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From page 35... ... 35 C H A P T E R 4 Geophysical Investigations Introduction Geophysical investigations are used to estimate the physical properties of the subsurface by measuring, analyzing, and interpreting seismic, electrical, electromagnetic, gravitational, and magnetic fields measured at the ground surface or within boreholes. Investigations conducted from the ground surface typically provide information about the subsurface both laterally and to some depth; while most of the borehole investigations, with some exceptions, provide detailed information about subsurface materials only in the immediate vicinity of the borehole or between boreholes. Read the entire page →
From page 36... ... 36 • The interpreted subsurface conditions may not be unique for many geophysical methods; there may be multiple, physically plausible interpretations for the stratigraphy or location and size of anomalies that all yield the same measured geophysical response. For example, a structural low in bedrock topography; a small, air-filled void in the bedrock; or a larger, water-filled void in the bedrock may all produce the same magnitude of gravity anomaly. Read the entire page →
From page 37... ... 37 Table 4-1. Matrix of surface geophysical methods in relation to investigation objectives Objective Seismic Electrical and Electromagnetic Potential Field R ef ra ct io n an d R ef le ct io n Su rf ac e W av e R es is tiv ity El ec tr om ag ne tic G ro un dPe ne tr at in g R ad ar M ic ro gr av ity M ag ne to m et ry Se lfPo te nt ia l Lithology and stratigraphy      Bedrock topography        Water table    Rippability  Shear wave velocity profile  Fault detection      Void and cavity detection       Subsurface fluid flow   Ferrous anomalies    Conductive anomalies     Corrosion potential  Sources: Fenning and Hasan (1995) Read the entire page →
From page 38... ... 38 Geophysical Method ASTM Guide or Standard Time-domain electromagnetic D6820 Ground-penetrating radar D6432 Microgravity D6430 Source: Sirles (2006) 4.3.1 Seismic Methods Seismic methods use measurements of the velocity of mechanical (i.e., stress) Read the entire page →
From page 39... ... 39 Source: Bourbie et al. Read the entire page →
From page 40... ... 40 often of limited value for determining the properties of the soil itself. In these cases, the shear wave velocity is a more useful quantity because it is mostly unaffected by the presence of fluid in the voids. Read the entire page →
From page 41... ... 41 Source: Geosyntec Consultants, Inc. Figure 4-2. Read the entire page →
From page 42... ... 42 • Correcting for normal move-out • Migrating to correct for dip of subsurface reflectors Subsequent data interpretation can be subjective. Other data, such as boring logs, may be used to generate synthetic seismograms to correlate with measured seismic reflection data to check the reasonableness of interpreted sections. Read the entire page →
From page 43... ... 43 Source: Miller and Steeples (2008) Figure 4-5. Read the entire page →
From page 44... ... 44 The processing steps consist of (i) calculating an experimental dispersion curve that shows the variation of Rayleigh wave phase velocity with frequency (or wavelength) Read the entire page →
From page 45... ... 45 4.3.2 Electrical and Electromagnetic Methods Electrical and electromagnetic geophysical methods use the flow of electrical currents through the ground to evaluate subsurface characteristics. Electrical (or galvanic) Read the entire page →
From page 46... ... 46 electromagnetic coupling; and ease of field implementation, automated data acquisition, and interpretation. Additional array configurations are available for specific objectives (Zonge et al. Read the entire page →
From page 47... ... 47 4.3.2.2 Electromagnetic Methods Electromagnetic methods can be broadly divided into two groups: Frequency-domain electromagnetics (FDEM) methods and time-domain electromagnetic (TDEM) Read the entire page →
From page 48... ... 48 Source: Spotlight Geophysical Services, LLC Figure 4-10. Example terrain conductivity results A common TDEM resistivity sounding survey consists of a square transmitter coil laid on the ground and a receiver coil located in the center of the transmitter coil. Read the entire page →
From page 49... ... 49 • Mapping underground utilities and other buried objects such as tanks and drums GPR antennas are designated by their center frequency and range from approximately 10 megahertz (MHz) for mapping deep subsurface stratigraphy to approximately 3,000 MHz for shallow rebar mapping. Read the entire page →
