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[Research Guide]

 

 


Geo-Imaging & Geo-Nerve Laboratory

***本人提供研究獎學金給學經歷優異的研究生,特別是考慮攻讀博士但擔心經濟問題與機會成本的學生,歡迎與我聯絡洽詢助研金與研究方向。***

To prospective graduate Students: I offer research assistantship to those with good academic qualification. Especially if you are considering pursuing a PhD degree and worried about the finance, you are welcome to contact me for details about assistantship and research direction. 

Our visions:
Geo-Imaging: Imagine how modern medical doctors practice without technologies, such as X-ray, ultrasonic, X-ray CT (Computed Tomography), MRI (Magnetic Resonance Imaging), and PET (Positron Emission Tomograpy)? Civil engineers are specialists that deal with problems of our infrastructures. However, without subsurface imaging techniques, engineers, especially the geotechnical engineers, often have to make judgments in the lack of information. With the advancement of computational technology, our ability to numerically model or simulate the domain is far beyond our ability to characterize the domain. Subsurface imaging by geophysical imaging techniques, borrowed from earth science and petroleum industry, is desired and given much hope by engineers. Geophysical exploration methods have been applied to geotechnical problems since their early developments. However, the results often do not live up to engineers' expectations. In pursuing high resolution accurate methods for engineering applications, limitations of geophysical methods are identified and possible countermeasures are being developed. The challenges include: the lack of standard in data reduction, non-uniqueness of data inversion, limitations of exploration depth and resolution, field conditions violating model assumptions, and the weak link between geophysical parameters and engineering parameters. It is our goal that one day civil engineers will have an arsenal of high resolution subsurface imaging techniques at their disposal to help solve traditional problems as well as emerging problems associated with sustainability and hazard mitigation.

Geo-Nerve: Human body is built in with a complex never system. It continuously monitor the functions of our bodies and provide early warning to potential problems. Our infrastructures are analogous to major organs in human bodies, each playing an important role in our living environment.  However, they are not built in with a monitoring system. Failures, such as bridge collapse, landslide, levee breach, often occurs without timely warning. Monitoring system for infrastructures can be engineered by sensing technologies. Electrical sensing technology has advanced tremendously in the past century. Analogous to the peripheral nervous system, distributed sensing can be developed by waveguide and wireless sensing network (WSN) technology. A versatile waveguide sensing is the time domain reflectometry (TDR). TDR technique is based on transmitting an electromagnetic pulse through a coaxial cable connected to a sensing waveguide and watching for reflections of this transmission due to changes in characteristic impedance along the waveguide. Depending on the design of the waveguide and analysis method, the reflected signal can be used to “feel” various engineering parameters. When embedded in a geotechnical structure, TDR can be seen as a “geo-nerve” system. Unlike other techniques having a transducer with a built-in electronic sensor, TDR sensing waveguides are simple mechanical devices without any electronic components, and the monitoring system has a self-diagnosis function by examining the pulsing waveforms. Innovative measurement techniques based TDR are being developed, ranging from soil moisture, suspended sediment concentration, water level, extensometer, localized shear deformation, scouring, to dielectric spectroscopy.


Research directions (Categorized by geotechnical activities):

  • Investigation:

    • 1D ==> 2D ==> 3D exploration by geophysical methods

    • Geo-environmental investigation by geo-electrical methods

  • Analysis/Design

    • Geophysical characterization of soils

    • Reliability-based analysis and design

  • Construction:

    • Quality control/Quality assurance methods

  • Monitoring

    • Monitoring techniques based on electrical sensing (in particular, time domain reflectometry, TDR)

    • Integration with information technology


Laboratories Facilities:

  • Seismograph systems for refraction, reflection, surface wave and borehole seismic (downhole, cross-hole) testing

  • Electrical resistivity tomography systems

  • Ground penetrating radar system

  • Borehole geophysics system (caliper, acoustic televiewer, optical televiewer, PS-logger)

  • Bender element testing system

  • Time domain reflectometers

  • Optical time domain reflectometer

  • Data logging systems, monitoring servers

  • Data processing servers

  • Comsol Multiphysics® Modeling Software


Recent research projects:

1. Optimization and high-resolution method of seismic surface wave testing

Many geotechnical designs are based on the empirical SPT N values. Why can't we use shear-wave velocity (shear modulus) for the same purpose? It's more scientific and it can be measured non-destructively by seismic surface wave testing. Seismic surface wave methods measure shear wave (S-wave) velocity on the basis of the dispersive feature of Rayleigh (or Love) waves in vertically heterogeneous formation. There are three steps involved in the surface wave method: (i) field testing for recording surface waves, (ii) determination of experimental dispersion curve from field data, and (iii) inversion of S-wave velocity profile from the dispersion curve. 

The inversion of the experimental dispersion curve is based on the solution of the forward problem of surface wave propagation in a horizontally-layered system. This line of research deals with how to improve lateral and vertical resolution such that the geophysical measurement is in the same scale as in the geotechnical measurement. Specific research topics include high lateral resolution survey method, unified wavefield transformation and wavelength-controlled dispersion analysis, optimization of field survey and dispersion analysis, etc.

2. Model appraisal and optimization of electrical resistivity tomography

Electrical resistivity tomography (ERT) is a convenient subsurface imaging technique, which is gaining popularity in the engineering community. The resistivity method injects electrical current into the ground through a pair of current electrodes and measures response of electrical potential through a pair of potential electrodes.  By altering the distances between the electrodes, different volumes of the subsurface are sensed and additional information about resistivities at different depths is obtained. The resistivity method has evolved from 1D to 2D and ultimately 3D measurements, with 2D electrical resistivity tomography (2D ERT) being the most practical and popular.

ERT has good depth coverage and can easily produces 2D/3D resistivity image. However, its resolution degrade significantly with depth. Model appraisal and limitations of the more common 2D ERT are investigated. This line of research continues to focus on how to enhance imaging resolution by optimizing electrode array and utilizing surface electrodes as well as borehole electrodes.

Various survey configuration for ERT

 

3. Comprehensive modeling of TDR and TDR-based dielectric spectroscopy

Time domain reflectometry (TDR) is a measurement technique based upon transmission line theory. It is finding more and more applications in engineering measurements and monitoring in both laboratory and field works, driving the demand for an accurate TDR wave propagation model. We have been working on the comprehensive modeling of TDR measurement system, based on new measurement and monitoring applications can be developed.

The dielectric permittivity as a function of frequency (dielectric spectrum) contains rich information for composite materials such as soils. It may serve as a finger print for characterizing composite materials, with special interests on characterization soil properties and contaminated soils. Measurement of broadband dielectric permittivity is not an easy task in the laboratory, let alone measurements in the field. Practical approach of dielectric spectroscopy is being developed based on TDR.

 

4. Applications of geophysical exploration on engineering problems

  • Assessment of liquefaction potential by seismic surface wave method

       

 

  • Investigation of abnormal seepage using electrical resistivity tomography

 

  • Inspection of concrete dam by seismic traveltime tomography

       

 

  • Investigation of soil and groundwater contamination

 

  • Imaging of unknown foundation

       

 

  • Imaging of grout columns and assessment of ground improvement

       

 

5. Engineering monitoring based on TDR

  • Constructing geo-nerve by cable radar

 

  • Monitoring of soil moisture profile

       

               

 

  • Monitoring of sediment transport

       

       

 

  • Bridge scour monitoring

       

 

  • Water level sensing by pulsing TDR

       

 

  • Dielectric spectroscopy of soils

  • Instantaneous water content and density determination of compacted soils by multi-physical data fusion