey according to the purposes of the survey. 

3.5.1.   FIELD GEOMETRY: 

3.5.1.1. Three major types of parameters are the most important, see photo below:   


• X1-the source offset  

• dx - the receiver spacing  

• D - Length of the receiver spread 

3.5.1.2. The length of the receiver spread (D) is directly related to the longest wavelength (λ max) that  can be analyzed, which in turn determines the maximum depth of investigation (Z max). 

D≈ λ max ≈ Z max 

3.5.1.3. On the other hand, receiver spacing (dx) is related to the shortest wavelength (λ min) and  therefore the shallowest resolvable depth of investigation (Z min). 

dx ≈ λ min ≈ Z min 

3.5.1.4. In practice, however, λ max (therefore D) in an active survey is usually limited by the seismic  source because it is the primary governing factor.  

3.5.1.5. The source offset(X1) needs to change in proportion to the maximum investigation depth (Z max). 

3.5.1.6. NTERVAL (dSRC) OF SOURCE –Receivers Configuration (SRC) Movement (Rolling): 

An interval between 1dx -12dx is recommended. 4dx is most used in the case of 24– channel acquisition. 

Below field parameters proposed to be used during the fieldwork:

• No. of geophones         : 20 – 24 Geophones 

• Geophone Spacing       : 1 m  

• Shot offset                     : 4 - 12m  

• Spread Movement       : 4 - 10m 

 

3.5.2. RECORDING PARAMETERS: 

- Sample Interval: 0.5 to 1.0 msec. (Over –sampling is fine). 

- Record Length: 1 to 2 sec. (Should be long enough to capture distance arrival). 

- Stacking: as needed to increase signal to noise ratio 3 to 10 Times for each shot point. 

- Delay: -10 ms allows the first break on the near geophones to be more easily viewed. 

- Acquisitions Filter: acquisitions filters are not recommended because effect is irreversible should be 

carefully applied to filter signal .also not recommended to use 60 Hz power line noise. 

- Preamp Gains:  Highest setting 

- Display Gains: Fixed gain ( same gain over time for a given trace , but variable from trace to trace, 

traces far from the source will need a higher gain setting than those that are near). 

 3.5.3. START AND END OF THE POINTS OF THE SURVEY LINE: 

Start and end of the points of the survey line should be determined in such way that the final 2-D shear–  velocity map can cover certain portion of the normal area on both sides of the target (anomaly) zone to maximize the mapping effectiveness. A rule of thumb is that center of the receivers (or array) starts and ends at a quarter length (L/4) before and after the zone of the length respectively as illustrated in  diagram shown in photo below.  



4. FIELD PROCEDURES FOR MASW DATA.

4.1. SURVEY LINES AND SITE MAP 

4.1.1. Survey lines and or a survey grid will be established to provide a location for geophysical measurements. 

Measurements must be made along straight lines. The survey parameters are specific for each project and will be identified in the Form/GF-01H along with a site map presenting the configuration of survey lines on the site.  

4.1.2. Over small areas of a few acres or less, a survey line is commonly constructed using a tape measure marking the beginning of the line, equal intervals along the line and the end of the line. Over larger  areas, a land surveyor or GPS may be used to establish the survey lines and grid. 

4.1.3. The survey line locations will be referenced to surface features, survey markers. Station marks will be made using surveyors pin flags or paint and referenced in distance along the survey line. The geophone spacing is determined on a specific project-needs basis depending upon the need for a reconnaissance or detailed survey. 

4.1.4. The site map should be updated by the Field Geophysics(s) to show key site features, along with the actual survey lines or survey grid and returned along with site worksheets and data to the office  geophysicist. 

4.2. INITIAL SET-UP PROCEDURES.  

The operator will conduct the following procedures prior to the start of a survey: 

4.2.1. Unpack the seismograph, cables, geophones, and source to be used, and check for any damage. 

4.2.2. Make sure all batteries are fully charged. 

4.2.3. Follow the manufacturer’s instructions for connecting the seismograph to the power source, geophone cables, and trigger. 

4.2.4. Turn on the noise monitor and connect at least one geophone to the cable. 

4.2.5. Manually tap the geophone and check for a response on the seismograph. 

4.3. DATA ACQUISITION. 

After the functional checks, the operator will conduct the following procedures for data acquisition: 

4.3.1. In case of roll-along spread configuration, the land streamer should be moved out from the vehicle and tied from one side on the vehicle’s back hook and adjusted on the start point of the MASW profile. In case of using fixed receiver spread, the geophones should be adjusted first along the MASW profile. 

