Previous Section Field Sampling Procedures Manual Chapter 8 – Page 1 of 46 Return to Main TOC Chapter 8 Geophysical Techniques Table of Contents 8.1 Introduction 8.2 Ground Penetrating Radar 8.2.1 Fundamentals 8.2.2 Advantages 8.2.3 Limitations 8.2.4 Instrumentation 8.2.5 Survey Design, Procedure and Quality Assurance 8.2.6 Data Reduction and Interpretation 8.2.7 Presentation of Results 8.3 Magnetics 8.3.1 Fundamentals 8.3.2 Advantages 8.3.3 Limitations 8.3.4 Instrumentation 8.3.5 Survey Design, Procedure and Quality Assurance 8.3.6 Data Reduction and Interpretation 8.3.7 Presentation of Results 8.4 Gravity 8.4.1 Fundamentals 8.4.2 Advantages 8.4.3 Limitations 8.4.4 Survey Design, Procedure and Quality Assurance 8.4.5 Data Reduction and Interpretation 8.4.6 Presentation of Results 8.5 Electrical Resistivity 8.5.1 Fundamentals 8.5.2 Advantages 8.5.3 Limitations 8.5.4 Instrumentation 8.5.5 Survey Design, Procedure and Quality Assurance 8.5.6 Sounding Mode 8.5.7 Profiling Mode Figure 8.1 Common Arrays 8.5.8 Profiling-Sounding Mode 8.5.9 Resistivity Data Reduction and Interpretation 8.5.10Presentation of Results 8.5.10.1 Sounding Mode 8.5.10.2 Profiling Mode 8.5.10.3 Profiling-Sounding Mode 8.6 Induced Polarization 8.6.1 Fundamentals 8.6.2 Advantages Field Sampling Procedures Manual Chapter 8 – Page 2 of 46 8.6.3 Limitations 8.6.4 Instrumentation 8.6.5 Survey Design, Procedure and Quality Assurance 8.6.6 Sounding Mode 8.6.7 Profiling Mode 8.6.8 Profiling-Sounding Mode 8.6.9 Data Reduction and Interpretation 8.6.10Presentation of Results 8.7 Electromagnetics 8.7.1 Fundamentals 8.7.2 Advantages 8.7.3 Limitations 8.7.4 Instrumentation 8.7.5 Survey Design, Procedure and Quality Assurance 8.7.6 Data Reduction and Interpretation 8.7.7 Presentation of Results 8.8 Very-low Frequency (VLF) Electromagnetics 8.8.1 Fundamentals 8.8.2 Advantages 8.8.3 Limitations 8.8.4 Instrumentation 8.8.5 Survey Design, Procedure and Quality Assurance 8.8.6 Data Reduction and Interpretation 8.8.7 Presentation of Results 8.9 Seismic 8.9.1 Fundamentals 8.9.2 Instrumentation 8.9.3 The Seismic Refraction Method Figure 8.2 Seismic Refraction 8.9.3.1 Seismic Refraction Advantages 8.9.3.2 Seismic Refraction Limitations 8.9.3.3 Seismic Refraction Survey Design, Procedure and Quality Assurance Figure 8.3 Observer’s Log 8.9.3.4 Seismic Refraction Data Reduction and Interpretation 8.9.3.5 Seismic Refraction Presentation of Results 8.9.4 The Seismic Reflection Method Figure 8.4 Seismic Reflection 8.9.4.1 Seismic Reflection Advantages 8.9.4.2 Seismic Reflection Limitations 8.9.4.3 Seismic Reflection Survey Design, Procedure, And Quality Assurance 8.9.4.4 Seismic Reflection Data Reduction and Interpretation 8.9.4.5 Seismic Reflection Presentation of Results 8.10 Borehole Geophysical Methods 8.10.1 Introduction 8.10.2 Advantages 8.10.3 Limitations 8.10.4 Types of Borehole Tools Field Sampling Procedures Manual Chapter 8 – Page 3 of 46 8.10.4.1 Gamma Ray and Self Potential (SP) Devices 8.10.4.2 Electrical Resistivity and Induction Devices Figure 8.5 Lateral Resisitivity Sonde Figure 8.6 Normal Resisitivity Sonde Figure 8.7 High Frequency Electromagnetic Energy 8.10.4.3 Porosity/Density Devices Figure 8.8 Basic Sonic System 8.10.4.4 Mechanical Devices 8.10.4.5 Acoustic, Radar and Optical Devices Figure 8.9 Magnetically oriented, acoustic-amplitude image of borehole wall generated from an acoustic televiewer. Figure 8.10 “Virtual core” wrapped (left) and unwrapped (right) images of a bedrock fracture at a depth of 29.4 meters collected with a digital television camera. 8.10.5 Quality Assurance 8.10.6 Presentation of Results References URLs for Surface and Borehole Geophysical Methods Field Sampling Procedures Manual Chapter 8 – Page 4 of 46 Field Sampling Procedures Manual Chapter 8 – Page 5 of 46 Return to TOC Chapter 8 Geophysical Techniques 8.1 Introduction The use of various geophysical techniques for the investigation of hazardous waste and ground water pollution sites is often a rapid, cost-effective means of preliminary evaluation. The information obtained from a geophysical investigation can be used to determine the subsurface conditions at, and in the vicinity of, a site. Various geophysical techniques reveal physical properties of the subsurface which can be used to determine hydrostratigraphic framework, depth to bedrock, extent of concen- trated ground water contaminant plumes, the location of voids, faults or fractures, and the presence of buried materials, such as steel drums or tanks. Geophysical investigations are most effective when used in conjunction with a drilling or boring program, and should not be considered a substitute for such programs. The information gained from a surface geophysical survey can be used to chose optimal locations for the placement of boreholes, monitor wells or test pits, as well as to correlate geology between wells and boreholes. The informa- tion derived