Understanding DC IP/resistivity surveys
Six Commercial Survey Arrays
and what they measure
on their first shot.

Questions to ask your
"3D" geophysical survey provider.
Five critical elements
required for TRUE 3D results.

A slide show explains the True 3D data set
vs E-SCAN's strongest competitive method.  
The one-picture summary.

E-SCAN® 3D earth geo-electric mapping
(continuing in preparation 2019)

3D earth
test model
This project would have been easier on crew and equipment if it had started as planned in January, instead of in August's maximum heat.

Time: Completed on schedule.
Cost: Fixed budget, no additional charges allowed.
Risk to client: Zero. All risks to contractor.
The use of reduced-information derivative values to constrain 3D inversion model development is a significant disadvantage, proportional to the number of derivatives involved in each datum... dipole-dipole data are second-order derivatives, remember?

It is useful to reinforce the fact that the fundamental pole-pole data are individually more effective in guiding (forcing) the developing 3D earth model inversion. A compounding of this advantage occurs when the pole-pole array data set steps out from its conventional along-the-line survey arrangement and through E-SCAN technology measures cross-line, and diagonally, in fact everywhere, at every station on the property-wide grid setup. A few hundred one-directional data become thousands of all-directional, dense and uniformly sampled data, quite a different set of conditions to be matched by any 3D model inversion processing.

Conventional surveys deliver a single strip of gradients (voltage differences, as n=1 to 7 or n=1 to 10) always in one direction, radially outward from the centre of the potential field. For the same potential field, established by current injection at the same grid station, 3D E-SCAN technology allows measurement (not of gradients, but) absolute voltages at all of the grid stations surrounding the injection point, effectively mapping and defining the nature of the entire potential field as it surrounds the current injection point.

The information/data difference is immense. The multi-directional E-SCAN pole-pole data set would allow calculation of equivalent strips of dipole data that could be plotted radially outward in all directions from the current station,- the effective equivalent of having run four or eight separate, complete dipole surveys of the property, in varying survey line orientations. But with the advantage of not having derivative (dipole) data, but full-information, maximum-depth fundamental pole-pole data.

Effect on interpretation: The inversion process repeatedly (every iteration, for 20, 30, maybe 100 iterations) solves for a surface potential field (2D field of voltages) which it then compares to the observed field data, to see how close it is getting to reproducing a 3D resistivity model that could cause what we saw at surface in the field survey results. When the algorithm compares it's model-development progress to conventional pole-dipole or dipole-dipole field data (the sequence or strip of gradients or differences in a line out from the current site), it selects the few absolute voltage values that it needs to calculate derivatives (gradients, differences) that are equivalent in location to those we measured in the field. So with derivative data, one (or a few) sequence of gradients (not even absolute values) is the basis of comparison to see how the latest inversion model is doing in matching the field data. Because the field data are derivatives, the the matching can be done only after the computed model voltages field have been calculated (degraded) into equivalent derivatives (gradients, differences) for comparison. How inefficient,- and remember that the data we are trying to match are the dipole-dipole or the pole-pole measured values, when we could be comparing, matching and adjusting the model each iteration with the actual fundamental potential field voltages. Efficiency aside, which model will be most objective, most comprehensively constrained... one matching 256 short strips of one-direction voltage differences, or one matching 256 complete, area-wide absolute-voltage-mapped potential fields?

The example pictured here shows the best-alternative conventional data set (pole-dipole), and the 3D E-SCAN data set.
Absolute voltages, all relative to a the
same single remote point of reference.
The term "derivative" is used loosely here to describe a real and precise effect. A "difference" field or a "difference" voltage (potential difference) would also suffice, perhaps more accurately. ut this is not intended to be a working mathematical characterization,- just an easily repeatable reminder of the consequences of electrode positioning in a survey: the loss of signal strength and information by the subtraction or cancellation of primary or "fundamental" values... leaving a difference or, loosely, a derivative.

