  A method to diagnose the gas distribution in the Galactic halo using a shock
wave from the Galactic Centre (GC) is presented. Propagation of a shock wave
caused by central explosion with released energy $\sim 10^{55}$ erg is
calculated for various models of the gaseous halo, and the shock front
morphology is compared with the observed North- (NPS) and South-Polar Spurs
(SPS). The observed bipolar hyper-shell (BHS) structure of the spurs is
reproduced, when a semi-exponential halo model with radially-variable scale
height is adopted. A spherical halo model ($\beta$ model) is shown to be not
appropriate to explain the observed BHS shape. Asymmetry of the spurs with
respect to the rotation axis and to the galactic plane is explained by
large-scale density gradient in the halo across the Galaxy. Such gradient may
be produced by ram pressure of the IGM during motion of the Galaxy in the Local
Group. A halo model having sinusoidal density fluctuations (clouds) can also
explain the asymmetry, given appropriate cloud parameters are chosen. Further
irregular features such as filaments and multiple curvatures superposed on the
spurs are also understood as due to density fluctuations in the halo.
