Geophysical Technique Capability Review Table v002
Instrument
Resolution (H)
Resolution (V)
Shallow FDEM
+/- 1m
n/a
(upper 5-7m)
Shallow TDEM
+/- 1m
n/a
(upper 3m)
1/10th depth
1/10th depth
GPR
Micro-gravity
Magnetic total
field/
gradiometry
(surface)
Electrical
resisitivity
tomography
Surface-nuclear
magnetic
resonance
1/5 depth
1/3 depth
Depth penetration /
detection resolution
Averages the properties of
the upper ~3 -7m of
ground
Averages the properties of
the upper ~3 m of ground.
Dependent upon
frequency, e.g.
1GHz – 1m
400MHz – 2m
100MHz – 6m
No depth restriction.
10 micro-gals
(Equivalent to e.g. a 2 meter
cylinder void at 8 meters
depth)
No depth restriction.
0.1 nT
1/5 of depth
1/3 depth
e.g. from soil variations
associated with
archaeological remains
200m is a typical
maximum – most surveys
are in the upper 50m.
Principal compromising factors that would reduce the ability to
detect the target^*
As sensitive to above ground conductivity contrasts (esp. any
metal) as to below ground features; Strong local EM fields (e.g.
power cables, transmitters, mobile phones, etc.)
Strong local EM fields (e.g. power cables, mobile phones, etc.)
Electrically conductive ground conditions may limit penetration
depth; uneven surface may cause air gaps beneath the antenna
which will compromise data clarity;
Vibration noise; soft/unstable ground; strong free-earth
oscillations;, rapidly varying topography; inversion to determine
the position and nature of the causative body requires a simple
geometry, and little or no other signals in the data.
Horizontal resolution dependent upon body geometry and survey
design. Vertical resolution often requires additional constraints
from other geophysical or investigation data.
As sensitive to above ground ferrous objects as to below ground
ferrous objects; Lateral resolution dependent upon the signal to
noise ratio, so will be compromised in areas of high magnetic
variability (e.g. areas of high anthropogenic materials, or certain
geological environments). Vertical resolution depends upon the
causative body being an isolated feature of known geometry
otherwise depth inversions are non-unique.
1/4 depth
1/10 depth
Limited by the maximum
distance between electrodes
and ability to drive sufficient
current through the ground.
Requires good access to set out long electrode spreads to achieve
large depth penetrations. Interpretation relies on robust inversion
of data sets, and as such is less certain in areas of high contrasts. In
particular very high or low electrical resistivity in the shallow
subsurface and significantly affect the reliability of data acquired
from greater depths.
Few m
dm (shallow)
to several 10m
(depth)
150m is a typical
maximum – most surveys
are in the upper 50m.
High and incoherent electromagnetic noise around 2kHz resulting
in low S/N is the main limiting factor. High magnetic susceptibility
and small pore sizes (e.g. clay) can prevent the detection of NMR
Depends on loop
layout and distance
(NMR)
between
measurements
Depth of penetration is
limited by loop size and
Electrically conductive
ground conditions
signals. Electrically conductive ground conditions (<100 Ohmm)
affect the sensitive volume and thus need to be considered during
inversion.
^All techniques seek to detect physical contrasts (density, elastic or electrical) between the target object and the surrounding ground materials. Greater contrasts are more easily detected, as
are larger, shallower targets. Deeper, smaller and less contrasting targets are correspondingly more difficult to detect.
*All anomalies of interest may be masked by the signals/responses generated by other features in the subsurface (or for some technologies also above surface) that may represent equivalent
or greater contrasts, and which would therefore mask or compromise the signal detectable from the target feature.