Summary: Avoiding using a measurement dipole avoids signal cancellation, thus allowing maximum-signal (maximum information) measurements. When the four-electrode measurement setup involves two dipoles, the current dipole delivers a subtracted net field - the first derivative - that is left over after a negative field is overlapped onto a positive field, from which a second dipole measures a voltage difference - effectively a derivative of a derivative, or, a second order derivative.

For our purposes: Derivatives mean reduced information,- subtracted remainders of fundamental information. Fewer is best. The dipole-dipole array uses two; the pole-dipole array uses one; the pole-pole array uses none.
Each pole-dipole measurement contains more information than the dipole-dipole array, covering a greater vertical sampling interval with higher signal resulting from a simpler stimulation patterns - a single current point potential field that is not decimated by a nearby opposite-polarity field. The dipole-dipole measurement is actually a derivative of these two pole-dipole measurements - one can calculate the dipole-dipole plotted value by subtracting the first pole-dipole measurement from the second, effectively subtracting what happens due to one current injection from what happens due to another.... which is precisely what is going on when the measurement is made in the dipole-dipole setup. Much of the deeper pole-dipole information, and a lot its signal strength, is subtracted away, leaving the smaller signal plotted at a shallower depth. This is another way of showing that there is more "information" in each of the pole-dipole measurements, representing what's going on across the deeper and wider sample interval (the 50% bar). When it comes to inversion modeling processing, far better to have two separate deeper-information data to work on, to accommodate in the developing model, than just the single dipole-dipole derivative value.
The dismal operating economics of conventional pole-pole surveys are completely overcome with 3D E-SCAN's high degree of automation of field processes. This enables E-SCAN to acquire a 15,000 datum field measurement set in 10 days, as compared to the four years (at 7 days per week) required to obtain the same data with conventional pole-pole array operations. Conventional pole-pole survey is not only difficult to set up initially, but it is also time consuming to to reposition the additional long wire needed for every measurement.
Up to 2000 watts of transmitter capability can be backpacked along survey lines to get the full benefit of the dipole-dipole array's operating efficiencies. This includes many battery powered units, and backpack motor-generator-powered transmitter systems (Phoenix Geophysics, formerly McPhar) up to 1.5, even 2 kilowatts. Once the required power levels exceed this level, stationary transmitters are used with pole-dipole arrays. Motor-generators powered systems (Walcer-Huntec, Scintrex, Zonge, others) providing up to 20 kilowatts (20,000 watts) of power may be employed. The 35+ kilowatt systems (Anaconda, Asarco, Premier, various USSR entities), most originating from the porphyry copper boom days many decades ago, are today mostly irrelevant, strongly outperformed in every aspect by 3D E-SCAN.
Edwards*, L.S. 1977. A MODIFIED PSEUDOSECTION FOR RESISTIVITY AND IP: Geophysics 42, 1020 (August 1977)
*U.N. Development Program, Rangoon, Burma
ABSTRACT: Dipole-dipole induced-polarization measurements are commonly presented as pseudosections, but results using different dipole lengths cannot be combined into a single pseudosection. By considering the theoretical results for simple earth models, a unique set of relative depth coefficients is empirically derived, such that measurements with different array parameters will "mesh" smoothly into a combined pseudosection. Application of these coefficients to a number of theoretical and field cases shows that they give reasonable results when applied to more complicated models. The empirical coefficients are compared with Roy's theory of "depth of investigation characteristic," and support that theory, if a modified definition of "effective depth" is accepted. This leads to an absolute depth scale for the modified pseudosection. It is shown that rough estimates of the depth to the top of an anomalous body can be made directly on the pseudosection, at true vertical scale. This definition of effective depth is applied to other electrode arrays. It is shown, by examples, that the resulting pseudosections give consistent estimates of depth to top, within the characteristic anomaly patterns of each array. The effective depths for various arrays are compared; the results agree with the traditional applications of each array.
For all arrays, the nominal depth of investigation, or effective penetration, is proportional to the distance separating the current input electrode(s) from the measurement electrode(s). Wider spread = deeper sampling.

