Soil-Bedrock Mapping

This is one of the most common type of MASW applications.  Actual applications are grouped according to the depth range of bedrock; shallow (<
10m), intermediate (< 20 m), deep (< 30 m), and
very deep (> 30 m).  Data acquisition geometry and interpretation of the obtained 2-D shear-wave
velocity (Vs) cross section are briefly presented for each example.

Shallow Bedrock (< 10 m)

Data Acquisition Geometry

Soil layer has s-wave velocities (Vs’s) in 100 m/sec ≤ Vs ≤ 300 m/sec, indicating soft-stiff soil.  Top about 1-m thickness shows a higher velocities (e.g.,
Vs ≥ 250 m/sec) due to the ground surface roughly treated with gravels and sands.  Bedrock surface highly undulates within a depth range of 2 – 8 m.  
Bedrock velocities are in 300 m/sec ≤ Vs ≤ 1000 m/sec, indicating a bedrock with highly variable stiffness (i.e., variable degree of weathering).  Those
areas of the lowest velocities (e.g., Vs ≤ 500 m/sec) are indicated by dotted lines and they may represent usually soft bedrock.   

Data Acquisition Geometry (Original)

Data Acquisition Geometry (Roll-Along Format)

All forty-nine (49) dispersion curves analyzed from the 24-channel roll-along records are used to produce a 2-D shear-velocity (Vs) cross section
displayed below.  A velocity (Vs) scale of 100-1500 m/sec was used for the color scheme shown on top of the map.  In general, overall velocities of
bedrock are in ~1000 m/sec ≤ Vs ≤ ~1500 m/sec, indicating a relatively competent  (or slightly weathered) rock.  Bedrock topography indicates its
depth changes slightly between ~13-m  (for most of front half) and ~16-m (for most of back half).  The overburden velocities are mostly in ~250 m/sec ≤
Vs ≤ ~300 m/sec except for a layer of about 5-m thickness occurring right above bedrock where velocities are very low (e.g., ~150 m/sec ≤ Vs ≤ ~200
m/sec).  Relatively isolated velocity inversions are also observed throughout the entire overburden.     
Deep Bedrock (< 30 m)

Data Acquisition Geometry

All forty three (43) dispersion curves analyzed from the 24-channel roll-along records are used to produce the 2-D shear-velocity (Vs) cross section
shown below.  A velocity (Vs) scale of 100 - 1000 m/sec was used for the color scheme shown on top of the section.   In general, bedrock depths are
approximately between 20-m and 50-m.  However, considering that the lowest frequency of surface waves observed in most of dispersion images was
about 5 Hz, which corresponds to the maximum investigation depth (Zmax) of about 20 m, bedrock depths deeper than 20 m are less reliable.  Bedrock
velocities are observed approximately in 700 m/sec ≤ Vs ≤ 1000 m/sec.  However, considering uncertainties in dispersion curve picking at the lowest
frequencies (e.g., ≤ 5 Hz), bedrock velocities could be under-estimated.  Overburden velocities are approximately in ~150 m/sec ≤ Vs ≤ ~300 m/sec
corresponding to those of soft  stiff soil.   Significant velocity inversions are observed within the overburden, especially right above bedrock with an
approximate thickness of 10-m for most of the mapped areas.