Passive and Active Combined Survey
It is often useful or necessary to combine dispersion images processed from active and
passive data sets for two reasons: (1) to enlarge the analyzable frequency (therefore
depth) range of dispersion (Fig. 1a), and (2) to better identify the modal identity of
dispersion trends (Fig. 1b). While the active survey provides a dispersion curve in a
relatively high frequency (short wavelength) range (20-50 Hz, for example, corresponding to
wavelengths of 1-30 m), the passive survey can fill the dispersion trend at lower
frequencies (long wavelengths) (5-20 Hz, for example, corresponding to wavelengths of
30-100 m). Traditionally, it was assumed that passive surface waves consist
predominantly of M0 of Ray¬leigh waves (Aki, 1965). However, recent studies aided by the
imaging method revealed a strong possi¬bility of frequent higher mode domination (Park
et al., 2005; 2006). For this reason, Park et al. (2005) suggested a combined
active/passive survey to be followed by combining disper¬sion images of both types for a
more reliable modal identification (Fig. 1b).
The passive image in Fig. 1a obtained from a remote survey using a 48-channel cross
receiver array deployed over a surface dimension of about 120 m (Park et al., 2004) shows
a prominent dispersion trend in 6-17 Hz range. In addition, the active image from a
24-channel active survey conducted with 1-m receiver spacing at the center of the passive
cross array shows another dispersion trend in the higher frequencies (16-50 Hz). When
these two images are combined by vertically stacking both sets of image data, two trends
are merged naturally to make one continuous trend over a broader bandwidth (6-50 Hz).
On the other hand, the passive dispersion image in Fig. 1b obtained from another remote
survey conducted over a different soil site shows a trend prominent in the 5-20 Hz range
that was originally interpreted as the fundamental mode (M0). When this image was
combined with the active image obtained from an active survey at the center of the passive
array, its modal nature is reinterpreted more likely as a higher mode (M1) (Park et al.,
Necessity of separate active survey in passive remote survey: Because a passive survey usually operates with much a larger receiver spacing than normally used in an
active survey, a processed dispersion image usually lacks information at shallower depths (higher frequencies). Although in theory this missing information can be filled by
multiple surveys of progressively smaller receiver-array dimensions (D’s), higher frequency components of passive surface waves may not be recorded effectively because of
their relatively rapid attenuation properties. The best way would be to perform a separate active survey, preferably at the passive array center, using a sledge hammer. Then,
two separate dispersion image data sets can be combined to form a broader-band image. Combining active and passive dispersion images can also help modal
identification. An alternative approach instead of a separate active survey would be to apply active impact(s) at a place close to (but outside) the passive receiver array during the
recording of a passive record. If this is the case, however, followings are to be noted:
1. Analyzed shear-velocity (Vs) information at shallow depths can only be associated with those near-surface materials near the impact point.
2. Analyzed dispersion trend for those high frequencies generated by the active source can be slightly distorted (up to 10%) because of a violation of the plane-wave assumption.
3. Dispersion images for those low-frequency passive waves can be adversely influenced and result in a degraded definition, or slightly distorted trend, or both.
|Fig. 1. Dispersion images from passive remote survey (top), active survey (middle), and
combination of the two (bottom) for (a) the same and (b) different modes.