Each measurement contains more information, covers a greater vertical sampling interval with signal resulting from a simpler
stimulation patterns - a single current point potential field that spreads uniformly in all directions. The dipole-dipole
measurement is actually a derivative of these two pole-dipole measurements - one can calculate the dipole-dipole plotted value
by subtracting the first pole-dipole measurement from the second,
effectively subtracting what happens due to one current
injection from what happens due to another.... which is precisely what is going on when the measurement is made in the dipole-
dipole setup. Much of the deeper information, and a lot of signal strength is subtracted away, leaving the smaller signal
plotted at a shallower depth for dipole-dipole. This is another way of showing that there is more "information" in each of
the pole-dipole measurements, representing what's going on across the deeper and wider sample interval (the 50% signal bar).

When it comes to inversion modeling processing, it is advantageous to have two separate deeper-information data to work on,
to force details to be accommodated in the developing model, rather than just the single dipole-dipole derivative value.

The colored bar represents the vertical distribution of 50% of the measured signal, and the plot point is the
median of that 50% interval - the nominal effective penetration "Ze" always assuming a uniform resistivity earth.
The more familiar NDIC (normalized depth of investigation characteristic) of Roy and Apparao uses the mean as
opposed to the median. The difference between the Ze and NDIC or DIC is insignificant for our purposes.
The signal in millivolts is based on an earth resistivity of 20 ohm-metres, and input current of 2 amperes, for comparison.
The comparative signal strength is based on a factor of "1" for the dipole-dipole array measurement.
The other current electrode has been positioned at the remote "infinite" location where it would remain for the duration of a pole-dipole survey.
Not to dismiss the fact that some normally arid areas may be surveyed most cost-effectively in the good weather immediately following the rainy season, when the ground is still damp near surface, providing easy electrode contacts.
With 3D E-SCAN's almost two-orders signal advantages, up to 100 times less current is needed for measurements, compared to a dipole-dipole survey. This means that transmitter voltage levels will be proportionally lower. What may have required 3000 volts for a conventional survey may only need 150 volts for E-SCAN, greatly reducing possibilities of current leakage in wet conditions. Transmitted voltages in wet conditions can be deliberately lowered even further if desired, by stacking signal a little longer,- a keyboard option that can be invoked at any time.
If conventional geophysics is "a bunch of things trying to go wrong at the same time", imagine 3D E-SCAN. That's why any normal deployment of an E-SCAN system includes many special supplies intended to avert or quickly fix the many things that can come up on a "normal" day.

For example, we don't count on line-of-sight radio contact,- we always bring a repeater station and the means to power it up in any location. We can't count on not finding wire-eating animals in an area, so we always bring a variety of supplies and devices so that, for example, where bite damage is repeatedly occurring, we can elevate (or re-route) the wire and avoid the next bite from the rabbit that claims that 50 metre stretch of desert as his own, every night. Armored cable, for road crossings where there are no trees to go up and over, and no culverts to go under.

We don't know when we will encounter an unexpected obstacle, so we bring wire flinging tools and supplies, and enough wire to re-route around. Sometimes a 250 gram lead fishing weight in your cruiser vest is the only thing keeping you from losing 15 minutes searching for a decent throwing rock. Extra batteries, and backups for every key instrument, ATV, radio, GPS and generator. Not ten rolls of tape,- 200.

Once we start, with all that wire spread out, we don't want to stop for anything, least of all to make a 100 mile drive to Wal-Mart for something we didn't bring, and know we should have, (only to find that Wal-Mart is sold out).

In rainy conditions, there may be no alternative other than to replace entire sections of wire that defy quick diagnosis and repair... the cost of replacement is often much less than the cost of delays in trying to detect a small insulation breach in a long wire. Having spare wire at the survey site is essential.