from a geophysical survey can also be used to reduce the risk of drilling into buried drums or tanks. The use of geophysical methods at hazardous waste and ground water pollution sites is a fairly recent development. In the past, many of these techniques were used in the mineral, geothermal and petro- leum exploration industries. In recent years, the need to conduct ground water pollution investigations has coincided with improvements in the resolution, acquisition and interpretation of geophysical data. This process is ongoing; therefore, outlines of geophysical techniques and procedures are subject to revision as improvements are made in the instrumentation and interpretation algorithms. Each geophysical method has its advantages and limitations. The combination of two or more tech- niques in an integrated interpretation results in a reduction of the degree of ambiguity. A comprehen- sive knowledge of the local geology and site conditions is necessary in order to select an effective geophysical method or methods, to plan a survey, and to interpret the data. In some instances, site conditions may preclude the successful use of most or all geophysical tech- niques. These conditions include the presence of factors that degrade the ability of the geophysical instruments to measure various physical parameters. For instance, the presence of strong electromag- netic fields at site may preclude the use of some geophysical techniques. Under such instances the use of geophysics may not be recommended. However, the application of geophysical methods should not be entirely dismissed until an experienced geophysicist evaluates the site. Although geophysical techniques may not be directly applicable on-site, a geophysical survey of the area surrounding the site may be useful to assist in the understanding of the hydrogeology of the impacted area. Each site must be considered unique. The project geophysicist should therefore evaluate all material at his or her disposal prior to the implementation of a geophysical survey plan. In addition to visiting the site, an examination of aerial photographs, geologic maps, well data, and other information is recommended. A “generic” approach to work plans should be avoided. Another practice that should be avoided is attempting to apply geophysical methods when inappropriate, merely because a poorly written proposal states that a geophysical survey must be performed to satisfy contractual obligations. Performance guidelines for a total of eight surface geophysical techniques, in addition to borehole methods, are presented in this chapter. The surface methods include ground penetrating radar (GPR), magnetic, gravity, electrical resistivity, induced polarization (IP), electromagnetic (EM), very-low Field Sampling Procedures Manual Chapter 8 – Page 6 of 46 frequency electromagnetics (VLF), and seismic methods. Other methods, not widely used in ground water pollution investigations, are not in this Chapter; these include spontaneous or self-potential (SP), controlled source audio-magnetotellurics (CSAMT), infrared (IR), and airborne geophysical methods. The reader should consult the literature for more information on these methods. Metal detectors are not included in this Chapter because most are essentially electromagnetic systems whose response is an audio or visual feedback that is rarely recorded. These instruments may be useful immediately prior to excavation to relocate some anomalous areas. Although radiometric devices (scintillation counters and Geiger counters) and organic vapor analyzers can technically be considered geophysical instruments, they are more commonly referred to as health and safety moni- toring devices, and are therefore not included in this chapter. The reader is advised to consult the literature if additional information on a particular method is needed. The use of new geophysical techniques or algorithms is encouraged if the investigation addresses the problem and the work plan is within budgetary constraints. The expertise of the Geophysics Section of the New Jersey Geological Survey is available to other State or Federal agencies. Assistance can be given in the following areas: preparation of Requests for Proposals, review of proposals, field quality control, and review of reports. Geophysical surveys may be performed on a case-by-case basis. A reasonable lead-time is a necessary courtesy required on all requests. Requests for assistance should include all pertinent information, including a project activity code, and be sent in writing to the State Geologist, New Jersey Geological Survey, NJDEP. 8.2 Ground Penetrating Radar 8.2.1 Fundamentals Return to TOC The ground penetrating radar (GPR) method has been used for a variety of civil engineering, ground water evaluation and hazardous waste site applications. Of all geophysical techniques available, it is one of the most highly used and successful.