Use of Geophysics for Transportation Projects (2006)

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8INTRODUCTION This chapter begins by defining geophysical methods and techniques, includes results from a literature search, and dis- cusses the geophysical methods and techniques most com- monly used for transportation projects. As related by Telford et al. (1), applied geophysics can be divided into the following seven general methods of explo- ration: Magnetic, Electrical, Electromagnetic, Seismic, Gravitational, Radioactivity, and Well logging. Each geophysical method can be used for many different applications (e.g., mining exploration, oil and gas explo- ration, engineering, and environmental). The division of each method is based on the physics that governs it; therefore, geophysical techniques (e.g., refraction) within each method (i.e., seismic) are designed primarily for applications of the method to a given problem (e.g., rippability). This synthesis will use the terminology of methods to refer to those seven major divisions and techniques to identify specific applica- tions of the methods. Material properties can be measured indirectly through the use of engineering geophysical methods such as seismic, elec- trical resistivity, EM and ground penetrating radar (GPR), to name only a few. The capability of conducting geophysical in- vestigations in difficult or remote terrain and with greater sam- ple density, demonstrates the potential geophysics has to sig- nificantly affect design efforts and construction activities. The most important factor geophysics can help address is to reduce the risk associated with unknown subsurface conditions and to avoid related costly claims and repairs. Before publication of this synthesis, it was well known that the success of any geo- physical investigation requires that appropriate techniques be applied that address specific engineering objectives. For ex- ample, if karstic (e.g., pinnacles or sinks) limestone bedrock is expected to be encountered at shallow depths, applying a mag- netic method would not be appropriate. LITERATURE SEARCH AND TRAINING RESOURCES A multitude of literature sources exist on geophysics. Ex- isting works either deal purely with the theory and specifics of the physics regarding the variety of methods or they are segregated into the application of geophysics to specific fields. The standard of the geophysical industry for textbooks that contain all the geophysical methods are Telford et al. (1) and Dobrin (3), both published in 1976. Both books are still used in universities for geophysics coursework. It was not until the mid-1980s that enough demand for en- gineering geophysics resulted in the publication of the Hand- book of Engineering Geophysics—Vol. 1: Seismic (5), and Vol. 2: Electrical Resistivity (6). In the 1990s, a distinct need for specific, application-related books became apparent to present the state of the art for all the geophysical methods and techniques. Consequently, two “best-sellers” related to the use of engineering geophysics became available. The Society of Exploration Geophysicists produced one of the first books dedicated to shallow geophysics, Geotechnical and Environ- mental Geophysics—Vols. I–III (7). In 1995, the U.S. Army Corps of Engineers produced a comprehensive document, their engineering manual, Engineering Design—Geophysical Exploration for Engineering and Environmental Investiga- tions (8). In these last two references, the practical application of geophysics was brought to the forefront through requests to authoritative contributors in specialized fields to develop chapters based on their expertise (Dr. Gregg Hempen, per- sonal communication, 1995). Both books have remained key components of the early application of geophysics to shallow, engineering, and environmental studies. The Corps’ engineer- ing manual is available for unlimited public distribution at http://www.usace.army.mil/inet/usace-docs/eng-manuals/ em1110-1-1802/ toc.htm. With the publication of these two resources, available lit- erature became much more specific as it dealt with particu- lar geophysical methods or engineering applications. One such book, published by The Geological Society of London, Modern Geophysics in Engineering Geology (9), makes ap- plication of the geophysical techniques central to the theme of the book. More recently, FHWA published Application of Geophysical Methods to Highway Related Problems (10), CHAPTER TWO GEOPHYSICAL METHODS

