Koichi Hayashi is presently a Senior Technical Manager at OYO Corporation and Geometrics, Inc. in San Jose, California. Over the past 26 years, he has worked as a research geophysicist focusing on providing better tools and algorithms for near-surface geophysical methods. He earned a BS degree in Earth Sciences from Chiba University in Japan, an MS degree in Earth Sciences from the Massachusetts Institute of Technology, and a PhD in Earth Resources Engineering from Kyoto University in Japan. His main research areas are seismic refraction, active and passive surface waves, finite-difference seismic modeling, and traveltime inversion. He is the author of the SeisImager data analysis suite of programs and has incorporated many of his theoretical developments into the software, making SeisImager one of the premier surface wave, refraction, and downhole data processing packages available today. He regularly presents papers at the major meetings, publishes in the journals of SEG, SEGJ, EEGS, and SSA, and serves on scientific commissions.
2014 SEG Honorary Near-Surface Global Lecturer
Integrated geophysical methods applied to geotechnical and geohazard engineering: From qualitative to quantitative analysis and interpretation
The "Near-Surface" is a region of day-to-day human activity on the Earth. It is exposed to the natural phenomena which sometimes cause disasters.This lecture covers a broad spectrum of the geotechnical and geohazard ways of mitigating disaster and conserving the natural environment using geophysical methods and emphasizes the contribution of geophysics to such issues.
The lecturen focuses on the usefulness of geophysical surveys in providing information to mitigate disasters, rather than the theoretical details of a particular technique. Several techniques are introduced at the level of concept and application. Topics include various geohazard and geoenvironmental applications, such as for earthquake disaster mitigation, preventing floods triggered by tremendous rain, for environmental conservation and studying the effect of global warming. Many geophysical techniques discussed with the applications of active and passive surface-waves, refraction, and resistivity methods highlighted. Several related issues, such as performance-based design, standardization or regularization, internet access, and databases are also discussed.
The lecture discusses the application of geophysical methods to engineering investigations from a "nonuniqueness" point of view and introduces the concepts of "integrated" and "quantitative." Most geophysical analyses are essentially nonunique and it is very difficult to obtain unique and reliable engineering solutions from only one geophysical method. The only practical way to improve the reliability of investigation is the joint use of several geophysical and geotechnical investigation methods, an "integrated" approach to geophysics. The result of a geophysical method is generally vague—here is a high-velocity layer, it may be bed rock; this low-resistivity section may contain clayey soils. Such vague, qualitative and subjective interpretation is not worthwhile in general engineering design work. Engineers need more quantitative information, such as bedrock depth is 10.5 m and permeability of this sand layer is 1.5 x 10-3 cm/s. In order to apply geophysical methods to engineering design work, "quantitative" interpretation is very important. The lecture introduces several case studies from different countries around the world from the "integrated" and "quantitative" points of view.
The "Near-Surface" is a region of day-to-day human activity. It provides benefits from nature that are indispensable in our lives. It is also exposed to the natural phenomena that sometimes cause disasters. Hurricane Katrina (2005) revealed the bright and dark side of nature in the near-surface. The Mississippi River and adjacent canals played an important role in the development of the city of New Orleans, but levee failures on the canals and consequential flooding due to the hurricane caused a great loss of life. Is near-surface geophysics able to contribute to protecting cities surrounded by rivers or canals against floods? The answer is yes. Several levee failures in New Orleans were caused by soft subsoil layers beneath the levees or flood walls. Using geophysical methods, we have the ability to detect such soft subsoil layers. This lecture covers a broad spectrum of the ways we can mitigate disaster and conserve the natural environment using geophysical methods, and emphasizes the contribution of geophysics to geotechnical and geohazard issues.
The usefulness of geophysical methods in providing information to mitigate disasters, rather than the theoretical details of a particular technique, will be discussed. As well, this lecture will cover the use of geophysical methods together with techniques from other science and engineering areas, such as soil mechanics, architectural engineering, earthquake engineering, etc., applied to a range of topics. The topics include various geotechnical, geohazard, and geoenvironmental applications, necessary for earthquake disaster mitigation, preventing floods triggered by tremendous rain, environmental conservation, and studying the effects of global warming. Since for most near-surface problems, the results from geophysics cannot solely provide the answers, integration of geophysical results with those from other science and engineering areas is essential, and can further provide a more robust answer with independent corroboration of results. For example, seismic methods would be able to detect the soft subsoil layers in New Orleans as slow S-wave velocity layers but geophysical methods do not answer the question of what levels of water pressure the levees and their foundations can withstand.
