| It is well-known that the local seismic site effects may have a significant contribution to the
intensity of damage and destruction (e.g., Hough et al., 1990; Regnier et al., 2000;
Bonnefoy-Claudet et al., 2006; Haase et al., 2010).
The thicknesses of sediments, which play a large role in amplification, usually are derived
from seismic velocities. At the same time, thickness of sediments may be determined (or
defined) on the basis of 3D combined gravity-magnetic modeling joined with available
geological materials, seismic data and borehole section examination. Final result of such
investigation is a 3D physical-geological model (PGM) reflecting main geological
peculiarities of the area under study. Such a combined study needs in application
of a reliable 3D mathematical algorithm of computation together with advanced
methodology of 3D modeling. For this analysis the developed GSFC software was
selected.
The GSFC (Geological Space Field Calculation) program was developed for solving a
direct 3-D gravity and magnetic prospecting problem under complex geological conditions
(Khesin et al., 1996; Eppelbaum and Khesin, 2004). This program has been designed for
computing the field of δg (Bouguer, free-air or observed value anomalies), δZ, δX, δY ,
δT , as well as second derivatives of the gravitational potential under conditions of rugged
relief and inclined magnetization. The geological space can be approximated by (1)
three-dimensional, (2) semi-infinite bodies and (3) those infinite along the strike closed, L.H.
non-closed, R.H. on-closed and open). Geological bodies are approximated by horizontal
polygonal prisms.
The program has the following main advantages (besides abovementioned ones): (1)
Simultaneous computing of gravity and magnetic fields; (2) Description of the terrain relief
by irregularly placed characteristic points; (3) Computation of the effect of the earth-air
boundary by the method of selection directly in the process of interpretation; (4) Modeling of
the selected profiles flowing over rugged relief or at various arbitrary levels (using
characteristic points); (5) Simultaneous modeling of several profiles; (6) Description of a
large number of geological bodies and fragments. The basic algorithm realized in the
GSFC program is the solution of the direct 3-D problem of gravity and magnetic
prospecting for horizontal polygonal prism limited in the strike direction. In the developed
algorithm integration over a volume is realized on the surface limiting the anomalous
body.
It is necessary to note that when we apply a series of interpreting profiles, we can compile
several detailed maps of thicknesses of sedimentary or intrusive associations for the area
under study. Such an experience was obtained for Carmel and Maanit areas (Eppelbaum and
Katz, 2012a).
Taking into account that seismic site effects must have an obvious correlation with
tectonic pattern (in regional, middle and detailed scales), satellite (gravity), airborne
(magnetic measurements at 1 and 5 km levels) and land (both gravity and magnetic) data
were processed by the use of different methodologies.
For instance, it was shown that magnetic gradient computations from airborne magnetic
observations (1 km level) enable to classify the region under study to areas with thick
sedimentary cover and areas with shallow intrusive rock location. Self-adjusting and adaptive
filtering of gravity satellite obtained and magnetic airborne (1 and 5 km) data enabled to
reveal the areas with quasi-homogeneous characteristics.
Satellite derived gravity data were processed by the use of numerous algorithms: entropy,
adaptive filtering, wavelet, and information approach (Eppelbaum and Katz, 2015a, 2015b,
Eppelbaum et al., 2014), and strike angle and virtual deformations (KlokoÄník et al., 2014).
Application of these methods was effective not only for tectono-geological setting
sharpening, but also for calculation of such parameters as ‘dominant location of subsurface
masses’, areas of ‘compression’ and ‘dilatation’.
Land gravity and magnetic data were processed by the use of abovementioned algorithms
including procedures of downward continuation and computation of third derivatives of
gravitational potential.
For this investigation was utilized the recently constructed map of the Neogene-Quaternary
structural stage (indicating thicknesses of these deposits) of Israel and the eastern
Mediterranean (Eppelbaum and Katz, 2014b). Results of other map compilation (Palaeogene,
Late and Early Cretaceous, Jurassic and Triassic structural stages, hypsometric
map of the base of the newest (Post-Jurassic) tectonic complex as well as map of
Lower Mesozoic wells and outcrop locations) (Eppelbaum and Katz, 2011, 2012a,
2012b, 2014a, 2014b, 2015a, 2015b) were taken into consideration for 3D PGMs
construction.
Acknowledgements
This research was carried out under grant No. 214-17-016 from the Ministry of
Infrastructure, Energy and Water Resources of Israel.
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