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.   
Intermediate Bedrock (< 20 m)

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.   
Bedrock Velocity (Vs and Vp) Evaluation

By using the same MASW data set used to generate the 2-D velocity (Vs) cross section displayed above, an approximate evaluation of bedrock
velocities (Vs and Vp) has been attempted based on the “frequency-summation (FRQ-Sum)” method (
Park, 2016) that sums all the energy in the
stacked dispersion image along the frequency axis.  This method, when effective, generates a summed curve in which the last two peaks represent Vs
and Vp of bedrock, respectively.  The effectiveness of this method varies significantly depending on the velocity structure of the site as well as
acquisition parameters such as source offset (X1), receiver spread length (L), etc.  

The last peak at 3540 m/sec is interpreted as Vp of the bedrock.  Then, the next peak at the lower phase velocity of 1660 m/sec is interpreted as Vs of
bedrock.  These two values give a Poisson’s ratio of 0.359 (i.e., POS ≈0.359).  The strongest peak at the lowest phase velocity of 340 m/sec is
interpreted as energy from air waves.