Surface wave, or ground-roll wave, is usually attenuated or removed as a strong amplitude noise that interferes with the reflected signal. However, the basic idea of this task is to use the dispersion characteristics of surface wave to analyze surface wave, so as to invert formation velocity.
Multichannel analysis of surface waves (MASW)——active and passive methods
The discussion here is about near-surface geophysics, which generally explores tens of meters to hundreds of meters deep. This task usually uses wavelets larger than 50Hz as signals. MASW, i.e. multichannel surface wave analysis, usually deals with low frequency (such as 1-30Hz) surface waves. Generally, it detects several to tens of meters, which is shallower than the above.
S wave = shear wave = secondary wave = shear wave
P wave = pressed wave = primary wave = longitudinal wave
The shear wave velocity is closely related to the hardness of the material, so we need to invert the shear wave velocity.
Compared with other methods, ground-roll inversion is relatively economic. Surface wave has the strongest energy, and its velocity is basically determined by the shear wave velocity of the medium.
The sampling depth of a particular frequency component of surface waves is in direct proportion to its wavelength, and this property makes the surface wave velocity frequency dependent, i.e., dispersive.
That is to say, the sampling depth of a frequency component is directly related to its wavelength, so the velocity and frequency of surface wave are related, that is, the dispersion effect is produced.
Here we introduce MASW, so-called MASW, which uses the dispersion effect of surface wave to do Vs profiling. Generally speaking, this work aims at several Hz to dozens of Hz frequency spectrum, using multiple channels to record, the range of geophone array is about 2-200 m order of magnitude. The difference between passive and active is that they usually use sledgehammer, hammer and tap to get signals, while passive uses natural surface waves (such as traffic or lightning tides, etc.). For active, it can only be measured within 30 meters, while passive can be measured within several hundred meters.