"A few hundred pounds of prevention... "

For all its complexity, 3D E-SCAN maintains an astonishingly high historical record of productive "up-time" in the field.
Animals damaging wire insulation is the principal
cause of wet-weather maintenance delays. Here -
sheep, cattle, deer, and elk farms are everywhere.
This survey was done under a daily rate contract... slower progress cost the client more, as did the need for two extra crew members to minimize rain repair delays.

Time and cost: probably 30% higher than would have
  been the case in the drier summer.
Risk to client: Some... in any intensively farmed
  areas there are many uncontrollable variables that
  affect the wiring system, especially with rain.
Premier Geophysics welcomes your questions and discussion on all technical issues of concern.

Transmitter-delivered current is digitized and recorded over exactly the same integration intervals as the measured signal. This means that if current pulses are irregularly tapering off due to arc-caused heating, the measured voltage remains in perfect correlation (for every pulse) so that the stacked signal/current ratio remains free of error.
Result:    Normal high-quality field data, from difficult current-injection conditions, with no extra demands on the field system operator.

Innovations like this don't come from "lab-coat" engineers.
E-SCAN's system designers bring over 75 years of hands-on field experience in low-power and high-power DC resistivity (and IP) to every aspect of instrument design.
Planning for this high-elevation Chilean project (which did not proceed for other reasons) included both the Pionjar gas hammer
(rock breaker) and the VibraCore hammer to drive electrodes through a caliche layer estimated to lie up to several metres below surface.
     Applying an ohm-meter (like the one on the front panel of most commercial IP/resistivity receivers) to an electrode in poor contact conditions may show an "acceptable" resistance (under the 1.5 to 6 volts DC stimulation) that is not present when ordinary millivolt (1/1000 volt) level signal is present. This leads to possibly strongly-attenuated data being accepted as valid, introducing false conductive values.
      Further, application of such high voltages (1.5 to 6 volts DC) often upsets the electrode-earth contact dynamics, setting up a signal-swamping electro-chemical voltage offset that may take minutes to settle out before a measurement can take place. That will slow survey progress.
     Frequent settling delays caused by too-high applied voltages tends to inhibit the routine use of contact testing by a production-driven operator, exposing a risk that attenuated (falsely conductive) measurement data may be acquired, unnoticed.
      Employing hundreds of automated electrodes demands systems that can routinely verify contact resistance quality, preferably without causing delays, and certainly without introducing new sources of measurement error. E-SCAN measures contact resistance as it actually occurs at the millivolt signal level, ensuring that any high-resistance signal attenuation is quantified and documented, and the means for data correction is preserved.    In the meantime, crew can be alerted to service a dried-out electrode (add some salt water) when convenient.
      There is no other responsible way to manage a survey, using any number of electrodes, in conditions of questionable electrode contact, i.e. in most hot/dry survey settings.
Veterans of Nevada geophysics will recognize this area northwest of Winnemucca
as being hostile to any geophysical methods requiring current injection.
Several IP/DC resistivity survey attempts were abandoned after failure to get
sufficient current into dry sand cover conditions, in order to make measurements.
For 3D E-SCAN, operating in this area was absolutely routine. No special equipment,-
just the usual 2-pound hammer and two electrodes per station. E-SCAN's fundamental
pole-pole array data require a small fraction of the current that is required by
conventional surveys that measure derivatives (pole-dipole and dipole-dipole arrays),
which makes all the difference in operating conditions like these.
The VibraCore system is most frequently used
for driving drill tubing for deep soil
sampling or tailings testing. Premier uses
the same electrode-driving adapters that
were made for the Pionjar gas hammer.
There are remaining limitations, just as with the X-ray CAT-scan. To name one: the issue of discrimination of a "stack of pancakes" feature from a single anomaly of similar overall proportions. Even E-SCAN can't help in this task.

