MASW Data Acquisition

1
MASW - Data Acquisition

• See Field Procedures in Appendix for more
  detailed descriptions

2
Field Parameters

• Offset
• Source and Receiver
• Recording Parameters
• Station and Record Numbers

3
Optimum Offset

• Avoid Near- and Far-Field (or Offset) Effects
• Near-Field Effects Nonhorizontal propagation
  of surface waves
• Far-Field Effects Contamination by body waves
  and ambient noise

4
Near- and Far-Field Effects

• Near-Field Effects
• Non-planar Surface Waves
• Irregular Direction of Propagation
• Weak Amplitude
• Wavelength Dependent (The Longer Wavelength, the
  Further Offset Needed)
• gt 10 m ? A Rule of Thumb for Common Soil Site
• Far-Field Effects
• Body-Wave (e.g., Refraction) Domination
• Ambient Noise Domination
• Higher Mode Domination
• Low S/N
• lt 100 m ? A Rule of Thumb for Common Soil Site

5
Near-Field Effects

• Reduced Coherency
• Weak Amplitudes
• Most Wavelengths stabilized beyond 10 m from
  source

Shot Gather
Swept-Frequency
6
Far-Offset Effects
S-Surface Wave, B-Body Wave and Ambient Noise,
FM-Fundamental , HM-Higher Mode
7
Optimum Offset

• Minimum Offset (the Source Offset) (X1)
• Anywhere in 5-10 m range
• Maximum Offset (the Farthest Offset) (X2)
• Anywhere in 50-100 m range
• Receiver Spacing (dx)
• Anywhere in 1-2 m range

8
Seismic Source

• Sledge Hammer
• Best Source ? Cheap, Portability, Easy to use,
  Investigation depth of 10-20 m
• Heavier Hammer (gt 5 kg) for deeper range (e.g., gt
  10 m)
• Lighter Hammer for shallower range

9
Seismic Source - Sledgehammer
10
Receivers

• Lower-Frequency Geophones
• 4.5 Hz Phone Best in price, performance
• Higher-Frequency Phones (e.g., 40 Hz) Limited
  depth range (lt 10 m)
• Phones higher than 100 Hz Absolutely Not
  Recommended

11
Receivers
Shot Gather
Dispersion Curve
Comparison made by using a weight drop source
All with Weight Drop Source
12
Receiver Coupling (Spike vs. Plate)
Plate Coupling
Comparison made by using a weight drop source
and 4.5-Hz geophones
All with 4.5 Hz Geophones and 20-lb Sledgehammer
13
Recording Parameters

• Total Recording Time (T)
• 1000 ms (1 sec)
• Increase when length of receiver spread becomes
  longer than normal
• Increase if slower materials (i.e., softer soil)
  
• Sampling Interval (dt)
• 1 ms
• Decrease (e.g., 0.5 ms) if body waves are to be
  utilized
• Low-Cut Filter Out!

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Field Parameters - Rules of Thumb

• Source 20-lb (10 kg) Sledge Hammer
• Receiver 4.5 Hz Geophones
• Total Channel At least 12 Channel
• Offsets
• 5 m minimum offset
• 0.5 m receiver spacing
• Maximum offset not to exceed 100 m
• Recording Parameters
• 1 ms sampling interval
• 1000 ms total recording time
• Low-cut filter out

15
Optimum Acquisition Parameters Table
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Quality of Data (Records)

• Good Data Well-defined pie-shaped envelop
• Bad Data Irregular or incoherent waves

17
Station and Record Numbers

• Station Numbers
• Arbitrary numbers for surface coordination
• For example, 1001, 1002, 1003, etc., for line 1
• Record Numbers
• Data ID to correlate with field parameters
• Arbitrary numbers assigned by seismograph (file
  name)
• Can be reassigned at the beginning of processing
• Field Note
• Record number associated with station numbers for
  source and receivers
• All acquisition parameters
• Site map and photos
• Comments

18
Field Note Example Format
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Station Number Field Coordinate