4.3.2. The contact between the geophones and the ground is critical. Each geophone must be firmly pushed into the soil or in the case of using the land streamer set-up, set on solid ground. 

4.3.3. Make sure all geophones have been connected to the cable. 

4.3.4. If using geophones planted in the ground, connect the geophone cables to the roll box. 

4.3.5. Connect the geophone cables to the seismograph. 

4.3.6. Connect the battery to the seismograph. 

4.3.7. Turn the seismograph on and check that all channels are responsive on the noise monitor. 

4.3.8. Set up the source at the first shot-point. 

4.3.9. Test the seismic source and trigger cable. 

4.3.10. Setup the seismograph with the following parameters: 

- Turn off acquisition filters. 

- Select SEG-2 data file. 

- Recording window = 1.0 seconds (or more if needed). 

- Sample rate = 0.5 msec. 

- Equalize the display gain.    

4.3.11. Select the survey area(s). 

4.3.12. A survey line should be installed prior to the survey. Go to the start of a survey line and install the geophones at the locations and separations given in the work plan (4.1.1). 

4.3.13. The connection between the geophones and the ground will be checked utilizing the noise monitoring options. 

4.3.14. The hammer will be impact on the plate 5 times or more and the data from each blow “stacked” to enhance the data and improve the signal to noise ratio. The data will be viewed on the computer screen to check the quality and then saved on the hard disk. 

4.3.15. After finishing station number one, the geophones and shot point will be shifted for n.dx along the survey line (according to the specifications of the project – shot movement) by moving the vehicle and the tied land streamer then repeat the previous procedures. 

4.3.16. The pulling vehicle should back up to relieve strain on the land streamer or to eliminate vibrations traveling down the land streamer webbing. 

4.3.17. These procedures will be repeated with geophones and shot point until cover the line of survey. 

4.3.18. Field worksheet Form/GF-003 shall be filled to describe the field set up and any specific site  observations. The raw field data along with field worksheet(s) and site map are returned back to the  Office Geophysicist.

5. DATA PROCESSING AND INVERSION. 

Field data shall be processed the data to create a shear velocity image by using SurfSeis 4.0(Kansas Geological Survey)  software according to the following procedure: 

5.1. STEP 1−FORMATTING: 

Most seismographs and digital signal analyzers store data either in ASCII or SEG-2 format; however, SurfSeis 4.0  requires input seismic data in SEG-Y format. Thus, the seismic data must be converted into SEG-Y or 

standard KGS format. Therefore, the filed measurements shall be converted from SEG2 format into KGS –format using Utility 

Format → Open SEG2 data files → Output File Name → Run Forma, see below photo.  


5.2. STEP2 – FIELD SETUP: 

In this step, the location of source, receivers, receiver-array spacing, and SRC is entered. To input the  required information, Go to Utility → Field Setup, and open the formatted data file created in the previous step, when the formatted file selected a graphical dialog box will appear as shown in below photo allowing selection of MASW Survey Type. In our cases Active button is selected.


After selected MASW Survey Type (Active) two graphical dialog boxes will appear for the field configuration, see below photo.


The field acquisitions parameters are filled utilizing these boxes, such as Station number (Name of file record), Source Offset (x=), Unit of distance (m), receiver-array spacing, (dx=). After fixed the acquisitions parameters  and save the file as Field Setup.dat. After the process is complete, a window will appear with the encoded field geometry as shown in below photo.  


5.3. STEP 3−GENERATION OF OVERTONE (OT) RECORDS. 

After all data are formatted and the field setup is encoded with the data, the next step is to process the  dispersion information from each record that is also the first step in the dispersion analysis. This step consists of the generation of the dispersion image or the overtone image and extraction of the dispersion curve from the overtone images. Click on the “Analysis → Dispersion” button and open the field geometry encoded data file “test (Field setup).dat” created in the previous step. When the data file is selected, the data set is ready to process. The overtone is created utilizing the following parameters presented in below photo.  