9which is designed specifically for use by state DOT and federal highway engineering staff when particular applica- tions necessitate the use of geophysics. The manual was not developed or designed to be a textbook, and although it was published as one, its primary function is the website, http://www.cflhd.gov/geotechnical, which uses a solution matrix that guides users to particular engineering problems (e.g., rippability) and what geophysical method may best suit the objectives of the investigation. However, an MS Word text version as well as a PDF version of the FHWA manual is also available for download. The most recent textbook to be published on the subject comes from the Society of Exploration Geophysicists of Japan. This comprehensive book, Application of Geo- physical Methods to Engineering and Environmental Prob- lems, covers the most recent innovations for geophysical technology for 17 methods, and is available online from the website (11). Numerous opportunities exist for instruction through at- tendance at geophysics conferences. Since 1988 the Sympo- sium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP), the EEGS annual con- ference, consistently produces case-specific presentations on the use of geophysics. EEGS is dedicated to providing edu- cational opportunities for nongeophysicists about the value and use of the technologies available. The EEGS website, http://www.eegs.org/, can be searched for case histories pre- sented at SAGEEP by either geophysical technique or engi- neering application (e.g., geotechnical engineering). More than 600 case histories are available at http://www.eegs.org/ sageep/proceedings.cfm, and that only covers SAGEEP con- ferences between 1988 and 2000. Searchable CD-ROMs of the annual proceedings are made available for purchase, with approximately 30 to 40 case histories presented related to engineering geophysics. Additionally, cutting-edge tech- nologies are discussed and innovations in equipment and software made available by exhibitors and vendors. In 2000, an introductory EEGS short course designed for geologists, engineers, and environmental scientists became available (2). The success of this particular introductory course created a high demand for the course notes and the CD-ROM, which were reproduced and distributed interna- tionally. Simultaneously, FHWA became increasingly aware of the need for dialog and training; thus, it created a conference that deals specifically with the needs of the state DOTs; that is, a geophysical applications conference dedi- cated solely to transportation. Since 2000, three Interna- tional Geophysical Conferences have been hosted by FHWA, with proceedings published on CD-ROM (12–14). The content of the training sessions as well as the experience presented through case histories has provided significant help to transportation engineers. The proceedings from 2000 are no longer available through a website; however, the 2002 and 2003 proceedings are still available (13,14). The Topical Bibliography contains the papers, listed by topic, presented at each of the three FHWA conferences because they all contain useful information. Literally hundreds of articles have been published per- taining directly to the topic of this synthesis in professional (peer-review) journals available from the major geotechni- cal, engineering, geology, and geophysical societies. The pa- pers typically deal with one or two geophysical techniques and are dedicated toward solving a problem. Such articles can be accessed at TRB’s Transportation Research Informa- tion Service (TRIS) on-line library (http://trisonline.bts.gov/ search.cfm). It is the most comprehensive source for journal articles. The Topical Bibliography lists many of the articles germane to this synthesis from SAGEEP and the FHWA geophysics conferences, as well as journal articles obtained from the TRIS website. Engineering geophysics is a bur- geoning field, and these articles represent the latest innova- tions and the most practical solutions to the problems facing today’s engineers in the transportation industry. Therefore, the bibliography contains articles that are generally less than 5 years old at the time of this publication. Only one NCHRP synthesis has been prepared that is con- cerned specifically with geophysics. Although it deals solely with the use of GPR for transportation projects (15), it is a well-prepared document explaining the technique, instru- mentation, data, and applications for such projects. METHODS, TECHNIQUES, APPLICATIONS, AND STANDARDS As discussed in this chapter’s introduction, there is a diverse set of geophysical methods capable of subsurface imaging at a scale appropriate for exploration and for engineering in- vestigations. This section is intended to • Present brief, introductory-level information regarding the most commonly used methods for transportation- related (engineering) studies; • Identify the techniques associated with each geophysi- cal method; • Define general applications for the techniques as used for geotechnical investigations; and • Present existing standard test methods and guides used for geophysical investigations. Methods Numerous publications were prepared for the three afore- mentioned FHWA International Conferences. As part of the keynote address at the first two of these conferences, matri- ces of method versus applications were published and pre- sented (16,17). Another comprehensive matrix of methods, techniques, and applications is presented in The Code of

10 Practice for Site Investigation (18). Because this synthesis is not intended to be a training document, a simplified, com- bined version of a matrix for geotechnical practice is pre- sented here. For more thorough tables and matrices depicting specific engineering applications and comprehensive meth- ods/techniques see the referenced papers. Tandon and Nazar- ian (4) summarized the referenced FHWA conference papers and presented all of their matrices. Techniques Appendix A includes Technical Briefs for the following most common methods: • Seismic Method: Refraction, reflection, Spectral Analy- sis of Surface Waves, Multi-Channel Analysis of Surface Waves, and crosshole techniques. • Electrical Resistivity Method: Profiling and sounding techniques. • EM: Time- and frequency-domain techniques. • GPR method. • Magnetic method. The briefs are presented as specific techniques (e.g., fre- quency- and time-domain EM). They are reproduced from a draft FHWA geophysics workshop that is currently in de- velopment, and intended to be an educational workshop en- Methods Seismic Electro-Magnetic Electrical Other Techniques Se ism ic R ef ra ct io n Se ism ic R ef le ct io n Se ism ic T o m o gr ap hy Sh ea r W av e Su rfa ce W av e (S AS W, M A SW , an d pa ss iv e) Fr eq u en cy –D om ai n EM (t err ain co n du ct iv ity ) Ti m e– D om ai n EM (m eta l d ete cto r) Ti m e D om ai n EM S ou n di ng s El ec tri ca l R es ist iv ity In du ce d Po la riz at io n G ra vi ty M ag n et ic s G ro u n d Pe n et ra tin g R ad ar Investigation Objectives Bedrock depth P P P P S S SS Rippability P P P Lateral and vertical variation in rock or soil strength P P P P P Location of faults and fracture zones S P P S S S S S S S S Karst features S P P S P P Near-surface anomalous conditions S P P S P Soil characterization and lithology S S S P P S P P Locating landfill boundaries, waste pits, waste trenches, buried drums S P P P P P P Water table S S P P P Water quality, fresh-saline water interfaces P P P S Notes: This matrix is intended to aid in the selection of an appropriate geophysical method and respective technique for typical geotechnical investigation objectives. The table does not account for geologic conditions, site cultural features, target size, and depth. Refer to Appendix A for additional information regarding methods and techniques. SASW = Spectral Analysis of Surface Waves; MASW = Multi-Channel Analysis of Surface Waves; P = primary technique; S = secondary technique; blank space = technique should not be used. TABLE 1 MATRIX OF SURFACE GEOPHYSICAL METHODS AND TECHNIQUES IN RELATION TO TYPICAL INVESTIGATION OBJECTIVES

