Passive Remote Survey

The passive remote (Fig. 1) method employs a two-dimensional (2-D) receiver array such as a cross or circular layout
to record passive surface waves.  This results in the most accurate evaluation of 1-D shear-wave velocity (Vs) (Park et
al., 2005) at the expense of more intensive field operation and the burden of securing an open-wide space for the
array.  This can be a good choice if a relatively regional one-dimensional (1-D) Vs profiling is needed and a wide open
space (e.g., 200 m diameter) is also available.

Any type of 2-D receiver array of fairly symmetric shape can be used.  An array of significant asymmetric shape, for
example an elliptical or elongated rectangular shape, is not recommended due to bias toward a specific direction of
incoming surface waves that do not necessarily coincide with the actual direction of major surface wave energy.  
Common array types may include the circle, cross, square, triangular, random, etc. (
Fig. 2).  A detailed study comparing
each different type of array and its effect on dispersion analysis has not been reported yet, as far as systematic and
scientific perspectives are concerned.  Intensive modeling tests, however, indicated an insignificant difference
between different types insofar as the symmetry of the array is maintained (
Fig. 3).  It is, therefore, the convenience of
field operation that determines the specific type to be used.  
Field experiments with circular and cross arrays indicate
the circle may result in dispersion images with a slightly higher resolution and better definition
.  Fig. 4a shows a
dispersion image processed from a data set of a passive remote survey that used a circular receiver array of 115-m
diameter (Park et al., 2004).  Two higher modes (M1 and M2) were identified on the image from a joint analysis with
another image processed from an active-survey data set conducted at the center of the array.  The corresponding 1 D
Vs profile analyzed from the multi-modal inversion of these identified dispersions is displayed in
Fig. 4b along with
corresponding theoretical curves (
Fig. 4c).
Fig. 1.  A field layout for the passive remote MASW survey.
Fig. 2.  Examples of 2-D receiver arrays
for the passive remote MASW survey.  
Fig. 3.  Modeling field records for three different types of receiver arrays and their corresponding dispersion
images to illustrate effectiveness.
(Right ) Fig. 4.  (a) Dispersion image from a passive
remote MASW survey using a circular receiver array, (b)
1-D Vs profile inverted from the two dispersion curves in
(a), and theoretical dispersion curves for the inverted Vs
profile.