There are several inherent difficulties in applying geophysical methods to near-surface engineering and environmental site investigations. These difficulties are often symbolically expressed as non-unique, low resolution, subjective, and qualitative. For example, underground structures delineated using geophysical methods performed on the ground surface are generally vague; if there is a high-velocity layer, it may be bedrock, and a low resistivity section might contain clayey soils. Such ambiguous, qualitative, and subjective interpretation is not worthwhile for general engineering design works. Engineers need more quantitative information, such as (bedrock depth is 10.5m and permeability of this sand layer is 1.5*10-3 cm/sec). In order to apply geophysical methods to engineering design works, unique, high-resolution, objective, and quantitative interpretation is very important. This lecture discusses these difficulties in the application of geophysical methods to engineering and environmental site investigations from a "Non-uniqueness" or "Uncertainty" point of view, and introduces the concepts of "Integrated" and "Quantitative."
Most geophysical analyses are essentially non-unique, and it is very difficult to obtain unique and reliable solutions without uncertainty from an individual geophysical method. For example, it is well known that a seismic refraction analysis based on first arrival traveltimes is essentially non-unique (Figure 1). In most cases, it is difficult to obtain a true velocity model from first arrival traveltimes observed only on the ground surface. Even with a one-dimensional, two-layer model, it is impossible to obtain true thickness and velocity without a priori knowledge that the model is two layers. This non-uniqueness is a problem not only for the seismic refraction method but also for most geophysical methods in which underground structures are estimated from geophysical data observed from the ground surface (Figure 2). All geophysical practitioners and algorithm developers have to admit this inherent limitation and try to develop alternative approaches.
There are many mathematical approaches to reduce the non-uniqueness. Using a priori information is one of the most promising approaches. It is well-known that the result of a non-linear least-squares inversion depends highly on the initial model. Constraints during an inversion are also very important. For example, a constraint that velocity is increasing with depth is generally very effective during the inversion of the surface seismic refraction method. The real question is how to obtain a priori information and constraints on analyses. The only practical way to obtain such information is the joint use of several geophysical and geotechnical investigation methods, an "Integrated" approach to geophysics.
From an engineering point of view, uncertainty is more complicated. Most engineering investigations in the near-surface region do not need geophysical properties, such as seismic velocities and resistivity, which are directly estimated from the geophysical methods. Engineering design works need geotechnical parameters, such as c, and permeability, etc., which have no direct physical or mathematical relationship to the geophysical parameters. Estimating geotechnical parameters uniquely from geophysical methods is very difficult even if geophysical parameters were uniquely obtained. A large part of qualitative and subjective interpretation is caused by the uncertain relationship between geophysical and geotechnical properties. Integrating geophysical and geotechnical investigations together is essentially important to reduce the uncertainty.
Consequently, there are mainly two problems in Non-uniqueness" or "Uncertainty. One is the essential non-uniqueness in the inversion of geophysical methods, and another is relationship between geophysical and geotechnical parameters. These problems cannot be solved only from an individual geophysical method. An integrated approach is essential to reduce the non-uniqueness or uncertainly and to estimate the quantitative results in engineering investigations.
The lecture introduces several case studies from different countries around the world from the Integrated and Quantitative points of view. Each case history demonstrates how to integrate several geophysical methods together with other geotechnical investigations and to perform quantitative interpretation from the engineering design works point of view (Figure 3). Among the geophysical techniques, the active and passive (microtremor) surface wave, refraction, and resistivity methods are highlighted (Figure 4).
Please tell us a little bit about yourself. (e.g. your education and work experience, why you became a geophysicist, etc.) My interest in geophysics began when I read a book about plate tectonics. I was really impressed that seafloor spreading theory was derived from the magnetic anomaly associated with oceanic ridges discovered by ocean magnetic measurements. I was born and raised in Japan where there is much volcanic and earthquake activity. Dynamic volcanic eruptions tragic earthquake disasters stimulated my curiosity about earth science and geophysics. I decided to study geophysics and went to a geophysics laboratory at Chiba University, Japan. The first code I wrote was traveltime calculation for 1D layered velocity model, and the second code was tau-p transform of waveform data. After I was granted a bachelor's degree, I joined the geophysical division of OYO Corporation, a geotechnical consulting company in Japan. Fortunately or unfortunately, my laboratory in the university belonged to a science department that related to geotechnical business rather than oil and gas business. That is the reason I belonged to the near-surface geophysics world.
In the OYO Corporation, I started my professional career as a field geophysicist. Subsequently, I started to develop a refraction tomography method and applied it to geotechnical site investigations. After several years, I came to realize the limits of my knowledge and went to the Massachusetts Institute of Technology (MIT) and subsequently to Kyoto University, Japan, to study the profound theories behind geophysics. I received a master's degree from MIT and a Ph.D degree from Kyoto University. I focused on the development of a finite-difference seismic simulation code when I was a graduate student at MIT from 1997 to 1999. I tried to understand reflection and refraction waves in a complex medium using the finite-difference simulation. However, the most important knowledge that I gained from the finite-difference simulation was that surface waves dominated near-surface wave propagation. During my stay at MIT, I did homework for a seismology lecture in 1998. It required the calculation of phase velocities from two waveform traces. It was my first calculation of a dispersion curve and it gave me the concept of a phase velocity. These two inspirations, from the finite-difference modeling and the homework, turned me toward surface wave methods in 1998.
