Geophysics concerns all that sector regarding the study of the seismic waves transmitted through the ground. Through the crossing of the wavelenghts has been possibile to study and analyze the internal of the Planet Earth, generally, deducting structure, texture, matter’s consistence and composition type, despite still much uncertainties could always arise.
Geophysics, particularly, in the last half century, has been more used for Civil Engineering purposes. Through the analysis of the crossing waves, inside the ground, It is possible to get, indeed, some important geophysical parameters that can be used to obtain, indirectly, other really important geotechnical values, sometimes pillars for the right geomechanical and seismic ground classification, as well as for the correct engineering seismic design and, then, for the sizing of the opera, which, for example, foundations, diaphragm walls, beams, structure weights to settle, building materials to choice and eventual heights to develop, in terms of design measures and composition of matters.
The use of Geophysics interests naturally also the seismic dimension, for earthquakes and their implications. The forecasted magnitude, in a particular area, is known as Base Seismic Hazard and, then, It is the first step of the knowledge to deduct by a geophysical study. Geophysics impacts the following Local Seismic Response dimension, really important to evaluate the dynamic response of a particular area, during an earthquake, or in an area subjected to continuous vibrations, with the corresponding possible amplification of the general local ground system.
Some of worldwide standards, included the fundamental Eurocode and also the Italian Standards for Construction, tell us about the correct behaviour to judge the local seismic conditions of the ground, in terms of response properties and of the significative volume interested, from the shallow site crust, i.e. the same volume, more or less, that the Civil Engineering usually has to consider to develop the infrastructure in the territory, a volume called, technically, “significative volume”.
The standard conditions regard a ground formation, in well mechanical conditions and with a simply maximum gravity acceleration calculated, waited on the ground, according the magnitude forecasted; they, also, tell us about the relative response of the ground in that well conditions considered.
After that the geophysical local studies contribute to get more resolution about site conditions in term of the geological, geostructural and geotechnical complex conditions, for any particular investigated site. The obtained models, finally, get an idea of the real local response, according geology, stratigraphy, geostructural geometries, geomorphological curves, geotechnical and mechanical properties, as well as following the detected geological hazards, reaching, at the end, the important Design Seismic Response.
The graphs then extrapolated are really representative for the hypothetical scenario in the ground response, for the planned civil structure during possible earthquake’s events, i.e. during dynamic conditions and according the design of the structures self. The graphs now, finally, are considering the amplified acceleration and the length waves reached in base to the height of the opera. For any different country, there are different identification letters to identify the conditions of ground category, that respects, each one, a hierarchy of quality, splitted, still, in different geophysical ground models.
Remembering, to complete the pillars of the seismic scenario culture, which are the fundamental types of seismic waves:
• Deep or volume waves, consisting in the P and S waves;
• Shallow waves, consisting in the Love and Rayleigh’s waves.
Let us try to get a general and quick view of the various different waves and their typical use.
Deep P and S waves – They are really important for different scopes, which, for example, regard the study of the deepest internal structure of the Planet. These waves are then good to be considered for scientific research purposes, as well as for the searching of hydrocarbons in the oil and gas production sector.
Particularly P-waves means “Primary” and S-waves means “Secondary”. Both waves are called also volume waves because they are transmitted inside the whole space of the ground, i.e. through the volume of the masses. It is easy, then, to understand that in the seismogram the P-waves arrive before than S-waves. The S-waves have more energy in terms of destruction capacity due their travelling mode way. Still P-waves have an oscillation parallel to the path undertaken from the waves itself, respect S-waves, that, these last, slip perpendicular respect to the direction they have been launched. For this reason these last secondary waves can own, easily, the capacity to cut foundations and plans of manufactured opera. The S-waves are always more researched to understand the complex stratigraphic situation of the ground, despite P-waves are really useful to complete the general view. It could also add that S-waves cannot travel through liquid matter respect the P-waves; so they are useful to understand better the consistency of the deep ground in particular conditions.
Primary waves Secondary waves
Shallow Love and Rayleigh waves – They are a simple consequence from the deep waves, that, for different optical reasons, are coming back from the deep levels, running just only in the shallow part of the ground. To be slightly more specific, Love’s waves fly directly shallow from their seismic source; instead Rayleigh waves derive from the interferences of the deep crossing waves, that are coming back upwards. Regarding their mechanical behaviour, the Love waves are similar to the S-waves, due they run with movements perpendicular to the principal propagation direction, but they are settled, in this case, only just by a shallow plane. The Rayleigh waves are really particular for their retrograde elliptical motion alongside and respectively to their principal propagation direction. For this reason, these last waves can be considered, at the same time, for their horizontal and vertical components, according the study to undertake and the used and scheduled geophysical investigation method.
The study of these both waves is really important for engineering purposes. Thankfully to them we are capable to have a general and good idea, indirectly, of the S-waves trend and, then, of the fragility of the ground when slightly or strongly stressed out from a cyclic vibrationally event, as the earthquake can be directly or indirectly.