5.4. STEP 4: EXTRACTION OF DISPERSION CURVES FROM OT IMAGE.  

The Overtone (OT) images previously save step-3 shall be used for extraction the dispersion curve for each record. Click on the “Analysis → Dispersion” button and open the Overtone (OT) images created in the previous step. When the data file is selected, the first overtone record in the input file will be displayed, and a new set of buttons will appear on the left side of the window.  For extraction of Dispersion Curve from OT image four tasks shall be performed: 

5.4.1. Task –1: Controls:  This dialog box where parameters related to dispersion extraction and some of the display attributes can be controlled, see below photo.  


5.4.2. Task –2: Bound: This window enables you to you to establish lower and upper limits of phase  velocities for the dispersion and input the guideline for extraction the dispersion curve automatically  by click several (5-10) (5-10) reference points on top of the dispersion trend that you identify as the  fundamental mode (M0), see below photo. The first and last points will determine the frequency range of the  dispersion curve. A pair of solid curves will be drawn as reference points are marked, and they  represent lower and upper limits (bounds) of the phase velocities to be extracted for the curve. 

 

5.4.3. Task –3: Extract: Clicking this button will extract a dispersion curve most likely within the bounds  specified in the previous Task -2, see below photo.   


After extracted the dispersion curve you can modify the bounds for a better extraction, you can do so by de- pressing and pressing again the ‘Bounds’ button (clicking twice normally). Furthermore, you can modify the picking point according to the S/N curve by selecting the point which required modifying and dragging by the mouse for the correct place. 

5.4.4. Task –4: Save: This will enable you to save the extracted curve with its own name, see below photo. Default name will be the input overtone file name with corresponding record number appended at the end. Once the extracted curve is saved, then the program will show the next overtone record being ready to repeat the previous steps. In most cases, the bound curves previously established will be reusable without any modification and you can start from the extraction Step.  



5.5. STEP 5: INVERSION FOR 2-D VS. PROFILE: 

Inversion of the dispersion curves extracted in the previous step is the last step in the data processing. In  this step, either a 1-D or 2-D shear velocity profile is constructed by the inversion of single or multiple  dispersion curves. To initiate the inversion process, Go to Analysis → Inversion’ and select dispersion curve files previously extracted and during the dispersion analysis. Parameters for this step include stopping criteria, initial Vs layer setup, and weight applied to data sets, all set using the “Control” tab shown in below photo. After setting the inversion parameters go to Run



5.6. STEP 6: INVERSION FOR 2D VS. PROFILE. 

Inversion of the dispersion curves extracted in the previous step is the last step in the data processing. In  this step, either a 1D or 2D shear velocity profile is constructed by inversion of single or multiple dispersion curves. The inversion has been carried out using SurfSeis ver.2.05 to provide quantitative interpretation of data after several iterations, a best fit model of the S-Waves distribution of the subsurface materials would be displayed automatically in two-dimensional (2-D) Surface Wave model for the subsurface for the data obtained from Active Seismic Surface Waves surveys. The final inversion will be 2D- Shear Velocity, see below photo.


The 2D Shear wave velocity model can then be used to estimate the statistical average shear wave velocity Vs of the  upper 100 ft (30m) {Vs-30} according to International Building Code IBC 2009 as described in below Table.


6. REPORTING: 

The final report will be prepared upon the completion of the field work stages and date processing. The report shall incorporate the following: 

6.1. Text outlining the objectives of the survey, filed procedures, equipment’s, Interpretations and Conclusions based on the interpretation of data presented and focused on establishing whether cavities are present or not, including any further necessary investigations. 

6.2. Planes and / or drawing showing the locations of the geophysical station’s surveys and boreholes.  

6.3. Summary  of  the  local  geology  for  the  site  (according to  the  geotechnical  report  and  verifications BHs). 

6.4. Summary of data collection procedures (methodology, quantity, and type). 

6.5. Quality and reliability of the acquired data. 

6.6. Summary of field investigation (equipment, and acquisition parameters). 

6.7. Summary of data processing (methodology and software). 

6.8. Summary of interpretation procedures, including verifications (ground truth or synthetic modelling). 

6.9. Summary of correlation between the results of MASW with available drilling (Calibration boreholes) or other subsurface data of the area, and the anomalies if any were highlighted and interpreted. 

6.10. Presentation of relevant interpretation in a form that is useful to end user 

6.11. Presentation the results of MASW as (2D or 3D). 

6.12. Presentation the results of MASW as cartography maps at different depths. 

6.13. Summary and recommendations. 

6.14. Soft Copy for the original filed measurements data for all tests.  








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