EXPLORATION GEOPHYSICS
Exploration Geophysics is a discipline that combines principles of physics with knowledge  of geology, engineering and digital signal processing to develop  non-invasive investigation methods and techniques aimed at studying the Earth's subsurface (from few centimeters to few kilometers) as well as human artefacts.
Originally, the survey methodologies were mostly "passive", based on the detection of anomalies of natural terrestrial fields such as gravity and magnetic fields. Thus, disciplines like Gravimetry and Magnetometry were primarily designed to study the medium- and large-scale terrestrial structures and to identify the "basement" on which layers of more or less thick sedimentary rocks rest. Active methodologies were later developed, to study the propagation in the sub-surface of elastic waves or electric currents generated by artificial sources. Today, the "passive" and "active" methodologies coexist and complement each other depending on the subject of investigation.

SOURCES OF ENERGY
The broad expansion and development of Exploration Geophysics is largely due to its employment in the search for energy sources and in particular for hydrocarbons and geothermal energy. Until the early 1950s, gravimetric, magnetometric and refraction seismic methods were used by the oil companies to contribute to the location of exploration wells.
Then, starting from about 1955, also thanks to two major technological innovations (acquisition of seismic signals in "multiple coverage", and advent of digital recording), which allowed to take a step forward in the quality of the results, the reflection seismic tool has gradually become the most widely used geophysical exploration method, since seismic reflection is able to provide a two-dimensional (distance-depth, Fig. 1), or even three-dimensional image of the subsoil (Fig. 2). This clear advantage has led the major oil industries and international geophysical service companies, over the past 40 years, to invest large amounts of money and resources for the development of this technology both in terms of scientific (also basic) research, and of large-scale application.

Figure 1. Reflection seismic section for oil research. The abscissa indicates a horizontal coordinate. Depths (0 to 5 km) in the ordinates. The color scale indicates the amplitude of the signals reflected by the sub-surface discontinuities. Geometry of the structures and presence of fractures (faults) is well evident. After the seismic survey interpretation, a well was drilled in the indicated position.

Figure 2. Example of 3D reflection seismics

Currently, the reflection seismics and other geophysical survey methods, including those applied directly in the exploration and/or production wells, are an extremely important tool for research and for the production of energy sources. Localisation of stratigraphic and/or structural traps favorable to the accumulation of hydrocarbons (Fig. 3), identification of geothermal deposits, petrophysical characterization of reservoirs, optimization of the production of reservoir resources are only some examples of activities in which geophysical exploration methods (both from surface and well), off-shore and on land, play a fundamental role.

Figure 3. Reflection seismic section for oil research. The abscissa indicates a horizontal coordinate. In the ordinate, travel -imes of the signal reflected by the rocks in depth. The color scale indicates the amplitude of the reflected signals. The two ellipses show two deposits of methane gas (present in Pliocene turbiditic sequences in onlap over the top of the Miocene unconformity), which were then drilled from wells B and C.

However, the significant economic relevance of these geophysical methodologies, and the frequent confidential character of the methods and techniques, combined with the need to dispose of large computing resources in terms of computers and equipment, confined this knowledge and methodology for a long time within the industry. Only later, thanks to the reduced cost of computers, software and equipment and to a somewhat osmosis between the industry and the outside world, there has been a merge of these methodologies, and in particular of industrial reflection seismics, in universities, research centers, professional offices and public authorities, with consequent enlargement of its applications to new sectors, such as environment, civil engineering and cultural heritage. This also explains why young graduates with a good specialized preparation find satisfactory jobs both in Italy and in Europe.

Finally note that the methodologies taught in this Master Degree Course, besides being used in the search for natural energy sources, find application in important environmental problems such as CO2 injection and monitoring (due to for example the hydrogen production or the CO2 emission) in depleted reservoir.

GEOPHYSICAL INVESTIGATIONS FOR ENVIRONMENTAL PURPOSES

For some decades, Applied Geophysics has provided various methodologies, such as gravimetry, magnetometry, electrical, refraction seismics, etc., useful to investigate the immediate depths of the subsoil for civil, environmental and engineering purposes. In recent years, however, more sophisticated techniques such as reflection seismics and Ground Penetrating Radar (GPR) have found increasingly widespread use in this sector, where surveys carried out to identify shallow or very shallow targets require an extremely high resolution (in the order of a few meters for seismics and tens of centimeters for GPR) (Fig. 4).

Figure 4. A Ground Penetrating Radar (GPR) profile (a), where different colors and color intensities corresponds to GPR-wave reflections in the subsurface. The horizontal axis shows distances along the line of the acquisition survey, the vertical  axis shows depths below the surface. In this case the GPR reflections were interpreted as stratigraphic beds showing angular discordances, consistent with a coastal-dunal setting. Intersection of GPR profiles (b) showing several reflections in the subsurface of a paved ambient. A map of GPR-reflection amplitudes (c), cut parallel to the surface at a desired depth (time-slice). The linear geometry of high-amplitude GPR-reflections was interpreted as buried walls and pipes. An example of time-slices draped on an aerial image (d). A 3D volume of GPR data intersected by a radar profile (e). The geometry and spatial continuity of high-amplitude GPR reflections (red zones) was interpreted as a buried wall.

The aims of surveys conducted using reflection seismics are to identify the geometries and elastic properties of the subsoil (speed, acoustic impedance), from which we can trace, thanks to an interaction with the geological knowledge of the area, the types of rock that compose the sub-surface and their characteristics of resistance, compactness, porosity, etc. These surveys are useful to evaluate the rock strength in critical locations before the construction of potentially dangerous plants or structures, to identify the depth of the rocky substratum for hydrogeological purposes or to detect the presence of any slip surfaces in areas characterized by landslides, etc.

Figure 5. Detail of the acquisition of the high-resolution land-sea seismic line in Ancona. In the foreground, a remote control unit and the final part of the on-land seismic are visible. In the background, the boat used for marine acquisition.

Some results of a project with the Municipality of Ancona are shown, aimed to determine the depth of the slip surface of the Great Landslide involving part of the city and to monitor its movements. Monitoring is conducted by radar interferometry, while reflection seismics (Fig.5) has been able to produce an image in times (Fig. 6) and an image in-depths (Fig. 7) of the probable slip  surface and of the syncline structure on which the landslide insists.

Figure 6. Final stack section in time to the final datum at 250 m a.s.l. From left to right: the land-land data, CDP 1421-954; the land-marine data, CDP 953-896; the marine-marine data, CDP 895-802. On the land-land data, continuous deep reflections overlaid by disrupted ones are evident. The correct interpretation of the line allows to identify potential slip surfaces, the area of their outcrop and the internal structure of the landslide in general..

Figure 7. Poststack depth migration of the stack section in Figure 6 overlaid by the velocity field derived from tomography. On the right, a portion of a pre-existing marine seismic line is included and a fair match with the new line is observed. On the depth section, the syncline structure, with its depocenter on land is clearly visible.

The use of small-scale reflection seismics such as that for environmental applications presents further problems compared to the well-established methodology adopted in the industry for the search of energy sources, where it originally developed. Indeed, the rapid variation of the geometries and of the speeds of the most superficial layers greatly increases the problem of their reconstruction through the "ultrasounds" (Figs. 2 and 3) from both a practical and a theoretical point of view. In this respect, the use of reflection seismics has been made possible only recently, thanks to technological development and to the reduced costs for equipment and processing. It is now possible to perform acquisitions and processing of seismic data with greater redundancy and at less prohibitive costs for 2D lines, thus opening up new professional opportunities in the field of surveys for civil and environmental purposes, where the resources available are generally more limited than those of the energy industry.