Quantitative review of existing techniques
To evaluate the potential benefit of quantum technology instruments, it is necessary to set out the capabilities of existing technologies.
We are looking for published examples that demonstrate the capabilities, but also the limitations of accuracy for different geophysical techniques and instruments.
With input from a number of experts, we have drafted the table below. It aims to set out the best resolution that a particular technology or instrument can deliver, and then to list the reasons why site conditions might mean that the ideal resolution cannot be achieved.
We invite the expert geophysics community to help us in 2 ways:
- Provide published quantitative examples of the capabilities and limitations of existing geophysical technologies.
- provide comments and additions for inclusion in the table below.
We have compiled a list of publications, and will also continue to update the table below with revisions.
Use the comments section below, or email to George Tuckwell.
Many thanks in advance from the SIGMA project team for your contributions.
Geophysical Technique Capability Review Table
(suggestions for a catchier title welcome!)
Version 002 (word version available here)
|Instrument||Resolution (H)||Resolution (V)||Depth penetration / detection resolution||Principal compromising factors that would reduce the ability to detect the target^*|
|Shallow FDEM||+/- 1m||n/a
|Averages the properties of the upper ~3 -7m of ground||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.)|
|Shallow TDEM||+/- 1m||n/a
|Averages the properties of the upper ~3 m of ground.||Strong local EM fields (e.g. power cables, mobile phones, etc.)|
|GPR||1/10th depth||1/10th depth||Dependent upon frequency, e.g.
1GHz – 1m
400MHz – 2m
100MHz – 6m
|Electrically conductive ground conditions may limit penetration depth; uneven surface may cause air gaps beneath the antenna which will compromise data clarity;|
|Micro-gravity||1/5 depth||1/3 depth||No depth restriction.
(Equivalent to e.g. a 2 meter cylinder void at 8 meters depth)
|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.
|Magnetic total field/ gradiometry (surface)||1/5 of depth||1/3 depth||No depth restriction.
e.g. from soil variations associated with archaeological remains
|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.|
|Electrical resisitivity tomography||1/4 depth||1/10 depth||200m is a typical maximum – most surveys are in the upper 50m.
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.|
|Surface-nuclear magnetic resonance (NMR)||Few m
Depends on loop layout and distance between measurements
|dm (shallow) to several 10m (depth)||150m is a typical maximum – most surveys are in the upper 50m.
Depth of penetration is limited by loop size and Electrically conductive ground conditions
|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 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.