Geothermal Geophysics Overview and Resistivity Techniques

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Geothermal Geophysics Overview and Resistivity
Techniques
by William Cumming Cumming Geoscience Santa Rosa
CA wcumming_at_wcumming.com
Cumming Geoscience 2006
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Geothermal Exploration Geophysics Questions

• When geophysics is integrated with complete data
• set in a consistent geothermal conceptual model
• What drilling target is lowest risk
• What is the capacity of the reservoir in MW
• Where should wells be targeted to prove that
  resource capacity
• What is the likelihood of success for the next
  well(s) based on analogous downside upside and
  most likely conceptual models
• What range of resource capacity is consistent
  with the geoscience data set based on analogous
  downside upside and most likely conceptual
  models

Cumming Geoscience
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Geothermal Geophysics Overview

• What surface methods are used
• What do they measure
• How do they contribute to the conceptual model of
  the resource
• Why are resistivity methods relatively popular
• Why is MT a popular resistivity method
• What are some of its pitfalls
• What are the advantages of conceptual targeting
  as opposed to anomaly targeting

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Geothermal Geophysics Methods

• Mainly adapted from the petroleum and mining
  industries.
• BUT
• Mining has shallower smaller targets.
• Petroleum has different imaging needs in a
  different geological setting making reflection
  seismic the preferred technique.
• Petroleum and minerals have more value per
  explored volume than hot water.

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Surface Geophysical Techniquesin Geothermal
Exploration

• Infer geothermal resource characteristics for
  well targeting and resource capacity estimation
  by remotely constraining rock properties such as
  
• Resistivity using MT TDEM VES CSAMT HEM
• Density using gravity and seismic
• Magnetic susceptibility using magnetic field
• Seismic velocity using active seismic and MEQ
• Natural electrical potential (V) using SP
• Crack density and stress using MEQ and active
  seismic
• Seismic impedance using reflection seismic

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Geophysical Acronyms
MT Magnetotellurics AMT Audiomagnetotellurics T-M
T Telluric-Magnetotellurics CSAMT Controlled
Source Audiomagnetotellurics HEM Helicopter
Electromagnetics TEM Time Domain Electromagnetics
(also TDEM) VES Vertical Electrical Sounding
(Schlumberger) DC-T DC Tomography (E-Scan and
Zonge) SP Self-Potential dGPS Differential Global
Positioning System MEQ Microearthquake
Cumming Geoscience
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Surface Geophysical Techniques in Geothermal
Exploration
Common (sometimes justified) assumptions Standard
MT with TEM for statics Case by case MT
VES CSAMT AMT TEM DC-T HEM Gravity SP
Reflection seismic Aeromagnetics Precision
Ground Magnetics Legacy Dipole-Dipole Tensor
Dipole-Bipole Research Reflection / Refraction
Seismic Development Microgravity
Microearthquake Subsidence Proprietary E-Scan
E-Map Suspect Seismic Noise Low Res Ground
Magnetics Plausible
methods with weak technical support
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Geophysical Exploration of Geothermal Systems
lt200C
gt200C
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Geothermal Resource Characteristics Affecting
Geophysics

• For gt200C Issue
• Reservoir top usually 300 to 1000 m deep Deeper
• Reservoir thickness 300 to 3000 m Thicker
• Testable wells usually gt1.5 million Wells cost
  more
• Commercial wells usually gt3 million
• For lt200C tabular Issue
• Reservoir top usually 100 to 800 m
  deep Shallower (for now)
• Reservoir thickness 100 to 1000 m Thinner
• Testable wells sometimes lt1 million Wells cost
  less
• Commercial wells usually 1.2 to 3 million

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Geophysical Exploration of Geothermal Systems
lt200C
gt200C
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Geothermal Resource Characteristics Affecting
Geophysics 2

• For lt200C upflow Issue
• Reservoir top usually 300 to 1000 m deep similar
  to gt200C
• Reservoir thickness maybe gt1000 m Fault zone
• Testable wells usually gt1.5 million Wells cost
  more
• Commercial wells usually 1.5 to gt3 million
• For HDR/EGS Issue
• Reservoir top created Dynamic
• Reservoir thickness created Dynamic
• Testable wells cost more Wells cost more
• Commercial well cost a research issue