11 titled Workshop on Geophysical Methods for Transportation Applications. Applications Geophysical techniques present incredible diversity in how they may be applied. A particular number of applica- tions tend to be used much more than others. Academic institutions and research organizations are constantly striv- ing for better use of existing technology (i.e., techniques) to solve engineering applications. Table 1 identifies a ba- sic or general approach that can be taken to select a pri- mary (P) or a secondary (S) geophysical technique that could be applied on common transportation project objec- tives. It is not intended to be inclusive of geophysical methods or techniques, nor geotechnical-related investiga- tion objectives. However, it covers the majority of needs for geotechnical engineers and for the state-of-the-practice geophysical methods. Often, multiple geophysical meth- ods (e.g., seismic and electrical) are selected to satisfy project objectives. In many cases it is because the physical contrasts may be better imaged with one method versus the other; however, under specific site conditions, the two methods may complement one another to meet the objec- tive. Table A1 (Appendix A) is another tool, developed for the previously mentioned FHWA workshop, that can aid in the selection of appropriate geophysical techniques. It is beneficial to go through the form, step-by-step, and arrive at a method that best suits the geologic and cultural setting, the surface conditions, and most importantly the type and size of the target. This tool for selection of geophysical methods and/or techniques is not comprehensive; how- ever, it will be useful to engineers and geoscientists to start a project with the same level of understanding regarding the site and objectives. Standards Mayne et al. (19) provides a good overview of geophysical pro- cedures for acquiring quality data. This report is a National Highway Institute publication that discusses the approach for field work and data processing, yet leaves flexibility for the varying site conditions that inevitably occur with field studies. Since 1995, ASTM has produced 15 documents regarding the acquisition and processing of geophysical data for both surface and borehole methods (20). Table 2 presents the ASTM Guides and Standards that have been published for the particular geo- physical techniques. Note that only two are Standard Test Methods (Crosshole—D4428 and Resistivity—G57), whereas the other 13 are identified as Standard Guides. The rationale for Geophysical Methods and Techniques ASTM Guide* Standard Guide for Selecting Surface Geophysical Methods (included in this guideline are the following techniques) • Seismic refraction • Seismic reflection • D.C. resistivity • Induced polarization (IP) or complex resistivity • Spontaneous potential (SP) • Frequency–domain electromagnetics (FDEM) • Time–domain electromagnetics (TDEM) • Very low frequency (VLF) electromagnetics • Metal detectors and pipe/cable locators • Ground penetrating radar (GPR) • Magnetics • Gravity D6429 Standard Guide for Conducting Borehole Geophysical Logging Seismic Refraction D.C. Resistivity Frequency–Domain Electromagnetics (FDEM) Time–Domain Electromagnetics (TDEM) Metal Detectors Ground Penetrating Radar (GPR) Gravity Seismic Reflection In press Mechanical Caliper–Borehole Logging Gamma–Borehole Logging Electromagnetic Induction Neutron–Borehole Logging Geophysical Methods and Techniques ASTM Standard* Crosshole Seismic Testing D5753 D5777 D6431 D6639 D6820 D7046 D6432 D6430 D6167 D6274 D6726 D6727 D4428 Soil Resistivity Testing G57 *Refer to Topical Bibliography for references. TABLE 2 GUIDES AND STANDARDS FOR GEOPHYSICAL INVESTIGATIONS

12 guides versus standard testing procedures is simple; geophysi- cal investigations require adaptation to the site conditions and the target to be identified. ASTM Standard Tests are rigid, whereas Standard Guides allow the operator some flexibility to acquire data that will meet the objectives of the investigation. Most people are not aware that ASTM Guides and Standards are available for geophysical testing. ASTM D6429 serves the industry as a guide for selection of appropriate geophysical methods based on objectives and setting. Several more are in development and review through the ASTM committee process. ASTM Guides and Standards referenced in Table 2 are listed in the Topical Bibliography. AASHTO published a com- prehensive manual in 1988 outlining the complex and diverse techniques for conducting subsurface investigations for trans- portation programs (21), and it includes the most common geo- physical investigation techniques used in the late 1980s. Cur- rently, there are no published AASHTO standards for acquiring or processing geophysical data.