After I left MIT and returned to work at OYO Corporation, I started to commercialize my refraction and surface wave analysis software and it was taking on a larger role in my work. Since 2010, I have been in the U.S. working at Geometrics, Inc., to focus on software development and research rather than field geophysics.
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 was a balance of curiosity, flexibility, intuition, and hard work. I believe that curiosity was most important. I have been curious about everything. I have been interested in computers and software although I have been a geophysicist. Developing software takes a significant part in my job right now. Flexibility played an important role in my career development. I studied mainly seismic methods in undergraduate studies and was most interested in the seismic methods. When I worked at OYO Corporation, my charge was mainly resistivity and gravity methods, and I was a little bit disappointed. My flexibility and curiosity, however, stimulated my interest in non-seismic methods. My inspiration to an integrated method owes a great deal to the knowledge and experience of non-seismic methods. Intuition was also important. As I mentioned above, from development of a finite-different method, I realized that surface waves dominated near-surface wave propagation from. The realization of surface wave propagation evoked my intuition that the seismic methods using the surface waves would play an important role in near-surface geophysics.
Why did you choose this lecture topic? Why is it important? Near-surface geophysics is generally used in other science and engineering areas. A single geophysical method is not solely applied to such investigations. Several geophysical and geotechnical methods are used together in actual projects. It is important that geophysicists understand other science and engineering areas in which the geophysical methods are used and how to integrate or combine several geophysical and geotechnical methods. I chose this lecture topic since the integration of geophysical and geotechnical methods are rarely discussed in geophysical or geotechnical societies and educational programs.
Could you tell us in a few sentences what your course objectives are? The primary objective is demonstrating how near-surface geophysics plays an important role in our lives. The secondary objective is to show the importance of integrated knowledge and a broad view. To increase interest in near-surface geophysics is another important objective.
Are there any more specific areas that you want to emphasize? I would like to demonstrate the difficulty and limitations in the inversion of geophysical data. I am developing and providing commercial geophysical processing software that includes various inversions. Many people tend to think the inversion is "automatic" and the inversion software always provides the best answer. The processing software does not always give the best answer to us. Geophysicists have to understand this inherent difficulty and limitation.
What do you hope people will have learned after they attend your lecture? How is it different from other lectures? I have been working as a geophysicist in the business sector and getting involved in a large number of actual businesses with which university professors are rarely concerned. I hope people will have learned to think of geophysical methods from a business point of view.
You have quite a busy year ahead. Do you enjoy traveling? Will it be difficult to balance the tour with your work? Yes, I enjoy traveling, and it will not be difficult to balance the tour with my work. I like to visit different places and meet different people with varying perspectives on the world. I am so excited to see and meet people on the tour.
Would you share with us one or two of your most exciting successes? One of the most exciting successes in my career was the data acquisition of a large-scale seismic refraction survey performed in Japan in 1996. The investigation site was located in a mountainous area, in the so-called "Japanese Alps," and the survey line was too steep to connect geophones with normal spread cables. The survey line was a length of 6km, and we deployed 120 geophones together with handmade data loggers, including a GPS clock with 50m intervals. We deployed geophones in daytime, and data acquisition was performed at midnight by remote operations. The sources were several explosions of 50kg of dynamite each. Since we could not bring large batteries that could last all night to the top of the mountain, small batteries were used in the data acquisition. The small batteries could last only one hour and handmade timers were used to wake up the data loggers just before the explosions at midnight. I was a little bit nervous about whether the handmade system would work for the data acquisition and I was thrilled the next morning after I confirmed that all 120 data loggers successfully recorded data.
How about a couple of disappointments? I had a bug in my 3D finite-difference seismic simulation code that was applied to estimate ground motion for future earthquakes. I forgot to add one component of the moment tensor, and it reduced the surface ground motion to about half of true amplitude. The bug in the source code was only one or two lines. It was lucky that I noticed the mistake in my code soon after we submitted the results to a client. We could fix the bug and re-submit the results. I shudder to think if I had not notice the mistake and the client had used our wrong calculation results for their decision making.
What advice would you give to geophysics students and professionals just starting out in the industry? Be curious about everything and try to get involved with a wide range of science and engineering. It is also important to be honest and objective in your work and admit the limitations of the accuracy and resolution of data processing. I think to be honest is the most important attribute for engineers and scientists.