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Geophysical Exploration of gt200C Geothermal
Systems

• Resource image area gt 1 km2 often
  gt 4 km2
• Exploration image area gt 4 km2 often
  gt 50 km2
• Depth to reservoir top 300 to 2000 m
• Access often rugged
• Environmental issues

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Conceptual Objectives for Exploration Geophysics

• Where is the reservoir How big is it
• Isotherm geometry the overall permeability
  constraint for reservoir simulation can be
  constrained using low resistivity temperature
  sensitive clay alteration
• for gt500 m depth MT. Possibly VES DC-T
  CSMT etc.
• for lt500 depth TEM CSMT AMT VES DC-T etc
• What can extend surface geology deeper
• Gravity lithology dense alteration basin
  geometry
• Magnetics near-surface sulfate alteration
  volcanics
• Where are specific well entries
• Rare validated success cases few partial
  technical successes and numerous under-reported
  failures
• Reflection seismic in geothermal often poor
  quality. Can characterize structure. However
  entries rarely imaged.
• Geochemistry and geology used with resistivity to
  map leaky clay cap alteration intensity and
  geometry (i.e. detect sweet spots not entries)

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Surface Geothermal GeophysicsOther Than
Resistivity
Magnetic Ground precision magnetometer with dGPS
mapping near-surface volcanicsAirborne mag maps
alteration of magnetite (sulfate) Gravity Standard
exploration gravity used to map volcano and
basin geometry and silicification Seismic Reflecti
on and refraction (active) seismic to map
structural setting and faults MEQ Microearthquake
(passive seismic) used after development which
can make seismicity reliable. SP Self-Potential
(natural surface voltages) for hydrology in areas
with low relief dGPS Differential Global
Positioning System made many of the other methods
cost-effective
Cumming Geoscience
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Geophysical Exploration of lt200C Geothermal
Systems

• Resource image area gt 1 km2 often gt 4 km2
• Exploration image area gt 4 km2 often gt 20 km2
• Depth to reservoir top 100 to 1000 m
• More like exploration for aquifers than for
  minerals or petroleum.

Cumming Geoscience
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Conceptual Objectives for Exploration Geophysics

• Where is the reservoir How big is it
• Isotherm geometry the overall permeability
  constraint for reservoir simulation can be
  constrained using low resistivity temperature
  sensitive clay alteration
• for gt500 m depth MT. Possibly VES DC-T
  CSMT etc.
• for lt500 depth TEM CSMT AMT VES DC-T etc
• What can extend surface geology deeper
• Gravity lithology dense alteration basin
  geometry
• Magnetics near-surface sulfate alteration
  volcanics
• Where are specific well entries
• Rare validated success cases few partial
  technical successes and numerous under-reported
  failures
• Reflection seismic in geothermal often poor
  quality. Can characterize structure. However
  entries rarely imaged.
• Geochemistry and geology used with resistivity to
  map leaky clay cap alteration intensity and
  geometry.

Cumming Geoscience
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Surface Geophysics Methodsfor Resistivity
Resistivity Using Natural Signals MT
Magnetotellurics (for gt1000 m) AMT Audiomagnetote
llurics (lower cost to 1000 m) T-MT Telluric-Magn
etotellurics (sometimes lower cost) Resistivity
Using a Transmitter Source TEM Time Domain
Electromagnetics (real 1D to lt500 m at lower cost
) CSMT Controlled Source Magnetotellurics (if
noisy) DC-T DC tomography mainly E-Scan and
Zonge especially for resistive zones like
sinter VES Vertical Electrical Sounding (good old
data)
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Geothermal Resistivity Pattern
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Resistivity Acquisition Issues

• Noise
• Pipes fences power lines and similar metal
  features usually require a standoff typically
  100 to 1000 m depending on method
• Near power plants passive methods like MT are
  doubtful and all methods suffer from higher noise
• DC power lines can limit MT depth of
  investigation to lt1000 m at 30 km distance and
  lt5000 m at 100 km distance
• Statics
• Static distortion affects all methods that use
  electrodes (all but TEM)
• Difficult to avoid in volcanics or rugged areas
• Static correction by inversion smoothing is
  sometimes unrealistic
• Access
• Cost rises steeply if access to sites is poor
• Faster methods like T-MT reduce cost only where
  access is easy
• Cable oriented methods require wide and
  continuous access so more suited to Nevada than
  New Zealand

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MT Method
Alteration
Clay Alteration

• 1 Hz is about 1 km down
• Shallow features like surface alteration result
  in different resistivity on Ex and Ey dipole.
  This is called static distortion.

