Acoustic point-source representation for vertical ground motions induced by buried explosions

Epicentral ground motions induced by underground explosions generate infrasound. Vertical ground motions change the volume of overlying air parcels, inducing pressure disturbances that propagate as linear acoustic waves in the atmosphere. Arbitrary volume changes in the source region can be represented mathematically by an infinite series of multipoles including monopole, dipole, quadrupole, and so on. If the source extent is considerably smaller than the wavelength of interest, higher-order multipole terms will have negligible impact on acoustic wave generation. The simple source model (monopole) has been widely used to represent natural and man-made explosive events. However, the ground motions may be more complicated than explosion sources which are relatively compact. Large underground explosions can drive strong vertical motions over large area, and its variation may not be negligible for sound generation. In addition, the large dimensions of epicentral area may cause directional infrasound wavefields which cannot be described solely by a monopole source model. Here, we investigate an acoustic source model that uses combinations of monopole and dipoles to represent ground motion induced infrasound. We apply the multipole source model to underground chemical explosions from Source Physics Experiments (SPE). The Rayleigh integral over the epicentral area of SPEs showed that SPE acoustic signals were direct response of vertical ground motions measured by surface accelerometers. We compare the multipole source model to Rayleigh integral model and find the best coefficients for monopole and dipole to fit the data. We find that the multipole source model describes the acoustic source characteristics better than a monopole model. Further, it provides a useful point source approximation for vertical motions that excite infrasound waves in the far field.