The SPAC Method
A technique of microtremor
exploration to analyze phase velocities.
There are two major categories of analysis methods
for phase velocities, namely the Capon method (also known as the FK method) and
the spatial autocorrelation (SPAC) method. The spatial autocorrelation method
was published by Keiiti Aki in 1957, whereas the
Capon method was published by Jack Capon in 1969. The spatial autocorrelation
method could be called more classic in that sense, but one had to await the
activities of Hokkaido University's Hiroshi Okada and coworkers (publication
years: 1983-200?) and the University of Tokyo's Kazuyoshi Kudo
and coworkers before one could see it becoming a practical method of microtremor exploration. It is only after they began their
activities that the spatial autocorrelation method came to be known by a
diminutive(?) acronym, the SPAC method.
The SPAC method is strictly constrained by the
requirement that the seismic array should be circular (a disadvantage), but it
is also characterized by the ability to analyze wavelengths that are fairly
long relative to the array size (an advantage). The number of seismic sensors
in the circular array can be relatively small, and may even be reduced to just
one central sensor plus one peripheral sensor (!?) in an ultimate case,
according to an idea that emerged recently. We have also endeavored to help
build a theoretical framework for this idea, which bore fruit in a recent
publication (reference [9]).
An overview of international publications suggests
that the SPAC method began to obtain worldwide recognition in the mid-1990s.