While seismologists have been using surface waves to image the Earth's crust since the ‘60s and geotechnical engineers developed surface-wave analysis methods to characterize the soil in the ‘80s, we have had to wait until the last decade to see surface-wave analysis becoming a standard method in seismic exploration. Nowadays, surface waves, which are traditionally considered noise in seismic gathers, have shown the potential of estimating the shear properties of the shallow subsurface. They are, hence, routinely used in many near-surface applications that range from seismic hazard studies to the building of weathering-layer velocity models for seismic reflection corrections.
Different kinds of surface waves can be gathered, according to the environment and the subsurface conditions: Rayleigh waves, from land surveys, Scholte waves, in marine environments, Love waves, when horizontally polarized sources and receivers are used, Lamb waves or P-guided waves, when very high-impedance contrasts are present. Regardless of the kind of waves, the most established analysis method is based on the inversion of the geometrical dispersion of surface waves. This approach, which exploits the relationship between the vertical distribution of the seismic properties in the subsurface and the phase velocity of different surface wave harmonics, only considers the kinematics of the propagation and is based on several simplified assumptions. I will examine the potentials and limitations of this approach and different technical solutions for acquisition, processing, and inversion in the context of different applications.
The most important assumptions that are usually made in surface-wave analysis are that the site is 1D and that the experimentally retrieved dispersion curve coincides with the surface-wave fundamental mode. Both assumptions are often violated and specific measures have to be taken to handle multimodal propagation in laterally varying environments. Several technical solutions have been proposed and I will discuss them to define the best practice for surface-wave analysis in complex geologic conditions. Recently, tomographic techniques, which are usually applied in seismology, have been adapted to active seismic data. Their potentials and requirements will be presented.
Please tell us a little bit about yourself (e.g., education, work experience, and why you became a geophysicist, etc.).
My Master's degree was in civil engineering at Politecnico di Torino (Turin University of Technology, Italy). I moved to geophysics for my PhD (Politecnico as well) and I worked on electro-osmosis. I was a postdoc for two years focusing on geophysical methods for civil engineering and after that I was appointed Assistant Professor in the same Department where I recently became Associate Professor. When I graduated in civil engineering I knew that the design of constructions was not my dream and I wanted to move to something more related to applied science, so, when a PhD was offered to me in geophysics I thought that was a very good occasion. The funny thing is that during my Bachelor's and Master's studies I tried to avoid all the subjects related to waves and vibrations and then they became my everyday life… never say never. [sic]
Would you like to mention anything about your personal attributes that helped you achieve the professional status you enjoy today? Was it self-belief, hard work, a mentor, or something else?
I think it has been a mixture of hard work, dedication, curiosity and a bit of luck. When I started to work on surface waves, in 1998, it happened by chance. I met some colleagues from the geotechnical engineering research team at the university canteen and they were interested in surface waves as a geotechnical characterization tool, but had no field data, so we offered them to use our seismic equipment and we started a zero budget joint project with my colleague Sebastiano Foti (now associate professor of Geotechnical Engineering). At that time I did not know the novelty and the potentials of the method. That was the starting point, and soon we discovered there was a lot of research to perform in that field. The successful outcomes that came later were also possible thanks to some smart fellows and students who were attracted by a young research team with lots of enthusiasm.
Why did you choose this lecture topic? Why is it important?
Surface waves have been used for decades in seismology, but only in the last decade they have become an established technique in near surface geophysical exploration. I started working on surface wave methods in applied geophysics when almost nobody was. So I have followed the tremendous development these techniques have undergone in the last 15 years. They have become standard methods, but are also somehow new with respect to other seismic techniques; hence they are not often taught in university courses and some education and divulgation initiatives on this subject are really important. Surface waves methods are a powerful tool to estimate S-wave velocity, which is a key parameter for many engineering problems ranging from seismic site response to geotechnical characterization. Recently several studies are pointing out also the possibility of retrieving reliable P-wave velocity models and this opens new possibilities, particularly related to near surface velocity models required in deep exploration processing. The idea of using the signal that is usually considered noise in the data to provide additional information is very attractive and represents an opportunity not only in engineering and environmental applications but also in oil and gas exploration.
Could you tell us in a few sentences what your course objectives are?
The first objective is displaying the complexity of surface wave propagation and its consequences on the methodological aspects. The second objective is to show, also through several field cases, which are the limitations and the potentials of this exploration method at the actual state of the art. The third objective is to give an idea of how recent research is working to overcome these limitations.
Are there any more specific areas that you want to emphasize?
I think that the challenges for research in surface wave analysis are at present the effective inclusion of higher and leaking modes of propagation of surface waves in the inversion process, and the capability to reliably recover 2D and 3D structures in spite of the 1D forward operator that is used in the inversion.
What do you hope people will have learned after they attend your lecture? How is it different from other lectures?
Honorary lectures can be devoted to students or to professionals. In both cases I hope to show the possibilities offered by surface wave analysis for addressing several near surface problems. I don't think that a lecture can provide exhaustive knowledge, but I hope to stimulate curiosity toward this geophysical technique. Surface wave analysis represents a well-established technique already applied to countless successful cases; still there is a lot of work to be done because its potentials are not fully exploited. I hope to be able to convey this message to the audience.
You have quite a busy year ahead. Do you enjoy traveling? Will it be difficult to balance the tour with your work?
Fortunately my teaching at Politecnico falls in the spring semester, otherwise it would have been impossible for me to make this tour. I like travelling a lot and this tour represents a fantastic occasion to meet colleagues and fellows all over the world and exchange knowledge and ideas. I am pretty sure I will learn more than I will teach. Of course it will be complicated to manage the travelling and the everyday work, but my university strongly supports me. The other difficult issue is handling family organization, but I am lucky and my family deeply sustains me and I hope they will be able to travel with me from time to time.
Would you share with us one or two of your most exciting successes?
Every time we are able to switch on a light in the subsurface I feel a strong enthusiasm and sense of discovery. Some datasets have been particularly challenging, others particularly easy and gratifying and I have been so lucky to work on data acquired all over the world for completely different purposes, ranging from fault detection to marine sediment characterization or to dune exploration. I think that my major achievement has been contributing to make surface waves a standard exploration tool for near surface problems in engineering and in oil and gas exploration.
How about a couple of disappointments?
The worst memory of my professional life goes back to the very beginning of my career, when I led a project aimed at identifying buried foundations in an abandoned area where new constructions were planned. We applied several near surface geophysical methods and we were pushed by the client to work in great hurry. Our interpretation was not deep enough and we missed a huge concrete foundation whose anomaly was masked by other shallow targets. I will never forget the telephone call of the client when they found it during the construction works. I came back on the data and the information was there: hidden but present and available. I lost the client but the whole team received a very important lesson: never get bored to look into your data because most of the times the information you search is there. I am pretty sure that also those young fellows who collaborated with me at that time and who are now well esteemed professionals in industry still remember that lesson. The other important lesson learned was that you should not allow the client to require the results in a time that does not allow for a thorough analysis of the data. Now I am very careful when I plan my work.
What advice would you give to geophysics students and professionals just starting out in the industry?
Never be frightened by something new; try to do what you like, but never decide whether you like something or not without knowing it; trust your fresh ideas and be ready to express them, but never be arrogant.