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A copy of the classroom presentation, in .pdf format, can be downloaded from here.

Textbooks referred to are:

Waltham: Foundations of Engineering Geology (W)
McLean and Gribble: Geology for Civil Engineers (MG)
Dennen and Moore: Geology and Engineering (DM)

Seismic | Electrical | Magnetic | Gravity

Geophysics

A science in which the physical properties of the interior of the Earth are studied, in order to infer information about the composition and structure of the interior.

• methods used to infer the distribution of rocks underground from physical measurements made at the surface
• especially for resource or development purposes, including environmental management

• Gravity field variations
  • sensitive to density distribution
• Magnetic field variations
  • sensitive to magnetic property distribution
• Electrical field variations
  • sensitive to conductivity distribution
• Seismic Velocity field variations
  • sensitive to elastic-property distribution

• Major: methods are surface-deployed (cheaper)
• Possible: responses average over larger volumes than test pits or cored boreholes
  • Geophysics often applicable within borehole to maximise hole value
• Disadvantage: interpretation equivocal (not for amateurs!)

Top | Electrical | Magnetic | Gravity

• Chiefly measure propagation of compressional (P) waves
• P velocity depends on elastic constants
• Energy may be transmitted and/or reflected at an interface
• Seismic Reflection and Seismic Refraction are two distinct methods

• Widely used in Engineering Geology
• Reports near-surface compressional-wave speed distribution
• Source of energy is mechanical impulse (hammer, explosives)
• Geophones (detectors) are "microphones"
• Example of recording system

• Waves from source critically refracted twice at interface, then detected by geophones
• Plot of wave traveltime vs distance from shot gives seismic velocities, depths of interfaces
• Closer analysis also gives variation in these

• Sudden collapse under house near old coal workings in Seattle.
• Where is the unsafe ground?

Final Interpretation

• Seismic refraction velocities often cited as guide to rippability (MG f 7.6, and see this case history)
• Fell (UNSW) pointed out that compressional wave speed alone cannot predict rippability
• Geological factors (discontinuity orientation/spacing, rock type and so on) are important modifiers
• Updated methods are available

• Better prediction comes by modifying seismic observations with geological observations
• Condition of rock mass, defect (discontinuity) distribution in rock material are important
• Quantified methods are available

• Travel-time, strength of "echoes" from layers below source measured
  • Travel-times give relative depth
  • Strength gives property contrast
• Repeated at close spacings along profile (compare echo-sounder)
• Result is seismic section, which mimics layer distribution in subsurface

• Interface (perhaps many) with depth variation not small compared with average depth
• Repeat experiment at frequent intervals

• Widely used in hydrocarbon search, because sedimentary layering can be reflection surface
• Not commonly used for geotechnical work, as reflections arrive "too early" and are confused with refractions, but abilities are growing
• More common in offshore work

Shallow seismic section

• Shallow seismic reflection widely used for marine surveys prior to dredging or raising offshore structures
• MG example is typical
• Note use of some drilling to control interpretation of seismic data
  • significant cost savings over grid drilling

possible groundwater sources

Another kind of resource case history...

• Major tool in oil,gas exploration
• Hydrocarbons trapped in deformed beds (for example, folds, domes)
• Relative travel-time of echoes shows relative depths
• Structural interpretation possible up to several km below surface

Top | Seismic | Magnetic | Gravity

• Electrical Resistivity: current introduced into the ground, and resulting potential field measured
• Electromagnetic Methods: time-varying currents are induced in the ground and fields radiated are measured

Definition:

The physical property which defines the resistance of a material to the passage of electrical current

• Most earth materials (silicates) very poor conductors of electricity
• Resistivity of earth materials (rocks, soil) depends on
  • porosity of material
  • resistivity of pore-filling fluid

Term "electrical resistivity" also used for process of measurement of the physical property

• Generally, rocks in situ are:
  • Porous, to a greater or lesser degree, and
  • Saturated,with a usually-saline water (pore fluid)

Four-electrode experiment used (MG f 6.12)

• If ground resistivity varies with depth, apparent resistivity varies with array spacing
• If ground resistivity varies laterally, apparent resistivity varies with array location
• Theory allows inversion of observations to construct (resistivity) model of ground
• Because resistivities strongly influenced by porosity, water quality, such models useful in geotechnical, environmental studies

• Client claimed landfill secured by clay seal (host rocks)
• Resistivity experiment shows low resistivity values below landfill ­p; clay?
  
  
• Subsequent drilling showed no clay, no seal
• Low resistivity below landfill results from leachate contaminating sandstone

Landfill study: Interpretation

Another example, from the web.

• Fundamentally similar to gold-detectors, airport metal detectors
• One loop carries AC, induces currents in nearby conductors
• Second loop detects EM waves from induced currents
• Signal levels reflect average resistivity of surroundings

• Similar application to "resistivity" method: changes in porosity, saturant are important
• Response also likely from metallic materials (wastes, UXO, services)
• No ground contact needed, so
  • rapid data acquisition on ground possible
  • airborne application possible

• Gravel deposits sought for road making
• Geological mapping shows gravel in river terraces near Broken River
• Clay overburden encountered, increases excavation costs
• Clay is electrically conductive, gravel more resistive

• EM method used to measure resistivities quickly
• Test proves correlation
• Mapping interpreted to show clay thickness directly

• EM analogue of seismic reflection
• Source is 80-300 MHz radar, at ground level, looking down
• Response affected by resistivity interfaces (including voids)
• Water affects response strongly
• Use limited to 1-30 m depth (that is, engineering scale)

• Strong radar reflections from rebar in concrete, here 10-20 cm in depth
• Rebar depth varies, result of improper construction
• Information useful for maintenance planning

Top | Seismic | Electrical | Gravity

• Two main components to Earth's magnetic field ­p;
  • Central field, from Earth's core
  • Local fields, from magnetised rocks
• Two chief sources of magnetisation in rocks ­p;
  • induced magnetisation of magnetite, widely but variably distributed
  • remanent (permanent) magnetisation, developed when rock forms

• Magnetometers measure variation in Earth's magnetic field, allow mapping at scale needed, from ground, aircraft, ship
• Applications vary widely, from
  • location of toxic-waste containers, to
  • mapping of depth of sedimentary basins

• Abandoned mineshafts were plugged with iron grid, then backfilled to surface
• Shafts on road easement pose collapse hazard during (and after) construction
• Iron (induced magnetisation) is physical target

• MG f 6.13 is good demo, but
  • Use should follow decision about cause of anomaly (estimate target properties first)
  • In this case history, the target was a metal object.
• Other applications
  • mapping firefronts in coal beds
  • mapping basalt (lava-flow) boundaries in water

• Pyroclastic flows have magnetite, so are magnetised naturally
• Tunnels are voids - non-magnetic
• Measurement of magnetic field variation interpreted to show location of voids
• Note use of test case!

Top | Seismic | Electrical | Magnetic

• "g = 10 m.s-2" is global average
• Local value is dominated by attraction of local masses
• If local rock density > average, then local g > average
• Variations in g then indicate subsurface density variations (geology)
• Gravimeters can resolve to 10 ppb of g

• Sparse borehole information extrapolated with gravity-field control
• Gravity var'n calculated from model section fits data

• Limestone landscape often develops cavities (caves, sinkholes) by dissolution
• Magnetics not appropriate (why?)
• Absence of rock ~ low density, so low gravity
• Detailed search shows location of voids
  
  
• Example from site evalution for runway construction

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