The town occupies part of a larger area grid. The town had no effect on survey
data. The deep anomaly that partially underlies the town is defined entirely from
data acquired from electrode pairs spanning the town at a distance,- well away
from any in-town, near-surface infrastructure. Proof of this is shown when all
field data that originate in and near the townsite are eliminated from the data
set, and an identical anomaly image is still produced by the 3D inversion algorithm.
With reasonable road access on both sides of the river, managing the wiring changes required by the river made little difference to time and costs. Only occasionally would we have a dead-time while some crew had to move from one side of the river to the other - a total of 20 metres, by driving around the long way for half an hour.

Time and cost: This project was a daily-rate contract.
   Total survey time and costs were
   increased by 10%, not more.
Risk to client: none.
... so that the station continues to occupy a position at a node on the 3D inversion mesh that will be used later to process the 3D data, thereby eliminating entirely the post-processing adjustments that would be needed for a randomly-offset position.

Note the small white square denoting the actual size of the eventual 3D mesh elements. The small grey circle shows originally proposed grid station. Each crew member's GPS has pre-programmed positions for every possible 1/4 offset, i.e. every 3D mesh node. If the suggested electrode offset as shown in this layout map can not be reached, another alternate can be called up on the GPS and an attempt can be made to wire to that point. While air photos are wonderful, it's the boots on the ground that tell the final story. Especially in bad weather conditions, we need to keep things simple, and waste no time.
Beyond throwing distance, a slingshot with fishing line could be used to pull a heavier wire across. A potato gun proved effective for a couple of areas where we couldn't even get close to the river due to thick brambles and treacherous slopes. The need for such crossings is minimized by being able to wire entire sub-areas on the other side from just one crossing.
A locally-hired skiff was used by one crew person with a handheld GPS to locate and install the 18 in-river electrodes, taking up about a half-day.

Time: negligible
Cost: no additional charges to client
Risk to client: none.
3D E-SCAN has not yet been deployed on this type of survey.
This is a plan for a high-speed, several-per-week helicopter-
supported program for the comprehensive evaluation of 10 to
200 AeroMagnetics or Airborne EM anomalies.

Time: Several sites per week with a crack crew.
Cost: To be determined.
Risk to client: Probably a per-day, best-efforts operation first time out.
    Fixed pricing could also be calculated, helicopter at cost.
This project was planned as a partly-lake survey, well in advance.

Time: Faster than a summer open-water survey; as fast as
   the best open-terrain ATV project.
Cost: No extra charges.
Risk to client: None. Fixed price guaranteed, including mine camp room and board.
This project was obviously planned as a partly-lake survey, well in advance.

Time: Between no ATV's (all on foot, helicopter setouts) and
   large water areas, twice the time normally needed.
Cost: About 50% premium for conditions.
Risk to client: Minimal. Helicopters already in the area for drilling.
       Fixed price guaranteed, but camp and helicopter at client expense.
When an expanded-spacing test of 300 foot grid data such as this is done, it actually involves four separate inversions using four separate and distinct data subsets. Using a nominal grid starting point, the first data subset is comprised of data that correspond to grid points located on 600 foot intervals on both X and Y axes. Then the origin-point is moved 300 feet in the X axis, and a completely different 600 foot data subset is sorted and filed. The origin-point is now moved 300 feet in the Y axis, for the third unique data subset. Finally, the origin-point is moved back 300 feet on the X axis to generate the fourth unique 600 foot data subset.

The point of this is to establish empirically, using the actual field measurements, that regardless of where a 600 foot spacing grid is started, the 3D inversion model results will be the same. In this case, they were the same, confirming that 600 foot data would do the job effectively and without missing any important details. If the four 600-foot data subset models had disagreed, this would show that 600 feet was too coarse a grid for the area, and that the wider-spaced (much lower cost) surveys were inappropriate for use.

