One Approach to Make the
Most of the Method's Potential
The
CCA method, one of the analysis methods adopted in our codes, allows one to
analyze waves of very long wavelengths when the SN ratios (signal-to-noise
power ratios) are sufficiently good. Presence of noise sources near the seismic array lowers SN
ratios, but it is difficult to control microtremor
noise sources in urban areas where humans live and industries are active. The
only solution would be to use "miniature arrays" with very
small sizes (if the array lies within the reach of arms, it should be easy to
ensure a calm environment in its surroundings, at least while the measurement
is active [about 15 minutes]). The pictures in the top page show how we
realized large SN ratios by using an array with a radius of just 30 cm. This
array realized SN ratios in excess of 10,000, which made it possible to analyze
wavelengths larger than 500 times the array radius (reference [6]).
A Knack for Setting
Parameters
One
major feature of the analysis methods adopted in our codes lies in their
ability to analyze long wavelengths. A knack for setting analysis parameters to make the most of
this feature is to make the segment duration as small as possible within the
tolerable range that depends on the frequency band of interest you wish to look
at. This measure is effective to extracting a large number of segments from observed
waveforms that are contaminated by non-stationary noise, and helps to make as
narrow as possible the bandwidth of smoothing with a spectral window. Spectral
windowing can cause biases, which becomes a particularly critical problem in
the analysis of long wavelength ranges (reference [3]).