1. Geophysical Exploration for Geothermal Resourcesby
William Cumming
Cumming Geoscience, Santa Rosa CA
Cumming Geoscience 1

2. Geophysics Outline Types of reservoirs Types of geophysical methods
High temperature versus low temperature
How resistivity methods work
MT, T-MT, TDEM, CSMT, VES
Applications of resistivity methods
Other methods
Gravity, SP, Magnetics
Cost of geophysics
Example pitfall in geophysics interpretation
New Methods and Research

3. Geothermal Geophysics
Paul Brophy’s “types” have similar rock physics
Almost all geothermal reservoir types host temperature sensitive clays that can be imaged using resistivity
O&G geophysics is dominated by seismic imaging of permeability “traps” and, recently, reservoir properties.
Geothermal geophysics is dominated by resistivity imaging of the permeability “traps” and a key reservoir property, the natural state isotherm pattern, that is the starting point for most geothermal reservoir models.
Surface resistivity cannot image individual entries but can image the permeable volume of the reservoir and, with geology, geochemistry etc, can significantly reduce well targeting risk in many cases.
Even if resistivity “works” for shallow low temperature resources, other approaches may be more cost-effective.
There are many “special” methods for “special” issues

4. Geothermal Development Characteristics Affecting Geophysics
For >210°C Issue
Production by flash lift of water-steam
Flash and/or binary generation
50 to 100% injection at new fields
Reservoir top usually 300 to 1000 m deep Deeper
Reservoir thickness 300 to 3000 m Thicker
Testable wells usually >$1.5 million Wells cost more
Commercial wells usually >$3 million
For <180°C Issue
Production by pumping hot water
Binary generation
100% injection
Reservoir top usually 100 to 500 m deep Shallower
Reservoir thickness 100 to 1000 m Thinner
Testable wells usually >$0.5 million Wells cost less
Commercial wells usually $1 to $2 million

5. Geophysical Exploration of >200°C Geothermal Systems
Resource image area > 1 km2, often > 4 km2
Exploration image area > 4 km2, often > 50 km2
Depth to reservoir top 300 to 2000 m
Access often rugged
Environmental issues
after Cumming et al. 2000
Cumming Geoscience 2

6. Geophysical Exploration of <180°C Geothermal Systems
Resource image area > 1 km2, often > 4 km2
Exploration image area > 4 km2, often > 20 km2
Depth to reservoir top 100 to 1000 m
More like exploration for aquifers than for minerals or petroleum.
Cumming Geoscience 2

7. Geothermal Geophysics Technology
Geophysical exploration technology is 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.
Cumming Geoscience 2

8. Geophysical Acronyms MT Magnetotellurics AMT Audiomagnetotellurics
T-MT Telluric-Magnetotellurics
CSAMT Controlled Source Audiomagnetotellurics
HEM Helicopter Electromagnetics
TDEM Time Domain Electromagnetics
TEM same as TDEM
VES Vertical Electrical Sounding
SP Self-Potential
dGPS Differential Global Positioning System
MEQ Microearthquake
Cumming Geoscience 2

9. Geophysical Techniques Geothermal Exploration
Standard: MT, T-MT, TDEM, Gravity
Legacy: Dipole-Dipole, Tensor Dipole-Bipole
Special: VES, AMT, CSAMT, SP, HEM
Aeromagnetics, Precision Ground Magnetics
Research: Reflection / Refraction Seismic Special Applications
Development: Microgravity, Microearthquake, Subsidence
Proprietary: E-Scan, E-Map
Unreviewed: Aquatrack
Suspect: Seismic Noise, Low Res Ground Magnetics Plausible methods with weak technical support
Cumming Geoscience 2

10. Geophysical Techniques in 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 reflection
Magnetic susceptibility: using magnetic field
Seismic velocity: Refraction and reflection seismic
Natural electrical potential (V): using SP
et al (e.g. crack density from MEQ)
Cumming Geoscience 2