Love waves Rayleigh waves
The use of Geophysics consists firstly in the right choice of the type of analysis. Geologists, Geotechnicians and Geophysicists can calibrate the correct use of the geoinvestigations, according the info collected and what they are searching in the area of study, in terms of characterization and following behaviour.
The geophysical analysis can be invasive or non invasive, according the capacity to let disturbed or undisturbed the ground investigated, with boreholes and / or excavations process.
Still, the process can use active and passive methods, according the generation of artificial seismic waves or the use of natural and / or artificial tremors present already in the ground.
Let us, also, list, briefly, the most important types of the geophysical investigations used for the various general purposes.
Non invasive and active geophysical investigations
• MASW – The Multichannel Analysis Surface Waves is largely used for civil engineering purposes. As per name, the analysis uses surface waves, Love and Rayleigh waves, to deduct direct and indirect measures of the geophysical and, then, geotechnical parameters of the ground;
• Refraction – The method uses the deep waves, P-waves and S-waves, to understand the geophysical behavior of the ground. Particularly important are the S-waves deductions for civil engineering goals, despite P-waves complete the general view of the ground state art;
• Reflection – The method uses the deep waves, P-waves and S-waves. It is mostly used for hydrocarbons research in the oil & gas sector, thank to the high capacity of the method to investigate deeper crust areas, that, commonly, are not much important for the civil engineering fields.
Non invasive and passive geophysical investigations
• MASW – The Multichannel Analysis Surface Waves is largely used for civil engineering purposes. As per name, the analysis uses surface waves, Love and Rayleigh waves, to deduct direct and indirect measures of the geophysical and, then, geotechnical parameters of the ground;
• HVSR – This analysis consists in the use of the Horizontal and Vertical waves. The relation between the waves indicates the Spectra Ratio deducted from the environmental microtremors, with differences in deviations and values. This method is called also Nakamura HVSR, from his inventor, and It is used for civil engineering purposes. It permits, indeed, to know the esteem of the seismic resonance of the site, as well as the wave velocity of the deep waves, deducted, indirectly, from the ratio between horizontal and vertical components of both deep and shallow waves self, i.e. SH-waves, P-waves, Love waves and Rayleigh waves’ horizontal component between SV-waves and Rayleigh waves’ vertical component. The method, practically, includes all components, from all waves, that derive from the natural and / or artificial environmental microtremors;
• Re.Mi. – The Refraction Microtremor analysis is much similar to the passive MASW experience, being both belonging to the SASW, Spectral Analysis Surface Waves sphere. The technic uses also the natural or artificial microtremor present in the environment, by the ground, to determinate, always indirectly, the elastic characteristics and the important velocity of the deep waves, especially the properties of the S cut waves, really useful for civil engineering purposes.
Invasive and active geophysical investigations
• Downhole – The DH analysis is very complete to determine geophysical and geotechnical properties. This analysis is invasive and must be made a perforation in the ground, a borehole developed until the necessary deep to investigate. The user can develop different type of investigations in the method, as well as It is possibile to determine RQD value of the rock, i.e. the Rock Quality Designation, or to sample some key parts in the borehole to analyze, subsequently, in the laboratory environment. Naturally all types of interventions should be evaluate according the rock or soil type of the ground. All these tests can be finally calibrated with geophysical investigations, that are composed alongside the deep vertical axis of the borehole self. The technician can measure with 3d geophone the arriving of the waves by steps pre-established of deep. The waves are generated and induced by artificial burst. This type of analysis is much expansive, but, surely, It is one of the more complete work that permits to compare geotechnical and geophysical properties to get a more truly model of the ground, according static and dynamic conditions. It can added, still, that with this method you can sample directly the real important S-waves, extremely decisive to get elastic and dynamic parameters of the ground for civil engineering scopes.
• Crosshole – The CH analysis consists in two boreholes, made and excavated in parallel, which can be both used for geotechnical and geophysical purposes. The great difference consists simply that the burst and the receivers are settled, step by step with the deep path, alongside at the same level of the boreholes self and the technician can measure, then, the waves through the ground between the holes. This analysis is really complete, but It requires much space and expensive costs to afford. It can be conducted, also, between various boreholes crossed each others by a large area.
From all these types of geophysical index It is possible, then, evaluate and assess the geotechnical parameters, arriving by a good resolution for the static, elastic and dynamic parameters, fundamental for the design activity of all the geopera forecasted.
The use of geophysics is really important, today, for civil engineering purposes, to guarantee safety, quality and right costs for the construction process and for the life of the opera self.
It needs to investigate and to invest strengths to assure success and to reach the minimum mistake events.
Ask to the Planet Earth; It will answer without hesitation.
Giovanni Paolo Amenta
Senior Engineering Geologist
M.Sc. Pro License EurGeol CGeol Prof.
EFG CNG ORGS
www.eurogeologists.eu
www.cngeologi.it – www.geologidisicilia.it
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