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).
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.