11. Geophysical Techniques in Geothermal Exploration
“Special”
“Standard”
CSMT for noisy areas or where limitations do not matter and low cost does
Magnetics for alteration & unit boundary patterns
SP for shallow <180°C
DC profiling and HEM for reconnaissance mapping
MT for base of clay cap
TDEM for statics and detail
Gravity for lithology and large structure
Cumming Geoscience 2

12. “Standard” Geophysical Plan >200°C Geothermal Exploration
MT to map base of clay “cap”
TDEM for MT statics and detail
Gas and fluid geochemistry for conceptual target
Maybe gravity for lithology and large structure
Cumming Geoscience
Cumming, 2006 2

13. “Standard” Geophysical Plan <180°C Geothermal Exploration
TDEM or other low-cost resistivity for clay cap
SP if target shallow and topography gentle
Other methods to support geology, geochemistry
Temperature Gradient Wells if access and drilling are low cost.
More like exploration for aquifers than for minerals or petroleum.
Cumming Geoscience 2

14. MT Objectives in Geothermal Exploration
Map structure and conductance of <180°C low resistivity smectite clay zone capping the relatively resistive >200°C propylitic reservoir
Integrate with geochemistry and geology to
Estimate resource capacity
Target wells for high temperature permeability .
Cumming Geoscience 2

15. MT Method E 2 dipoles ~100 m H 3 magnetometers
EM signal from sun and electrical storms
MT resistivity at 1 Hz is about 1 km down
Blue zone is low resistivity smectite
Topo and shallow conductors give different resistivity on 2 dipoles, i.e. statics
Cumming Geoscience

16. MT Physics
Geophys.washington.edu

17. MT Acquisition Issues
AC power line noise is usually mitigated by a ~200 to 400 m standoff
DC power lines and electric trains can limit depth of investigation to <1000 m
Pipes, fences and similar metal features usually require a 200 to 1000 m standoff
Although the equipment is portable, cost rises steeply if access to sites is poor
Cumming Geoscience

18. MT Field Layout Uses natural EM signal > 5 km depth
Records 7 to 20 hours
2-5 man portable system
One or two stations/day
T-MT uses 2 to 3 MT stations with 2-10 T-only stations for lower cost where lateral changes are smooth.
Cumming Geoscience

19. MT versus T-MT
Digging holes for magnetometers is time-consuming so costs are reduced by doing T-MT in areas with smooth near-surface resistivity variations.
Cumming Geoscience

20. T-MT Profiling
Quantech, 2003 2

21. T-MT Profiling Continuous line of T stations with one MT station
100 m spacing used in minerals is seldom cost-effective for deeper and/or larger geothermal targets
Cost is sometimes less than MT stations for smaller, shallower targets, like those in minerals exploration
Real time processing and display for noise reduction
Statics due to topography on continuous T-MT can be corrected when surface resistivity is uniform
Having T but not MT at some stations may limit resolution but this is seldom an issue in geothermal
Cumming Geoscience 2

22. TDEM / TEM
Pulse current in outer loop, measure signal in inner loop from “smoke rings” of current induced by magnetic field.
TDEM depth < MT
No electrodes so no static distortion
Focused so less 2D/3D distortion
Cumming, 2003
Cumming Geoscience

23. TDEM Record in minutes Very portable when using batteries
1 to 7 stations/day
Cost $200 to >$600 per station
From: Geosystem
Cumming Geoscience

24. TDEM Survey Types
From: MINDECO

25. TEM at Krafla Detects base of clay Maps reservoir top MT not needed
Shallow reservoir
300 to 1000 m loops
Cost >$600/station
1 to 3 stations/day
Geonics Protem / EM37
From: Arnason et al 2000
Cumming Geoscience

26. CSMT Profiling Scalar MT profiling using a wire transmitter
Costs < 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 < 200 to < 1000 m
Fewer imaging and processing options
Cumming Geoscience 2