• Ex Ey 2 dipoles 100 m
• Hx Hy Hz 3 magnetometers
• EM signal from sun and electrical storms
• Solar signal sometimes low

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MT Physics
Geophys.washington.edu
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MT Physics
Joe its movie time.
Geophys.washington.edu
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MT Method

• Ex Ey 2 dipoles 100 m
• Hx Hy Hz 3 magnetometers
• Horizontal magnetometers buried in shallow
  trenches

• Equipment portable
• gt8 hours so one station/day
• Two stations record at once to provide remote
  reference noise reduction

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T-MT Method

• Continuous line of 25 T stations 100 m apart with
  one MT station
• Real time processing can reduce noise from
  intermittent sources
• Statics due to topography can be modeled if dip
  is along line

Quantec 2003

• 100 m station spacing is expensive for large
  fields
• Improved imaging of base of clay is shallow in 2D
  geology

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TDEM / TEM

• Pulse current in outer loop measure signal in
  inner loop from smoke rings of current induced
  by magnetic field.
• TDEM depth oftenlt 300 m ltltMT

• No electrodes so no static distortion
• Focused so less 2D/3D distortion
• Noisy data or no signal is sometimes
  misinterpreted

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TDEM / TEM

• Equipment very portable if using batteries for
  lt300 m
• Generator needed for gt300 m

• Records in minutes so usually 1 to 7 stations/day
• Interpreting reliable data is usually simple

Cumming Geoscience
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Standard Geophysical Plan gt200C Geothermal
Exploration

• MT to map base of clay cap
• Gas and fluid geochemistry for conceptual target
• Maybe TEM for MT statics and detail
• Maybe gravity for lithology and large structure

Cumming Geoscience
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Resistivity Objectives in Geothermal Exploration

• Map structure and conductance of lt180C low
  resistivity smectite clay zone capping the
  relatively resistive reservoir
• Integrate with geochemistry and geology to
• Estimate resource capacity
• Target wells for high temperature permeability
• Estimate risk probabilities

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Geothermal Resistivity Pattern
Cumming Geoscience
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Salak Geothermal FieldMT Cross-sectionMT
Resistivity with MeB Smectite Isotherms from
Wells
Cumming Geoscience
from Gunderson Cumming Astra and Harvey (2000)
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MeB Analysis of Cuttings
Grind Cuttings
Suspend Powder
1.
2.
Add MeB Increments
Detect Excess MeB
3.
4.
from Gunderson Cumming Astra and Harvey (2000)
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Standard Geophysical Plan lt200C Geothermal
Exploration

• Lowest cost resistivity to reach base of clay
  cap maybe AMT CSMT DC etc.
• Temperature Gradient Holes if access and drilling
  are low cost.
• Ground magnetics and gravity for geology and
  alteration mapping.
• SP if target simple shallow and low relief
• Reflection seismic if structure is simple and
  manifestations are weak

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Geothermal MT Interpretation Pitfalls

• MT cross-section without distortion shows classic
  geothermal cap geometry
• Deep low resistivity zone (red) below Station 1
  misinterpreted as reservoir
• Vertical low resistivity contours below Station 2
  misinterpreted as fault
• MT imaging of resistivity distorted by
• noise near station 1
• static at station 2

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Exploration and Evaluation Stages and Styles

• Well Targeting
• Anomaly Stacking
• Anomaly Hunting
• Conceptual Model
• Case Histories Risk
• Statistical Database

• Resource Capacity
• Monte Carlo
• Anomaly Comparison
• Conceptual Simulation
• Case Histories Risk
• Numerical Reservoir Simulation with Cases

Cumming Geoscience
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What To TargetAnomaly or Conceptual Model

