Resistivity mapping for geothermal resources
SUMMARY REVIEW
Ground-operated DC resistivity techniques are applied on a regional reconnaissance scale over hundreds of square kilometers.
More detailed follow-up of identified target areas may span several tens of square kilometres.
A single geothermal system anomaly often spans many kilometres in width, and several in depth.
Conductive outflow plume anomalies have been mapped as far as 20 kilometres from their upwelling source locations.
In comparison to most mining geophysics, this resource mapping application is on a scale that is 1 to 2 orders of magnitude larger.
Appropriately scaled geophysical survey techniques are therefore designed and applied.
Until two decades ago, outlining a geothermally-prospective conductive anomaly represented the limit of geophysical capabilities.
Consequently, drilling (as an exploration tool) commenced at an early stage, often starting with thermal analysis "slim bore" holes, for example
absolute bottom-hole temperature, temperature gradient, heat-flow calculations. As the accumulating data and interpretations indicated the hottest and
most permeable target zones within the anomaly system, large bore test wells would then be drilled.
Today there are true 3D geo-electric survey techniques that can map structure and zoning within and beneath the typically broad conductive alteration
envelope of a geothermal system, providing more specific initial drill targeting guidance.
Expanding upon previously conductivity-focused investigations, the resolution of a
resistive core within a
conductive envelope is being proposed as a predictor of the centre of highest temperature (and possibly best productivity) in some geothermal settings (Anderson et al, 2000).
Electrical resistivity tools for geothermal exploration:
Reconnaissance stage: Reducing broad areas to manageable target zones of interest, e.g. 5 to 35 square kilometres in extent.
The traditionally successful tools are New Zealand style Schlumberger array mapping, and dipole-dipole array single-line traverse surveys.
Newer technologies build on their track record, improving both interpretability and performance, especially in rougher terrain.
The application of
airborne EM to some exploration phases is receiving increasingly favourable review.
Drill targeting stage: Resolving drill targets within a geophysically-indicated geothermal anomaly system.
Historically, every electrical geophysical technique has been applied for this purpose, with none appearing to demonstrate reliably consistent success in defining
sub-area preferential targets.
Newer technologies and interpretation approaches (such as 3D E-SCAN) are making progress.
Airborne electrical (EM) methods have been little used in the past, principally because the depth of
investigation has been considered "too shallow" for geothermal reservoir mapping purposes.
The use of airborne EM for reconnaissance purposes,
including outflow plume mapping where modest depth penetration is sufficient, has all but been ignored. Developments in deeper EM performance
(e.g. AFMAG-based ZTEM®)
and an understanding of how to employ shallow-depth airborne reconnaissance applications may be changing this.
An unsophisticated helicopter EM-33 survey mapped the valley-bottom outflow plume of the Meager Creek geothermal system,
effectively replacing (in a few hours) several months of hard-won dipole-dipole survey results. This demonstrates the possibility for
accelerated, low-cost reconnaissance exploration by airborne methods in equivalent or other specifically-calibrated geomorphic settings.
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Evolution of electrical resistivity technologies for geothermal exploration
The period 1960 through 1990 saw high levels of geothermal exploration world-wide, peaking in the 1980's with the oil shortage.
In North America, the accepted method for geothermal exploration geophysics was the dipole-dipole resistivity traverse,
which was applied across widely-spaced lines where possible, and in single lines where terrain would permit nothing else.
Single line dipole-dipole surveys produced exploration successes, despite very sparse data and ambiguities in interpretation.
In New Zealand, the presence of many roads and trails in the Taupo Volcanic Zone provided access for a
locally-developed method of continuous single-depth resistivity mapping
which continues to be used today, employing overlapping, fixed-spacing Schlumberger array measurements.
Schlumberger mapping, tuned specifically for local geologic conditions, produced many exploration successes in New Zealand.
Other more complex approaches were tried, but none attained widespread use.
Both of these reconnaissance methods were adapted and employed elsewhere.
Dipole-dipole survey results appear in reports from western Canada, Japan, Italy, the Philippines, Indonesia, Central America and many other places.
