Geophysical data acquisition - OGS

Ground penetrating radar for environmental problems
J.M. CARCIONE
F. CAVALLINI
G. PADOAN
G. SERIANI
The ground-penetrating radar (GPR) has unique capabilities as a subsurface
exploration device; as such, its applications (since 1992) include archaeology,
mining, glaciology, hydrogeology, and environmental remediation. But its
performance is highly sensitive on the knowledge of the physics of the medium and
of the instrument itself. Therefore, basic research in this field is of crucial
importance for pracical applications as well. Many concepts and techniques that
were devised for seismic waves can be applied to the processing of GPR data, in
virtue of the mathematical analogy between electromagnetic and elastodynamic
equations. This research, funded by the Regional Government of Friuli - Venezia
Giulia (northeastern Italy), aims at exploiting these methods for the monitoring of
a polluted area.
The exploding-reflector method, originally developed for seismic waves, has been
adapted to produce zero-offset synthetic radargrams without need to compute
common-shot records. The basic underlying idea consists in assuming that each
reflecting point acts at the initial time as an instantaneous source of motion with
a magnitude proportional to the normal-incidence reflection coefficient. The
magnetic permeability is used as a free parameter to obtain a constant-impedance
model and, so, to avoid multiple reflections. Moreover, the condition that the phase
velocity remain unchanged also requires the scaling of the permittivity and of the
conductivity. Thus, the method generates normal-incidence reflections, i.e., those
having identical downgoing and upgoing wave paths. In an example with
transverse-magnetic equations, this method has shown more accuracy, but less
efficiency, than the plane-wave technique.
Analytical solution for lossy anisotropic media has been developed. Microstructural
features, fine layering, and fluid-filled cracks give rise to electric and magnetic
anisotropy, while mineralized water in the fractures makes conductivity
anisotropic. Water relaxation and ferromagnetism are responsible of energy
dissipation. This motivates an orthotropic constitutive law in which attenuation
stems from the Debye model. The corresponding solution is obtained in the
frequency domain from a change of coordinates that turns Maxwell’s equations
into Helmholtz equations, which are solved analytically. Then, the solution in time
domain is recovered by numerically computing the inverse Fourier transform. The
results are in agreement with a plane-wave analysis of the slowness, the
attenuation and the energy velocity.
Simulation of the ground motion caused by earthquakes
and site response analysis
E. PRIOLO
G. LAURENZANO
Synthetic seismograms computed as a solution of the full-wave propagation
through a realistic geological structure can reproduce accurately much of the
effects of medium heterogeneity and local soil conditions on the ground motion
caused by earthquakes. They are a powerful tool for building seismic hazard
scenarios as well as for performing site response analyses. For this kind of studies,
we currently use two methods, i.e. the 2-D Chebyshev spectral element method
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