• Anomaly hunting
• Works by analogy
• Depends on unexamined conceptual relevance of
  analogies
• Other data can be stacked but not consistently
  integrated
• Not directly tested. Drill a 5 ohm-m anomaly and
  its still 5 ohm-m
• Anomaly stacking
• Uses some geoscience and more psychology to deal
  with uncertainty of resource decisions
• Coincident patterns supporting a decision are
  comforting even for experts who suspect patterns
  are redundant or irrelevant
• Conceptual model targeting
• Requires expert integration of details of
  geophysics geochemistry geology and reservoir
  engineering
• Decision making more reliable if evaluated with
  respect to actual experience in conceptually
  analogous situations
• Allowance can be made for conceptual differences
• Directly tested by wells

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Uncertainty in Geothermal Resistivity
Interpretation

• Noise and 3D Distortion
• Anomalous parts of images should be checked for
  underlying data quality issues
• Acquisition and interpretation should be done by
  different entities
• Conceptual Interpretation
• Resistivity methods can image the intensity of
  hydrothermal clay alteration and the geometry of
  the base of the low resistivity clay cap
  conforming to the geothermal reservoir
• However the apex of the clay cap may be over the
  shallowest permeability but not over the deep
  high-temperature upflow which must be inferred
  from less reliable alteration intensity.
• so
• Check conceptual advantages of other methods
• Integrate with geochemistry and geology
• Drill a conceptual model NOT an anomaly
• Validate geophysical interpretation after drilling

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Glass Mountain Geothermal FieldMT 1D-2D-3D
Resistivity and Well 17A-6
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Audit Geophysics with Well Data
e.g. Joe Moore pointed out lithologic
permeability at Bulalo
Litho/Structural Facies Model of Bulalo Reservoir
Moore 2006
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Cost for Geophysics
Includes acquisition some imaging but not
integrated interpretation. MT lt0.02 to gt300 Hz
Low cost Sites lt 500 m from vehicle. lt 1 hr
to easy camp etc. High cost
gt30 sites gt 1 km from vehicle. gt 1 hr to camp
etc. Method Cost / data unit Mob
misc MT 1.4k - 3.5k / MT 5k - 30k T- MT
0.4k - 1.4k / T 8k - 35k T-MT T-MT
Profile 4k - 12k / line km 5k - 45k CSAMT 2k
- 8k / line km 3k - 30k TDEM 0.4k - 2.0k /
TDEM 3k - 15k DC-T (E-scan) 7k - 11k /
km2 7k - 20k GravitydGPS 30 - 90 /
station 2k - 15k
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Geothermal Geophysics Overview and Resistivity
Techniques
by William Cumming Cumming Geoscience Santa Rosa
CA wcumming_at_wcumming.com
Cumming Geoscience 2006
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VES and Dipole-dipole Resistivity at Cerro Prieto
Charre-Meza et al 2000
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VES Resistivity

• Vertical Electrical Soundings ( also known as
  Schlumberger or DC Soundings ) transmit current
  in one expanding dipole and measure voltage
  across a smaller centered dipole.
• Use 2D images from VES for well targeting and
  resource capacity single dipole spacing for
  reconnaissance
• In geothermal areas depth of resolution is about
  15 to 25 of transmitter dipole length.
  Transmitter dipoles sometimes must be gt5 km long
  to resolve top of relatively resistive reservoir.
  
• Reprocessing old VES data to 1D/2D smooth images
  is often worthwhile if transmit dipole large
  enough (AB/2 gt 2 km)
• Environmental issues cost and logistics limit
  new surveys

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VES and Dipole-dipole Resistivity at Cerro Prieto
Charre-Meza et al 2000
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CSMT Profiling

• Scalar MT profiling using a wire transmitter
• Costs lt MT
• Active source better near some noise sources
• Cannot as reliably detect or correct static and
  2D/3D distortion
• Near field transmitter distortion
• Higher frequency so depth lt 200 to lt 1000 m
• Fewer imaging and processing options

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SP

• Self Potential (SP) profiling measures voltage
  across a dipole to map V/m.
• Low cost requires 2 people with wire
  volt-ohmmeter and electrodes.
• SP pattern mainly reflects electro-kinetic
  effect water flow in shallowest aquifer.
• In geothermal prospects thermo-electric effect
  is significant but ambiguous.
• SP anomalies may indicate faults or aquifer
  geometry.

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SP

• Case histories show SP can characterize upflow
  and shallow outflow aquifers in areas with gentle
  topography.
• Near-surface groundwater signal is strongest so
  even rainfall significantly changes SP patterns.
• Cost is relatively low but so is relevance
  especially for deeper resources.
• SP mainly used to characterize shallow low
  temperature systems.

