Poisson’s ratio ( σ ) of the near-surface materials
is one of the key parameters in various types of geotechnical projects. It
is usually associated with the integrity of the materials from the engineering
perspectives. A two-dimensional (2-D) distribution map of σ , therefore,
would have an invaluable value. Seismically,
σ can be determined if P- (Vp) and S-wave (Vs) velocities are known.
This would indicate that two separate (P- and S-wave) surveys should be performed
in order to obtain the separate maps for Vp and Vs. Running both types of
survey for one project will be expensive in terms of equipment, data processing,
and overall time. In addition, S-wave survey is generally known as being much
more difficult to acquire good quality data than the P-wave survey.
We can obtain Vp and Vs fields by using several methods. However, each
of these methods requires individual data acquisition and processing techniques.
For example, the reflection method is not suitable for studying the very near
surface because the shallower we aim our target of investigation the more
expensive it becomes and the more our data will be contaminated by waves considered
as noise. On the other hand, the refraction method is incapable of detecting
“hidden layers” (Burger, 1992) such as a high velocity thin layer or a low
velocity layer “sandwiched” between two high velocity layers. Such “hidden
layers” cause erroneous interpretation of the data. An improved way of interpretation
of refraction data is by using refraction tomography. Still, we need an initial
model that is close to the true Vp distribution as well as smoothing constraints
(Stork and Clayton, 1991.) in order to achieve reliable results during inversion.
Reflection and refraction Vs methods share the same problems as with
the Vp field. In addition there are equipment and acquisition difficulties
and there is the possibility for misinterpretation due to S-P-S wave conversion
(Xia et al., 1999). Recently, an economic seismic method has been
used to produce a Vs profile (plot of Vs vs. depth) by analyzing surface waves (ground roll) on a multichannel
record. This multichannel analysis of surface waves (MASW) method (Park et
al., 1999a) can produce a 2-D near surface Vs map when the multichannel records
(shot gathers) are acquired in a consecutive manner similar to conventional
reflection survey (Miller et al., 1999a). Since the MASW method SAGEEP 2000
employs the conventional seismic approach in which vertical source and receivers
are used the near-surface Vp information can be associated with the first
arrivals from the shot gathers. Refraction
tomography, for example, can be applied to the shot gathers to obtain the
near surface Vp map. The goal of this
paper is to propose a method for obtaining accurate and reliable 2-D σ
distribution map in the near surface in an economic manner. The economy is
achieved by performing a single near surface seismic survey using a vertical
source for acquiring data for two wave fields. The accuracy and reliability
is achieved using the analysis results of one wave field (surface wave) as
a priori information for the inversion of another (body wave).