27. 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 >5 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 > 2 km)
Environmental issues, cost and logistics limit new surveys
Cumming Geoscience 2

28. VES and Dipole-dipole Resistivity at Cerro Prieto
Charre-Meza et al 2000 2

29. Resistivity Imaging in Geothermal Exploration
Map base and conductance of low resistivity clay zone capping relatively resistive reservoir
Integrate with geochemistry and geology to
Estimate resource capacity
Target wells for high temperature permeability 2

30. Awibengkok Geothermal Field MT Cross-section MT Resistivity with MeB Smectite & Isotherms from Wells
1000
Meters
-1000
1 Km
Cumming Geoscience
from: Gunderson, Cumming, Astra and Harvey (2000) 12

31. Karaha Bodas MT (Moore,2006)
from: Moore (2006)
Cumming Geoscience 12

32. 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) 5

33. El Tatio Schlumberger Profiling 1973
Lahsen and Trujillo (1976) 2

34. La Torta Conceptual Cross-section with MT Resistivity
Cumming, Vieytes, Ramirez and Sussman (2002) 2

35. La Torta 3D MT Resistivity Structure
(Elevation of
base of clay)
Cumming, Vieytes, Ramirez and Sussman (2002)

36. Gravity
1 or 2 people
Scintrex automatic meter reduced error compared to L&R (which are OK)
dGPS reduced cost and error by half
Responds to rock density variation, mainly related to rock porosity.
Interpreted for lithology, structure and alteration.
Cumming Geoscience 2

37. Gravity Interpretation
Density in geothermal exploration models is determined by porosity and, to a lesser extent, mineral grain density.
Pore fluid changes detected by precision gravity for development monitoring are usually insignificant in exploration surveys.
2D interpretations focus on lithology, structure and alteration.
Large, shallow density contrasts overwhelm subtle ones so sinter may be undetectable near lava domes surrounded by pumice tuff.
Use top-down interpretation in models because the gravity effect of a deeper density contrast is more spread out and indistinct and, more importantly, rock density contrasts decrease with depth:
At m, 30°C, lava can be 2.7 and tuff 1.1 g/cm3
At 1000 m, 250°C, lava can be 2.7 and tuff 2.4 g/cm3
Contrast at 100 m is ~10 times larger than at 1000 m.
Because of its greater ambiguity, gravity is often more effective in extending models developed using sounding methods like MT.
Cumming Geoscience 2

38. Gravity Bradys Hot Springs and Desert Peak Interpretation 2
from Oppliger, 7 May 03 2

39. 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.
Cumming Geoscience 2

40. 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
Cumming Geoscience
Hochstein et al., 1990 2

41. Magnetic Surveys
Map local variations in earth’s magnetic field that, in volcanics, correlate with magnetite content
Aeromagnetic survey: magnetometer in plane
Draped is better, constant elevation is easier
Used to: 1) map structure and lithology; and 2) characterize extent of alteration, especially related to SO4 destruction of magnetite
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.
Cumming Geoscience 2

42. Cost for Geophysics
Includes acquisition & some imaging but not integrated interpretation. MT <0.05 to >300 Hz Low cost: Sites < 500 m from vehicle. < 1 hr to easy camp, etc High cost: >30% sites > 1 km from vehicle. > 1 hr to camp, etc.
Method Cost / data unit Mob & misc
MT $1k - $3k / MT $5k - $30k
T- MT $0.2k - $1.2k / T $8k - $35k T-MT
T-MT Profile $4k - $10k / line km $5k - $45k
CSAMT $2k - $6k / line km $3k - $30k
TDEM $0.2k - $0.6k / TDEM $3k - $15k
Gravity+dGPS $30 - $90 / station $3 - $15k
Cumming Geoscience 2

43. Geophysical Exploration of <180°C Geothermal Systems
Can geophysics be both useful and low cost.
Yes, if
Production aquifer is <500 m deep.
Method is matched to the situation; e.g. TDEM for <500 m, SP in gentle terrain.
Deep inferred from shallow
Cumming Geoscience 2