Mokai
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Hochstein et al. 1990
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Microearthquakes (MEQ)in Geothermal Exploration

• Numerous conventional (not noise mapping) MEQ
  exploration surveys had little or no success at
  wildcat geothermal prospects.
• Limited exploration successes (Simiyu and Malin
  WGC 2000) were on the margins of developed
  fields.
• Most geothermal fields that have been monitored
  prior to production are relatively aseismic.
• Tests to check if this was due to an unusual
  number of small events relative to larger
  earthquakes have not found this to be the case.
• After production most fields that have deep
  injection have an increase in local earthquakes
  but several large fields with deep injection and
  production remain relatively quiet. Most shallow
  fields remain aseismic.
• Although MEQ monitoring is a common geothermal
  development tool at fields where many MEQs are
  detected the episodic data and high cost make it
  a risky exploration tool.

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Magnetic Surveys

• Map local variations in earths magnetic field
• in volcanics correlates with magnetite content
• Aeromagnetic survey magnetometer in plane
• Draped is usually better constant elevation is
  easier
• Used to
• characterize extent of alteration especially
  related to SO4 destruction of magnetite
• map structure and lithology
• Ground magnetic survey 1 person walks profiles
• Proton precession magnetometer usually saturated
  and under-sampled near volcanics
• Cesium-vapor magnetometer data every 50 cm using
  dGPS can map near-surface geology.

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Reflection Seismic in Geothermal Exploration

• Dominates petroleum exploration
• billions in petroleum seismic research are
  making incremental progress on the issues that
  inhibit applications to geothermal exploration
• P attenuation by shallow gas like CO2 in clay
  alteration
• Shallow dense rocks like lavas
• Statics due to rugged topography with rapid
  seismic velocity changes (like lavas and tuffs)
• Poor velocity constraints near target depths
• Scattering from closely spaced deep faults
• Lack of rock contacts that coherently reflect
• S-conversion interference
• In these situations petroleum exploration
  companies use MT EM gravity etc

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Reflection Seismic Geothermal Applications

• When goal is to image permeability
• not all fault segments are permeable so ideally
  want stress setting not just one segment
• volume velocity imaged by bending ray tomography
  will usually be too low resolution to resolve
  discrete faults at reservoir depth unless
  geometry and velocity are ideal.
• Faults are imaged by reflection seismic in
  geothermal prospects with 1) layered geology 2)
  low gas flux 3) limited shallow lava and 4)
  discrete structures
• Case histories image some reservoir faults but
  very few validated entries.
• Field-margin injection wells are easier targets
• If CO2 is trapped in clay cap then perimeter of
  shallow reservoir permeability often matches bad
  data zone for reflection seismic
• Clay cap sometimes imaged by p-wave bending ray
  velocity analyses but poorer resolution and
  higher cost than MT resistivity
• S-wave splitting is still speculative
• Therefore still a research topic for geothermal
  exploration
• Reflection seismic is more cost-effective when
  acquisition cost is lower without compromising
  acquisition .

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Value of Geophysics

• Use decision trees to assess impact of new
  information
• Choose three likely outcomes of the resource
  decision
• Your best guess what you think it is.
• Typical downside considering similar situations
  that were disappointing.
• Likely upside best similar case or maybe better
  with justification.
• Assess probability of each case based mostly on
  100 prospects with Monte Carlo ranges providing
  context for exceptional cases. How many prospects
  with diagnostic information like this had an
  outcome better than this case
• Assess likely affect of new information on
  probability for each case considering
  consistency and breadth of experience using
  similar information elsewhere.
• Use decision tables to assess new information
• How much would the new information likely affect
  resource decision probabilities
• How much does sufficiently reliable information
  cost
• What other information would affect the same
  resource probabilities and is it more
  cost-effective

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Geophysical Confidence Levels
Routine Case histories illustrate best practice
and pitfalls in acquisition and conceptual
interpretation for this particular
application. Expert Science supported by cases
where both the conceptual interpretation and
outcome are validated by relevant well
data. Research Plausible science with few case
histories. Speculative Promotional claims with
too little scientific detail to allow an expert
to fully evaluate the method.
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