In a similar setting, 300 foot grid data for Paradise Peak were reprocessed as multiple 600 foot data subsets and 900 foot data subsets, all of which clearly and unambiguously mapped the three main mineralized zones. In the 900 foot data subsets, the subtle altered/mineralized areas (anomalous yellow in the 300-foot model) were not revealed. Decisions regarding district-scale survey dimensions would take that deficiency into account, perhaps settling on 600 foot grid spacing as the proven-effective, all-details-included grid spacing, or perhaps foregoing such detail in the interest of getting 40% greater area coverage for the same cost.
With commercial accommodation nearby, this became a routine 3D E-SCAN survey, very fast in and out. Completion on-time, on-budget.

Time: as predicted.
Cost: Fixed budget, no additional charges allowed.
Risk to client: Zero. All terrain risks to contractor.
For this debut 3D E-SCAN survey, extra time was budgeted for instrument calibrations, and some operating experiments. Dependence on daily helicopter lifts turned out to be over-estimated,- much of the survey layout and shooting was done entirely on foot, operating from the alpine camp.

Time: as predicted.
Cost: Fixed budget, no additional charges allowed.
Risk to client: ...who knew? First survey ever.
   No significant issues, no cost over-run.
Major funding/innovation contributors to the 12-year Premier Geophysics
   "extreme terrain mapper" (E-SCAN) research and development:
- the Geological Survey of Canada (GSC),
- the British Columbia Hydro and Power Authority (BCHPA),
- the British Columbia Department of Energy, Mines and Petroleum Resources,
- project managers for GSC and BCHPA - Nevin Sadlier-Brown Goodbrand Ltd. (NSBG)
- the Natural Sciences and Engineering Research Council of Canada (NSERC) with
- the University of British Columbia, Department of Geophysics and Astronomy (UBC).
Key individual acknowledgments - Dr. Andrew Nevin (NSBG), Dr. Jack Souther (GSC),
Josef Stauder (BCHPA), Dr. Douglas Oldenburg (UBC), Dr. Robert Ellis.
Weatherproof materials cases were lift-ready on the deck of the
ocean freighter when it arrived at Dutch Harbor from Seattle.
Dipole-dipole traverses (green lines) picked up conductivity (red dots) that was eventually
explained by 3D E-SCAN as originating far to one side of the traverses (red anomaly).
In the raw data image, the longer purple tics signify very conductive conditions, typical of the geothermal system already known in the area of the tics. Even in a single all-depths raw-data compilation plot like this, you can see a clear limit to the strong conductivity zone, at least two kilometres short of reaching the area (to the right) where road-building would be possible without extensive bridge construction. The issue had remained open due to the presence of normally-promising indications such as hot fumaroles and a hot drillhole (A-1, the black dot just below the word "bluffs") in the road-accessible area to the east. E-SCAN provided subsurface definition of permeability conditions in this area, reporting disappointing conditions. Favorable results would have looked like the area at the upper left... a dense area of long red tics, indicating low resistivity (high permeability) to go along with the high observed temperatures.
E-SCAN survey terrain, looking east from Fox Canyon through the deep canyon (gorge) at upper left.
The 250 metre deep canyon is near the centre of the grid layout. Electrodes missing from this area would cause gaps
in the measured deep data set for several kilometres in all directions, but not beneath the canyon area itself, which is
sampled using electrode pairs that are located far to all sides of the canyon.
Orange is simplex wire. Red is duplex wire for the bi-directional digital network communications,
plus the analog measurement circuit. The entire system as shown is set up before "shooting"
starts, to maximize completeness of the mapped fields.
This project was planned from a desk in San Francisco using maps and the client's description of site conditions. The project was a fixed-price contract, and was finished on time, on budget, and without excessive helicopter time.

Time: as predicted.
Cost: Fixed contract, no additional charges allowed.
Risk to client: none except possibility of helicopter cost overruns.

With no natural enemies, Aleutian foxes are remarkably unafraid. This fox
would happily remove electrical tape within easy reach of the crew person.
Edwards, L.S., 1977, A modified pseudosection for resistivity and IP: GEOPHYSICS, v. 42, p. 1020-1036.

