geophysical technologies – i geophysical technologies module
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Geophysical Technologies i
Geophysical TechnologiesMODULE AT A GLANCE
SLIDE GT-1 Geophysical Technologies
SLIDE GT-2 Magnetic SurveysSLIDE GT-3 Magnetic Surveys Primary Tools and ApplicationsSLIDE GT-4 Magnetic FieldSLIDE GT-5 Magnetic Surveys Survey DesignSLIDE GT-6 MagnetometersSLIDE GT-7 Magnetic Surveys InterpretationSLIDE GT-8 Magnetic Surveys CostsSLIDE GT-9 Magnetic Surveys SummarySLIDE GT-10 Magnetic Surveys Advantages and Limitations
SLIDE GT-11 Electrical ResistivitySLIDE GT-12 Resistivity Common ApplicationsSLIDE GT-13 Resistivity Physical BasisSLIDE GT-14 Resistivity InstrumentsSLIDE GT-15 Field Arrays for Resistivity SurveysSLIDE GT-16 Resistivity (Ohm-Meters) of Common MaterialsSLIDE GT-17 Vertical Electrical Sounding ExamplesSLIDE GT-18 Example of Electrical ProfilingSLIDE GT-19 Resistivity ProfilingSLIDE GT-20 Resistivity Cross SectionSLIDE GT-21 Resistivity Profiling CostsSLIDE GT-22 Resistivity AdvantagesSLIDE GT-23 Resistivity Limitations
EM Conductivity Surveys
SLIDE GT-24 Electromagnetic (EM) Conductivity SurveysSLIDE GT-25 EM Conductivity Surveys Primary Tools and ApplicationsSLIDE GT-26 Terrain Conductivity MeterSLIDE GT-27 EM Conductivity Surveys Survey Practice
Geophysical Technologies ii
SLIDE GT-28 Range of Electrical Conductivities in Natural Soil and RockSLIDE GT-29 EM Conductivity Surveys InterpretationSLIDE GT-30 EM Conductivity Surveys CostsSLIDE GT-31 EM Conductivity Surveys SummarySLIDE GT-32 EM Conductivity Surveys Summary (Continued)SLIDE GT-33 EM Conductivity Surveys LimitationsSLIDE GT-34 Time Domain Electromagnetic (TDEM) SurveysSLIDE GT-35 TDEM Surveys Primary UsesSLIDE GT-36 TDEM Surveys SoundingsSLIDE GT-37 EM Metal DetectorsSLIDE GT-38 EM61 SurveySLIDE GT-39 EM61 CostsSLIDE GT-40 TDEM Surveys CostsSLIDE GT-41 TDEM Surveys SummarySLIDE GT-42 TDEM Survey Limitations
Borehole Geophysical Methods
SLIDE GT-43 Borehole Geophysical MethodsSLIDE GT-44 Borehole Tools and ApplicationsSLIDE GT-45 Borehole ApplicationsSLIDE GT-46 Borehole Traditional MethodsSLIDE GT-47 Borehole Traditional Methods (Continued)SLIDE GT-48 Borehole Advanced TechniquesSLIDE GT-49 Borehole Image Processing System (BIPS)SLIDE GT-50 Full Waveform Sonic (FWS) LogsSLIDE GT-51 Instrument Packages and Coincident SurveysSLIDE GT-52 Borehole CostsSLIDE GT-53 Borehole Costs (Continued)SLIDE GT-54 Borehole SummarySLIDE GT-55 Borehole Limitations
Ground Penetrating Radar (GPR)
SLIDE GT-56 GPRSLIDE GT-57 GPR Surveys Physical BasisSLIDE GT-58 GPR Surveys InstrumentsSLIDE GT-59 Low-Frequency Bistatic GPR AntennaSLIDE GT-60 GPR Surveys InterpretationSLIDE GT-61 GPR CostsSLIDE GT-62 GPR Advantages
Geophysical Technologies iii
SLIDE GT-63 GPR LimitationsSLIDE GT-64 GPR Summary
SLIDE GT-65 Seismic Surveys Interpretation Types of Seismic SurveysSLIDE GT-66 Seismic Refraction and Reflection ApplicationsSLIDE GT-67 Seismic Surveys Physical BasisSLIDE GT-68 Refractive Seismic SurveySLIDE GT-69 Time/Distance PlotSLIDE GT-70 Refraction AnalysisSLIDE GT-71 Seismic ReflectionSLIDE GT-72 Processed Reflection RecordSLIDE GT-73 Seismic Survey (Refraction) CostsSLIDE GT-74 Seismic Survey (Reflection) CostsSLIDE GT-75 Seismic Refraction Advantages and LimitationsSLIDE GT-76 Seismic Reflection Advantages and Limitations
SLIDE GT-77 Geophysical Technology Selection SummarySLIDE GT-78 Electromagnetic and Borehole Geophysical Summary
GT-1Module: Geophysical Technologies
GT-2Module: Geophysical Technologies
Magnetometer surveys measure the magnetic field of the earth
Ferrous metals and minerals alter the natural field
A body placed in a magnetic field acquires a magnetization which is typically proportional to thefield. The constant of proportionality is known as the magnetic susceptibility. Susceptibility isvery small for most natural materials. However, typically, iron rich materials have relativelylarge magnetic susceptibilities.
