The knowledge of the geological and geotechnical characteristics of the subsoil

Question

The knowledge of the geological and geotechnical characteristics of the subsoil is a basic step, fundamental to the development of any type of research, evaluation, study or project in the areas of Civil and Environmental Engineering. In this sense are asked, from information available on the Internet:

a) Define what come to be geophysical techniques and the relevance of such employment in identifying geological and geotechnical characteristics of a given field;

b) provide a summary of the physical concepts associated with the use of techniques based on electrical resistivity, seismic refraction and penetrating radar;

c) provide advantages and disadvantages (or limitations) of using the techniques mentioned in item (b) as regards the definition of soil profiles (including water level position - NA) and detection contamination plumes / solid waste buried.

d) Identify the sources of information presented at Work.

Explanation / Answer

a) Geophysical techniques are the physical methods(such as seismic, gravitational, magnetic, electrical and electromagnetic) to measure the physical properties of rocks, and in particular, to detect the measurable physical differences between rocks that contain ore deposits or hydrocarbons and those without.

Methods devised for finding mineral or hydrocarbon deposits can also be used in other areas such as monitoring environmental impact,imagingsubsurfacearchaeologicalsites, ground water investigations, subsurface salinity mapping,civil engineeringsite investigationsand interplanetary imaging.

b)Electrical resistivity tomography (ERT)orelectrical resistivity imaging (ERI)is ageophysical techniqueforimagingsub-surface structures from electrical measurements made at the surface, or byelectrodesin one or moreboreholes.The technique evolved from techniques of electrical prospecting that predate digital computers, where layers or anomalies were sought rather than images.

Seismic refractionis a geophysical principlegoverned bySnell's Law. Used in the fields ofengineering geology,geotechnical engineeringandexploration geophysics,seismic refraction traverses(seismic lines) are performed using aseismograph(s) and/orgeophone(s), in an array and an energy source. The seismic refraction method utilizes the refraction of seismic waves on geologic layers and rock/soil units in order to characterize the subsurface geologic conditions andgeologic structure.

The methods depend on the fact that seismic waves have differing velocities in different types of soil (or rock): in addition, the waves are refracted when they cross the boundary between different types (or conditions) of soil or rock. The methods enable the general soil types and the approximate depth to strata boundaries, or tobedrock, to be determined.

Ground-penetrating radar(GPR) is ageophysicalmethod that usesradarpulses toimagethe subsurface. Thisnondestructivemethod useselectromagnetic radiationin themicrowaveband(UHF/VHFfrequencies) of theradio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks

c)

The suite of chemicals making up the contamination may be electrically conductive, either because it is acidic or because it contains salts. In this case, the plume is also electrically conductive and can often be detected using electromagnetic methods. If the contaminants are heavier than water, such as dense non-aqueous phase liquids (DNAPL), these chemicals will sink to the bottom of an aquifer. In this case, mapping the topography of the top of the aquitard (maybe a clay layer) that impedes the downward movement of the DNAPL may be valuable. Depending on the geologic conditions, this can be done using several geophysical methods such as electromagnetic, resistivity, refraction seismic, and possibly GPR. It is also possible that the suite of chemicals reacts with clay layers, altering their electrical properties. If that is the case, Spectral Induced Polarization (SIP) may be used. However, this application of the IP method is still being researched and should be considered experimental. Another method that can sometimes be used to detect chemically altered soils and overburden is GPR, since such changes can sometimes affect the dielectric properties of the overburden.

d)

http://en.wikipedia.org/wiki/Exploration_geophysics#Geophysical_methods

http://en.wikipedia.org/wiki/Ground-penetrating_radar

http://www.cflhd.gov/resources/agm/engApplications/SubsurfaceChartacter/633MappingContaminantPlumes.cfm

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