From page 50... ... 50 Source: Texas DOT Notes: CRCP: continuously reinforced concrete pavement Figure 4-12. Example GPR results showing pavement void detection 4.3.3 Potential Field Methods Examples of potential fields include gravitational, magnetic, and electrical fields. Read the entire page →
From page 51... ... 51 The processing steps for microgravity surveys include making corrections to the measured data for (i) instrument drift (using base station readings) Read the entire page →
From page 52... ... 52 Source: Spotlight Geophysical Services, LLC Figure 4-14. Example of a microgravity contour map 4.3.3.2 Magnetometry Magnetic surveys measure changes in the magnitude in the magnetic field of the Earth due to the presence of ferrous metal objects or by Earth materials that have high magnetic susceptibilities. Read the entire page →
From page 53... ... 53 time-varying changes in the magnetic field (and the value of the field at the base station location) Read the entire page →
From page 54... ... 54 Table 4-3. Matrix of borehole geophysical methods in relation to investigation objectives Objectives Se is m ic C ro ss ho le Se is m ic D ow nh ol e C al ip er R es is tiv ity Sp on ta ne ou s Po te nt ia l In du ct io n N at ur al G am m a G am m aG am m a D en si ty N eu tr on P or os ity A co us tic T el ev ie w er Se is m ic L og gi ng 1 Lithology       Seismic wave velocity profile    Fracture location and characteristics   Density, porosity, and water content   Borehole diameter  Source: Paillet and Ellefsen (2005) Read the entire page →
From page 55... ... 55 4.4.1.1 Seismic Crosshole Method The seismic crosshole method is conceptually simple. Compression and shear waves are generated in one borehole, and the arrival of those waves is recorded in one or more additional boreholes by a geophone at the same depth. Read the entire page →
From page 56... ... 56 4.4.1.2 Seismic Downhole Method For seismic downhole measurements, a source on the ground surface is used to generate compression and shear waves, and arrivals are recorded by one or more geophones placed in a borehole. For tests conducted in soils, the borehole is usually cased to avoid collapse. Read the entire page →
From page 57... ... 57 Source: ConeTec Figure 4-16. Example seismic downhole data 4.4.2 In-Hole Logging Methods In-hole logging methods use mechanical, electrical, electromagnetic, nuclear, acoustic (or sonic) Read the entire page →
From page 58... ... 58 be used in boreholes that are filled with water or drilling fluid. Caliper logs may be used to detect fractures and are commonly used to correct other in-hole logs for borehole diameter effects. Read the entire page →
From page 59... ... 59 Source: Geosyntec Consultants, Inc. Figure 4-17. Read the entire page →
From page 60... ... 60 A disadvantage of gamma-gamma density and neutron porosity logs is that the radioactive sources present a health hazard and require permits and certification for transportation and handling. Some states also restrict their use when testing in drinking water aquifers. Read the entire page →
From page 61... ... 61 Source: Geosyntec Consultants, Inc. Figure 4-18. Read the entire page →
From page 62... ... 62 Early geophysical applications of seismic logging methods focused on measuring only the travel time of compression waves. Since the 1980s, it has been more common to use full waveform logging methods that compression, shear, and Stonely waves to be measured to gather more information. Read the entire page →
From page 63... ... 63 Source: Geosyntec Consultants, Inc. Figure 4-19. Read the entire page →
From page 64... ... 64 Chapter 4 References AASHTO. Read the entire page →
From page 65... ... 65 Morey, R.M. Read the entire page →

From page 35...

... 35 C H A P T E R 4 Geophysical Investigations Introduction Geophysical investigations are used to estimate the physical properties of the subsurface by measuring, analyzing, and interpreting seismic, electrical, electromagnetic, gravitational, and magnetic fields measured at the ground surface or within boreholes. Investigations conducted from the ground surface typically provide information about the subsurface both laterally and to some depth; while most of the borehole investigations, with some exceptions, provide detailed information about subsurface materials only in the immediate vicinity of the borehole or between boreholes.