Materials containing iron have permanent magnetic moments in the absence of externalmagnetic fields. An object that exhibits a magnetic moment is characterized by a tendency torotate into alignment when exposed to a magnetic field. The susceptibility of a rock typicallydepends only on its magnetite content. Certain sedimentary rocks and acid igneous rocks haverelatively small susceptibilities whereas basalts, gabbros, and serpentinites usually have relativelylarger susceptibilities.
The magnetic field of the Earth originates from electric currents in the liquid outer core. Earthmagnetic field strengths are typically expressed in units of nanoTesla (nT). The Earthsmagnetic field varies during the day because of changes in the strength and direction of theplasma induced currents circulating in the ionospherethese changes are referred to as diurnalvariations in the Earths magnetic field, or the magnetosphere. Sunspot and solar flare activitycan create irregular disturbances in the magnetic fieldthese changes are referred to asmagnetic storms.
GT-3Module: Geophysical Technologies
In simple terms, magnetometer surveys are used in environmental site assessment for a varietyof objectives as provided in the next slide.
Where would magnetic surveys be considered in site assessment?
Where would you select magnetic surveys over other types of metal detection surveys?
GT-4Module: Geophysical Technologies
Magnetic Surveys Primary Tools and Applications
Underground storage tanks
Mapping of landfills
Magnetic surveys have become more widely used in recent years due to the increasedefficiency of magnetometer systems. The most commonly used magnetometers today are calledcesium vapor magnetometers. Other types of magnetometers are available, some withadvanced sampling rates (such as the overhauser proton magnetometer) and the older typeproton precession and fluxgate type systems. These magnetometers measure the magneticfield, in units called the nanoTesla otherwise called the gamma. Measurements can be obtainedwith these systems at a rate of 10 times per second or more, allowing for rapid data collection.
Magnetometers measure changes in the earths magnetic field caused by ferrous objects andgeologic formations.
Although there are other methods for metal detection, magnetometers are useful for a numberof site objectives. These include location of underground storage tanks, assisting in assessinglandfills for ferrous metal content, assisting in mapping lateral changes in geologic formations,and detection of ordnance in active and inactive military ranges. Another use formagnetometers is the detection of abandoned steel cased wells.
GT-5Module: Geophysical Technologies
Naturally occurring everywhere on earth and varying
Altered in the vicinity of a magnetic body or electrical medium that is carrying current
The earths magnetic field induces a magnetic moment per unit volume in buried ferromagneticdebris (bottom), causing a local perturbation (anomaly) in total magnetic field (top).
The total magnetic field measured is a vector sum of the ambient earths magnetic field, pluslocal perturbations caused by buried objects.
Note the variable spacing of the lines related to the objects geometry and the implications(gradient or flux) of making measurements in different regions around the object.
GT-6Module: Geophysical Technologies
Magnetic Surveys Survey Design
Data collected along transects
Spacing determined by objective
Base Station Measurement
0,0 10 20 30 40 50
000 06 12 18 24
Most geophysical surveys are conducted along regularly spaced lines. Surveys are normallyconducted along regularly spaced grid lines. Cesium vapor magnetometers are used inserpentine search or clearance patterns in addition to the grid-based data acquisition approach. Global positioning systems are used more often now to tie the geophysical measurements toground coordinates.
When total field measurements are being obtained, such as the case with proton and cesiumvapor instruments, a separate stationary base station magnetometer should be used. Thesecond magnetometer should be used to measure diurnal changes in the ambient magnetic fieldas well as possible magnetic storm effects. The next slide shows how the magnetic field varieswith time during the typical day.
GT-7Module: Geophysical Technologies
Marine Magnetometer SurveyEEGS SAGEEP 2002
Measure changes in field due to ferrous or magnetically susceptible objects
Also detects geologic variations due to magnetic minerals
Monitor of diurnal changes in the magnetic field
Magnetometers measure changes in the magnetic field due to ferrous objects, magnetic mineralsin soils and rock, and also senses the variability of the natural magnetic field of the earth. Thisfield can vary gradually during the day (1