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NOTE: These one-paragraph summaries do not replace
more complete coverage when learning about geophysical survey methods. They
are simply reminders. Each paragraph attempts to remind the reader about the
measurements, the relevant physical properties, typical surveying configurations,
and styles of interpretation.
If you are on the internet, a good summary (one long page) of
many shallow geophysical survey types is given at the website of Technos Inc.,
a geologic and geophysical consulting firm specializing in subsurface site
characterization for geotechnical, environmental, and groundwater projects,
based in Miami, Florida, USA. See http://www.technos-inc.com/Surface.html .
| magnetics | gravity | DC
resistivity | IP | seismic
reflection | refraction |
| GPR | EM terrain conductivity | FEM | TEM | VLF | RCPTU | others |
Magnetics:
Measurements of Earth's magnetic field will reveal subsurface variations
in magnetic susceptibility. The measurements record the sum of Earth's
field and fields induced in magnetic materials. More magnetic (ie
susceptible) materials have stronger induced fields. If the natural
field can be removed so that only induced fields remain in the data,
results reveal where magnetic material is, and to some extent, how
magnetic it is. Surveys are done from all conceivable platforms (ground,
vehicle, air, marine, satellite, in boreholes) and results are usually
presented as maps or profiles. Raw data may be Interpreted directly,
significant processing may be applied, and inversion to estimate
models of subsurface susceptibility distribution can be carried out.
This is one of the most versatile applied geophysics methods available. |
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Gravity:
Careful measurements of Earth's gravitational field
can reveal subsurface variations in density because gravitational
attraction depends upon the mass of materials. The measurements also
depend upon latitude, elevation, topography, and these need to be
recorded very accuratly for every measurement. Measurements require
meticulous care, but it is now common to carry out surveys on land,
on ships, using airborn platforms, within boreholes, and by observing
satallite orbit purturbations. Results are interpreted and managed
using procedures similar to magnetics, but data sets are necessarily
much sparser than magnetic surveys. |
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DC resistivity:
Electrical resistivity is measured by causing an electrical
current to flow in the earth between one pair of electrodes while
the voltage across a second pair of electrodes is measured. The result
is an "apparent" resistivity which is a value representing the weighted
averaged over a volume of the earth. Variations in this measurement
are caused by variations in the soil, rock, and pore fluid electrical
resistivity. Surveys require contact with the ground, so they can
be labour intensive. Results are sometimes interpreted directly,
but more commonly, 1D, 2D or 3D models are estimated using inversion
procedures. When the survey is conducted in order to obtain a 1D
model it is called a sounding. Profiles are used to build 2D models
of the earth. Multiple lines, or more complex electorde arragements
are used to obtain 3D interpretations. Soundings and profiles are
defined as follows: |

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Soundings are
surveys that are arranged so that measurements reveal vertical
variations in resistivity under one location. The earth is interpreted
in terms of flat lying layers. Results are often displayed rather
like a drill core. Roll your mouse over the image to see the
assumption. |
Profiles are
surveys that are arranged in order to be interpreted in terms
of vertical and lateral variations under a line of measurements.
Results are interpreted as a plane under the line. This is the
2D result. The assumption is that the structures extend without
change either side of the survey line. Roll your mouse over the
image to see the assumption. |
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Induced Polarization:
Induced polarization (or IP) is a secondary measurement
that can be made at the same time as DC resistivity if the correct
equipment is included. IP measurements respond to variations in the
capacity for subsurface materials to retain electric charge. The
principle materials that exhibit this property are clays, graphite,
and sulphide minerals. However, small changes in chargeability can
be detected when groundwater is contaminated with salt, hydrocarbons,
or other materials. |
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Seismic
reflection:
An impulse of compressional or shearing energy will travel from
it's source (such as a hammer strike on a metal plate) through the
ground. This energy will be reflected and refracted (bent) by changes
in the ground's elastic properties and density. Reflection surveys
are designed to record signals that have been reflected from boundaries
within the ground between materials with differing seismic velocity
(which is related to density and elasticity). Data require significant
processing before results are usable but seismic reflection surveying
represents over 90% of the geophysics done in support of exploration
for oil and gas. Results can be interpreted in terms of sub-horizontal
layering. Near-vertical contacts are difficult to image with seismic
reflection, although breaks in horizontal features can often be found.