The NZ method was adapted for use in laterally delineating the Meager Creek geothermal resource in British Columbia. The adapted method was also
applied on the flanks of Mt. Baker, Washington, and more recently on geothermal mapping projects in Oregon and Nevada.
Common to both of these reconnaissance approaches is the simple objective of locating anomalous conductive values of significant volume and lateral extent, within a much
larger prospective region. The detailed examination and characterization of the located anomalies was treated as a separate exploration issue, using different methods.
To the extent that geothermal system signatures were actually detected and laterally constrained, DC resistivity established itself as a useful exploration tool.
The sophistication of accurate 3D imaging of anomaly envelope shapes and internal structures, and the discrimination of (more resistive) high-temperature
centers within conductive alteration zones would not emerge until toward the end of the century.
These advanced detailing techniques still depend upon the prior identification and location of a geothermal anomaly by some less costly, regional-scale method.
Without this preliminary delineation of an area of interest, the cost of the necessary wider application of modern 3D imaging would be prohibitive.
As a result, The Schlumberger mapping and dipole-dipole profiling styles of reconnaissance geothermal mapping continue to be used today,
both in their original formats and in updated variants.
Today, 3-point Schlumberger "3pS" mapping extends the NZ capability into extreme terrain conditions, delivering more deployment flexibility
to this low-cost application. 3-point measurements add an element of vertical gradient mapping as an aid to interpretation in settings that are less predictable
than the original Taupo Volcanic Zone application area.
LINEAR E-SCAN updates and obsoletes most dipole-dipole traversing by providing much-needed en-route spatial verification of the source location for
any conductive anomaly observed along the traverse route. LINEAR E-SCAN also significantly increases the along-line
swept volume by an order of magnitude over dipole-dipole, covering more ground to greater depths on each line traverse.
Some of the applications now managed by these reconnaissance techniques may be supplanted by lower-cost, equally effective airborne EM programs.
In the area of anomaly characterization, a.k.a. reservoir delineation, the 3D E-SCAN mapping tool represents the state of art in identifying and resolving any
geo-electric zoning and linear variations within an anomaly, while providing high-resolution imagery of the outer surfaces of the anomaly. This can include the detection
and imaging, on the underside of a conductive anomaly, of a continuing-deep extension that may represent the site of an inflow conduit.
The integration of 3D E-SCAN follow-up as part of the LINEAR E-SCAN reconnaissance mapping system adds true 3D anomaly location and characterization,
en-route, in-situ, in real time, producing potentially drill-ready
anomaly targeting during a brief interruption to the ongoing LINEAR E-SCAN reconnaissance mapping progress. The LINEAR + 3D E-SCAN approach has the advantage of
compressing the usually separate stages of reconnaissance surveying and later anomaly confirmation and evaluation into a single, cost-effective field event.
In many cases, this will allow the compression of several seasons of traditional exploration into single month-long period.
In a more difficult adaptation, it is possible that some of the advantages expected for reconnaissance mapping with airborne methods may be applicable to the deeper,
more difficult challenges of conductive anomaly characterization, i.e. discrimination of a deep-going conductive anomalous body from an area of surficial conductivity.
Preliminary indications suggest that airborne AFMAG-based systems may have some advantage here, even if the initial purpose is simply to pre-qualify sites prior to a
commitment to more comprehensive (and expensive) ground-based evaluation with 3D E-SCAN.
For example, an OK-in-theory "roving bipole" method applied in Kenya was short-lived after it was demonstrated that
actual ground conditions were the opposite to survey's interpreted results. This outcome was later understood to be related to some assumptions made for
distant conditions that could not be verified due to a lack of hard data observations at the intermediate distances. This example supports E-SCAN's
core notion that objective, unambiguous interpretations
require a dense, uniformly-distributed, all-directional raw data set as the basis for interpretation.
Citation to be added.
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Schlumberger area mapping - 2D pattern generation
"Perhaps the most successful application of electrical resistivity techniques for delineating geothermal systems has been in the early exploration of the
Taupo Volcanic Zone of New Zealand.