See also: Roy, A., and Apparao, A., 1971, Depth of investigation in direct current methods: GEOPHYSICS, v. 36, p. 943-959.
Surveying the property with the pole-dipole array would supply only a small number of additional discrete data, but each pole-dipole datum contains more and deeper information than a dipole-dipole datum, helping to force the inversion algorithm to define a somewhat sharper, definitely deeper, and probably less-ambiguous earth model.
Note that this effective penetration varies when resistivity variables are present, as in cases such as suggested by the geologic outline behind.
Local volunteer Jim Fox arrives for work on the Makushin Volcano geothermal project. In these conditions, we'll take any help we can get.
Sparseness of observed data sets (and the limited directional information content of data sets) remain the key limiting issue in the EM methods. In any geo-electric earth modeling, the absence of sufficient hard data measurements with which to constrain and force the existing, competent inversion algorithms to resolve a low-ambiguity earth model constitutes a very serious limitation.
The value of inversions based on sparse, uni-directional field data such as these is strongly challenged within this website.
The paper refers to a geothermal exploration technique for circum-traversing volcanic edifices, seeking conductive anomalies only. Resistive epithermal gold orebodies lying along the route would probably not be detected, for all of the reasons elaborated and demonstrated on this website.
More than half of this website deals with demonstrating and proving this game-changing concept - that resistive targets in general have remained essentially undetectable by electrical resistivity survey tools, without any concurrent warning of the technical deficiency. Anomalies simply would not appear, while survey results looked competent, normal. We know that there has been some progress made in this area, but none of it comes even close to the performance demonstrated by 3D E-SCAN. Significance: most of the world's volcanic-hosted epithermal gold deposits will fall into this previously undetectable resistive-signature category. Implications are big, for finding additional orebodies of this type, under cover, where other exploration indicators are absent.
Which indeed they did... using the accepted state-of-art instruments and practices of the day. Only now we can see that those tools were deficient in several significant areas, so that only a narrow range of simple target geometries were actually reliably recognizable, with effectively no recognition of buried resistive targets at all.
Model example comparing 3D E-SCAN's true 3D raw field data set to that of a conventional, powerful pole-dipole DC resistivity survey.
The model site has been complicated by adding some topographic issues, and a large airport facility over the buried target bodies.
        See the slideshow above for full context and explanation. Step through the 15 slides at your own pace.
                For further explanation and illustrations see the "TECHNICAL BACKUP" link.

True 3D geo-electric models
      "True 3D" refers to a comprehensively hard-data-supported 3D inversion model, whose objectivity is secured provision of a genuinely 3D raw data set, one that is rigidly defined as (1) dense, (2) uniformly distributed, (3) all-directional, (4) consistently over-deep measured data, with inherent characteristics sufficient for the (5) objective recognition and correction (or elimination) of questionable data.

      Using the complete set of appropriately positioned and oriented raw data values, the True 3D process can actively define and constrain every part of the entire 3D earth model, including "uninteresting" background areas. Virtually no earth model aspect is left to the infill interpolation, extrapolation, smoothing or other programmed estimations that most inversion algorithms must employ to ensure a 3D model result through areas represented by sparse or incomplete raw data.
      It follows that, for an all-encompassing (True 3D data-based) survey, there can be no possibility of an initial conceptual error in survey parameterization (pre-survey choice of survey line orientation, depth estimate, model type estimate) because no such pre-survey guesswork is required. No pre-survey parameter selection is ever needed... 3D E-SCAN samples all parameters, every time.

      Non "True 3D", by this definition, would be those "3D-looking" earth models that:

    a) are directly 3D-inverted by a 3D algorithm using non-3D raw data, e.g. sparse, single- or non-directional in orientation, or having insufficiently deep raw data, at least one of which characteristics results from any line-type DC resistivity survey array and almost all EM methods,    or

    b) are cobbled together from 1D profiles or 2D sections, arithmetically merged or graphically stitched, based on the questionable assumption that an objective 3D model can be obtained from some number of adjacent 2D sections or 1D stacks.