Petroleum exploration work may involve investigation the ground to
a few kilometers of depth. Engineering scale surveys may involve
studing only the first few 10s of meters depth. |
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Seismic
refraction:
Refraction surveys are designed to record signals
that have been bent within the ground so that they arrive back at
the surface. The bending or refraction occurs owing to increasing
seismic velocity in the ground. The method is popular for mapping
sub-horizontal structure, but is not very effective at characterizing
sub-vertical features. Instruments are similar to those used for
seismic reflection surveys, but field layouts are different. Also
like reflection surveying, results can not be used without significant
processing. Surveys can be carried out at almost any scale from lines
a few 10s of metres long to lines many kilometers long. |
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GPR (ground
penetrating radar):
GPR is often characterized as "seismic" reflection with electromagnetic
energy instead of acoustic energy. Pulses of radio energy are emmitted
from one antenna and echoes are received at a second antenna. results
are plotted and sometimes processed, similarly to seismic reflection,
although data processing is usually much less intensive compared
to that required for seismic reflection work. The ground's electrical
resistivity controls the depth of signal penetration. Signals echo
at boundaries where electrical resistivity and/or dielectric permittivity
change abruptly. Penetration is usually less than 10 metres. |
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Electromagnetic (EM) terrain conductivity:
Electromagnetic methods involve using oscillating electromagnetic
energy which penetrates the ground and causes (induces) secondary
EM fields in regions of elevated electrical conductivity. Terrain
conductivity surveys usually involve a handheld instrument operating
at a single frequency. Some systems estimate terrain conductivity
at several frequencies. One transmitter coil generates the EM energy
and a second receiver coil detects EM fields caused by the transmitter
as well as fields induced in subsurface conductive regions. Large
data sets can be collected efficiently but results can not be used
directly to learn about variations with depth. Data are usually plotted
as maps or line profiles of apparent conductivity and interpreted
to find the ground positions directly above conductive features. |
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Frequency domain EM:
EM terrain conductivity discussed above is a form of frequency domain
EM (FEM). More sophisticated forms of FEM involve using more than
one frequency and/or coil configuration. Surveys can be carried out
from the ground or from airborne platforms (fixed wing or more commonly
systems towed by a helicopter). Results can be interpreted in terms
of one to 4-5 layers directly under sensor. Each set of measurements
at one location using a range of frequencies and/or coil configurations
represents a "sounding" (see DC methods
above). Information about the ground can be obtained from very shallow
to 100 or so metres |
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Time domain
EM:
Time domain methods are conceptually similar to FEM methods except
that instead of a fixed (or multiple fixed) frequency, the energy
source is a transient EM signal. The measurements involve recording
secondary fields that exist in the few micro- or milli-seconds following
the source signals transition. Like FEM surveys, each TEM measurement
is usually treated as a sounding, so many measurements must be carried
out to produce many soundings, which can then be intereted as a collection.
TEM surveys can be carried out on the ground or from airborne platforms,
and borehole TEM surveys are common for mineral exploration. Some
TEM systems can produce information about ground as much as 300 or
400 m deep, but most TEM surveys involve investigating only the first
few 100 metres or so. |
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VLF electromagnetics
(EM):
The VLF (Very Low Freqeuncy) band is a portion of the
electromagnetic spectrum that was used for very long distance communications
between the 1920's and 1990's. Some transmitters are still operational,
and these signals interact with shallow materials (within the top
20m-50m) in ways that can be measured. Results are useful for detecting
buried metallic objects, and (less reliably) for mapping variations
in electrical resistivity. |
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RCPTU:
RCPTU stands for "resistivity cone penetrometer testing unit". This
is a push-cone technology involving a system that pushes an instrumented
cone into the ground. The resistivity of the ground is measured using
a small scale 4-electrode system mounted just behind the cone, and
yields in-situ measurements of materials directly adjacent to the
instrument. Various other geotechnical and geophysical parameters
of the materials can be measured include shear strength, tip stress,
fluid permeability, pore pressure, friction, shearwave velocity (using
a surface source), and others. The image shows a CPTU truck operated
by the Department of Civil Engineering a
the University of British Columbia. |
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Other surveys:
Survey methods not discussed on this page include:
- Bborehole geopysical methods.
- Radiometric methods, which involve investigating the radioactivity
of ground materials.
- Natural source and controlled source magnetotelluric methods
(which involve low frequency EM sources).
- Sponteneous Potential methods, which involve measuring natural
voltages that occure in the ground due to movement of fluids or
chemical reactions between fluids and minerals.
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