"Systematic mapping using the Schlumberger resistivity arrays with fixed spacings has
identified 23 individual geothermal systems with well-defined boundaries." (Bibby et al, 2005)
Using available
roads and trails, the
decades-long New Zealand project collected resistivity data using simple Schlumberger arrays with AB/2 spacings of 500m and sometimes 1000m. The preferred AB/2 array
spacing of 500 metres (nominal depth of investigation of about 190 metres) was selected on the basis of observations that it produced the greatest contrast
in electrical resistivity across the boundary of a selection of observed high-temperature geothermal systems (Bibby et al, 2005).
The success of the method in New Zealand is in part due
to the uniformity of resistive young volcanic coverage throughout the area, providing good electrical
resistivity contrast between non-geothermal areas (at around 100 ohm-metres) and the highly-conductive altered conditions over geothermal systems (5 to 30 ohm-metres).
all within the first 250 metres of depth that is sampled by the AB/2=500m Schlumberger array measurements.
The array design was thus "calibrated" against known local
geologic conditions, an always prudent and sometimes
critical aspect of exploration planning.
In contemplating application of this exploration mapping method in different conditions, one must obviously consider the new setting's characteristics
and adjust survey parameters and interpretation considerations to maximize the chances of delivering useful exploration information.
Meager Creek Geothermal Area, British Columbia.
When Schlumberger mapping was considered as a means of closing off the upslope conductive anomaly at the Meager Creek geothermal project, it was known that there would
be no uniform resistive layer within which to easily see alteration anomalies. Exposed rocks in the Meager Volcanic Complex ranged from extensively weathered volcanics,
to fresher flow rocks, to areas of exposed crystalline rocks, interspersed with glacier ice, snowfields and loose debris. It was decided that if we were going to spend
the money and effort to access survey layout positions (mostly by helicopter), we would measure three points, not one, to maximize accumulated information.
We did not have the benefit of local examples of geothermal systems on which to test array results. However, at the time, we Canadians encouraged by Peter MacDonald
of the NZ
DSIR
to remember that (roughly recalled) - "an active geothermal system will have spent a long time vigorously assaulting the overlying rocks with
various hot acidic gases and thermal fluids that will in time substantially alter
any type of overlying rock.
The alteration signature should be there for the mapping." ...
Indeed it was.
3pS on Mount Baker, Washington State
Operating conditions on Mt. Baker are significantly better than those in the Meager Volcanic Complex. Conditions are similar to those at other Cascades
volcanoes like Rainier and Shasta, with road systems in most of the valley bottoms, and a tracery of logging roads providing access upslope in many areas.
One area of interest on the southwest slope of Mount Baker surrounds a hot springs camping area. A 3pS survey was conducted on logging roads throughout the
area of interest, but did not encounter any anomalous conductivities that would signal either a direct resource hit or an outflow plume to be tracked upslope.
3pS at Crump Geyser, Oregon
3pS survey at Crump Geyser covered a large area in the
valley, eventually establishing a partial boundary for the
conductive conditions that enclose the geyser site.
3pS in Nevada
A 3pS survey was undertaken at Salt Wells, near Fallon, Nevada, mapping conductive patterns through and around the
Bunejug
Mountains, and extending over the flats to Highway 50.
Numerous 3pS-suggested full Schlumberger vertical electrical soundings
(VES) helped confirm interest in certain
anomalous sub-areas, inviting further investigation. Part of this area was also subjected to a full 3D E-SCAN survey in a location selected on the basis of
an independent, geology-derived deep structural model.
At Black Warrior, 3pS survey confirmed an extremely large area of probable thermal conditions, probability based on interpretation of the absolute
conductivity values and the vertical ratios of the 3-point measurement sets. Complete Schlumberger vertical electrical soundings (VES) were applied in 3pS-indicated
areas of interest to verify conditions at depth. The 3pS and VES surveys were unaffected by the high-voltage
Pacific Intertie
power line that bisects the property.
