385 research outputs found
Inversion strategies for seismic surface waves and time-domain electromagnetic data with application to geotechnical characterization examples.
Geophysical methods are broadly used to map the subsurface. Their ability to investigate large areas in a short time and to reach significant depths with good resolution makes them suitable for a wide range of applications: from hydrological studies, mineral exploration, archaeological investigations to geotechnical characterization.
Unfortunately, most of the geophysical methods are ill-posed. Thus, to be able to effectively invert the geophysical data and get meaningful models of the subsurface a priori information needs to be included in the process. This is the basic idea behind the inversion theory. This thesis deals with the inversion of two types of geophysical measurements:
the Seismic Surface Waves (SSW) data and the Time Domain Electromagnetic (TDEM) observations.
The present work consists of two parts: (1) The first one is about possible implementations of the minimum gradient support stabilizer into a SSW inversion routine and its extension to the laterally constrained case. By means of this novel approach, it is possible to tune the level of sparsity of the reconstructed velocity model, providing a solution with the desirable characteristics (smooth or sharp) in both directions (vertically and laterally).
The capabilities of the proposed approach have been tested via applications on synthetic and measured data. (2) The second part of the thesis is about the joint interpretation of SSW and TDEM measurements for an improved geotechnical characterization of an area intended for construction. In this case, the SSW results, together with other ancillary data, are used as prior information for the subsequent inversion of TDEM measurements.
In this respect, the SSW results have been translated into pieces of information to be used in the TDEM inversion via a petrophysical relationship.
This work is coherent with one of the goals of the United Nations Agenda 2030 for sustainable development, specifically, the item 11b, as geotechnical characterization is one of the essential components for the design of civil engineering works, ensuring the necessary safety and resilience to natural disasters and climate change. However, the field of application of the proposed approaches is very broad as they can also be used, e.g., for groundwater mapping, as well as for the evaluation of aquifer contamination. In this respect, the present work is also in line with items 6.1, 6.3 and 6.4 of the 2030 UN Agenda
Integrated geophysical methods applied to some archaeological sites of North Sardinia
This study deals with the employment of a multimethod geophysical survey, electrical (Electrical Resistivity Tomography) and electromagnetical (Ground Penetrating Radar) carried out on two archaeological sites near shore archaeological sites.
For this purpose the settlements of Sant'Imbenia (Alghero, SS) and Santa Filitica (Sorso, SS), two Roman villas located in northern Sardinia were considered.
The focus of the paper is to assess as the geophysical response of different instruments might be impacted by different moisture conditions and archaeological contexts.
The electromagnetic surveys were conducted in reflection mode, using a monostatic GPR, working simultaneously by two transmitters (frequency of 200 and 600 MHz) and two receivers, while the electrical surveying was conducted with a Terrameter SAS 1000 device combined with ES 10-64 electrode selector.
The 2D and 3D high-resolution models revealed a distribution of shallow anomalies that indicate the presence of buried structures (walls and filled cavities) in the surveyed areas.
A comparison of the results was made, checking the relative merits and demerits of the different techniques in the two coastal archaeological sites.
The integration of geophysical methods has provided useful information to design a more efficient plan for further archaeological excavations
Lithospheric electrical conductivity structure across Southern Scotland and Northern England
Magnetotelluric soundings in the range (100-0.01Hz. ) have been carried out along a
140 km. profile across S.Scotland and N.England. Following discussion of the steps
taken to reduce cultural noise in the response functions, the results for 13 stations
along the traverse are presented. Since the study of dimensionality parameters
supports 1-D modelling, rotationally invariant resistivity-depth data have been
subjected to two 1-D inversion schemes and the different parameters of the model
derived have been examined using the Most squares approach. Two dimensional
numerical modelling has also been carried out considering both the present data and
those of earlier induction studies along the profile.The resulting geoelectrical model confirms the existence of a crustal conducting layer
(10-50 Ohm.m.) as found in earlier induction studies. Results of this study show
additionally that- a) the conductor rises to a depth of nearly 3.5 km. about 20km.
south of the SUF i.e., near the axis of the Eskdalemuir magnetic variation anomaly b)
the conductor is at a depth of about 8-10 km. in region of the Weardale granite in
N. England, c) there is a marked resistivity-depth variation below S.Scotland and d) the
surface resistivity along the profile decreases from NW to SE and corresponds well to
the Ordovician, the Silurian and the Carboniferous sediments. The geophysical
interpretation of the model has been discussed in association with the results of
earlier induction studies, seismic and gravity studies and the various tectonic models
of the Iapetus suture zone. Based on the present study and comparison with electric
structures of other tectonic regions, a hypothetical tectonic model has been proposed
for this study region. It comprises a resistive triangular wedge surrounded by a
conducting layer which is incorporated as a signature for the presence of a subducting
plate in an island arc environment.In addition to these field investigations, several computational analyses have also been
undertaken. The main studies were - a) procedures to reduce noise from the
recorded data using digital filtering techniques and b) determination of the
distortion of telluric field measurements due to a conducting hill using a conformal
mapping method. Digital filters have been discussed with special reference to
persistent noise signals, eg. power lines, electrical fences etc. Delay line filtering,
notch filtering and the maximum entropy method have been applied to both synthetic
and field data and the results are discussed. The effect of a two-dimensional
conducting hill on the telluric fields measured on a horizontal surface has been
studied using the Schwartz-Christoffel conformal transformation technique. The
results indicated that the distortion depends strongly on the inclination, height of the
hill and depth to the basement. The distortion of apparent resistivity values computed
from MT field measurements has also been discussed.In addition, some consideration has been given to - a) the processing of the data in
the time domain using adaptive filters, b) the application of joint inversion of DC-AMT
data to resolve shallow structures in N.England and c) the distortion effects of near
surface inhomogeneities in N.England.These various studies are presented in 7 chapters with 3 Appendices and the results
are discussed in chapter-8, where the conclusions and suggestions for further work
are also presented
- …