Plotting and reporting field data
Using a
standardized field procedure and standardized
coded icons
for plotting, the survey results offer several ways to visualize groupings and evaluate the significance of different measured responses across the
property. At right, the fully-annotated
"resistive-over-conductive, less than 5 ohm-metres" icon relates to station serial number 11.
AB/2 measured values for 250m, 500m and 1,000m are 5.7, 6.0 and 4.8 ohm-metres (apparent resistivity), respectively.
A
1D inversion of the three measured data delivered a two-layer earth model of 6 ohm-metres transitioning
at
685 metres below surface to 1.7 ohm-metres (true resistivity).
The strong vertical ratio of more than 3:1 earned a yellow emphasis annulus. The permissible 1.7 ohm-metre deeper zone is a potentially significant
response in a known geothermal area; it's possibility is not intuitively suggested by the very average looking measured values of 5.7, 6.0 and 4.8.
Mouse-over the image at right to see this station plotted in the context of several surrounding stations.
Plotted data may be grouped according to common aspects, especially those suggestive of geotehrmal conditions, whether deep, large volume bodies or lateral
outflow plume signatures. VES soundings are plotted in-position on the same maps. The intention is to identify a smaller percentage of the surveyed area as
geothermally prospective, as part of first-pass measures toward more efficient planning of temperature gradient drilling and other programs.
Department of Scientific and Industrial Research, the principal centre of geothermal research in New Zealand at the time, and a world leader in geothermal exploration expertise.
We acknowledge that a determination of a layer at 685 metres below surface from three points with a maximum AB/2 of 1000 metres
may (and should) generate questions, but the
model is found to be feasible, and is reported verbatim (as delivered by the 1D algorithm) under the always-overriding caveat "permitted model".
This may be an area where a full 1D inversion (to AB/2=3000m) would be employed to confirm a geothermally-interesting situation. Several other sites
near this one report similar deep conductivities beneath 400 to 500 metres of more resistive material, lending support to the model.
A 1D inversion using just three data points (where 10 to 20 are normal) obviously generates a non-unique layered earth model.
We aren't even sure that the situation at the site is layered in the first place. As a result, the suggested model is taken somewhat
lightly as "one possible or permitted layered model, if the area is in fact layered".
We are not entirely haphazard about this. Experiments were done with fully-populated 1D inversions
followed by repeat inversions employing fewer and fewer field data, until just the three at AB/2 = 250m, 500m and 1000m remained. Some
minor adjustments to the 1D algorithm controls revealed the best settings for simple 1, 2 or 3 layer model results that consistently provided
reasonable correlation with the larger data sets from which the three test sample data had been extracted. That process done, we have maintained the
same setup, for the same 1D algorithm, effectively normalizing for all projects those arbitrary elements of bias or trends that originate in the
processing algorithm itself.
Keeping in mind these limitations, we can comfortably compare, group, categorize and otherwise work with the computed layered result "suggestions" as plotted
beside each data site icon (and its three field data observations). When some area gains specific interest as a result of these layered
suggestions, a full Schlumberger sounding may be positioned in order to confirm and detail the vertical layering with a full 15- or 20-point data set.
Many measurements made at AB/2=1000m (effective depth 380m) began to encounter a source of
non-geothermal conductivity, a pervasive old ignimbrite layer which had over time undergone low-temperature diagenetic
alteration to conductive clays (Bibby et al, 2005). This layer presented a very conductive signature
which is indistinguishable from the surface-observed conductivity associated with active geothermal alteration.
Statistically, measurements at AB/2=500m avoided contact with the deeper non-geothermal conductor, preserving
a very reliable correlation between AB/2=500m conductive zones and in-situ geothermal systems.
This is a cautionary example for those who may strive for "depth" as a first survey objective.
Clearly, the application of one-parameter resistivity sampling across any wide area requires careful consideration of what is being (and might be) measured,
and how best to interpret the pattern of results to the true benefit of the survey objective.
Bibby et al (2005) is enlightening as much for its application caveats as for its confirmation of the face-value validity of (selected) New Zealand
Schlumberger mapping results.
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