44. Geophysics Uncertainty in Geothermal Exploration
MT -TDEM can image the base of the clay cap conforming to the
top of the reservoir for most geothermal reservoirs >140°C
but
Although the apex of this structure is often the shallowest permeability and sometimes becomes a steam cap, it is sometimes tight and it is often not located over the deep high-temperature upflow.
MT might not be the most cost-effective approach for shallow resources, especially for low-temperature cases. so
Check conceptual advantages of other methods
Integrate with geochemistry and geology
Drill a conceptual model, NOT an anomaly .
Cumming Geoscience 2

45. Value of Information Use case-oriented decision trees to estimate:
Value of resource based on risk weighted ENPV
Value of new information through its affect on case probabilities
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 redundantly affect the same resource probabilities and how does it compare with respect to the above questions?
Cumming Geoscience 2

46. Geothermal Geophysics Interpretation Pitfall Example
MT Observation
MT resistivity cross-section contours often appear to define a low resistivity zone extending near-vertically below 500 m depth.
Interpretation Pitfall
Vertically trending low resistivity zones at >500 m depth are commonly misinterpreted as evidence of deep reservoir structural permeability
Issue
Flaws in MT processing commonly produce false vertically-oriented low resistivity zones at depth.
Static distortion, noise, and inconsistent station projection are the most common problems.
Recognition
Large contrasts in resistivity over large depth ranges at adjacent stations suggest a statics problem. Check for a split between MT apparent resistivity curves at high frequency.
Check for noise in the apparent resistivity and phase curves for stations near the vertical feature.
2D inversions can be distorted when MT stations are projected onto the profile being imaged so that their relative geometry is not preserved.
Remedies
Correct statics using TDEM, smoothing inversions or surface geology consistency.
Edit noise so that it does not bias the inversion to low resistivity at depth
Correct inconsistent station projections.
Reliable imaging of resistivity is usually relatively smooth horizontally so be skeptical when interpreting near vertical resistivity contours.
Review the plausibility of resistivity values with respect to realistic reservoir properties.
Cumming Geoscience 2

47. Geothermal Geophysics Interpretation Pitfall Example
Vertical contours in MT cross-section show deep low resistivity in red
Erroneously interpreted as reservoir fault zone
MT imaging of resistivity distorted by:
noise near station 1
static at station 2
MT cross-section without distortion shows classic geothermal cap geometry
Cumming Geoscience 2

48. Geothermal Geophysics Research Topics
Reflection Seismic
Earthquake Tomography
Velocity
Attenuation
S-wave splitting
3D Integrated Resistivity
Cumming Geoscience 2

49. Reflection Seismic Dominates petroleum exploration
However, $ billions in petroleum seismic research have not solved problems with:
P attenuation by shallow gas like CO2 in clay
Shallow dense rocks like lavas
Statics due to rugged topography with rapid seismic velocity changes (like lavas and tuffs)
Resolving closely spaced deep structures
Lack of rock contacts that coherently reflect
S-conversion interference
So MT, gravity etc used by oil companies
Cumming Geoscience 6

50. Geothermal Reflection Seismic
Goal is usually to image permeable zones
Clay cap and possibly reservoir imaged by refraction tomography with resolution usually poorer than resistivity and cost that is higher
Reservoir volume imaged by reflection seismic in the sense that it is usually a “no data” zone
Large scale structural setting of fields imaged
Few reservoir faults or entries imaged
Therefore, still a research topic for geothermal exploration applications
Potential development applications such as field-margin injection well targeting would be more cost-effective if acquisition cost was reduced.
Cumming Geoscience 6

51. Reflection Seismic Coso reflection seismic section (annotated)
Pullammanappallil et al and Unruh et al. 2001 6

52. Refraction Seismic
Coso refraction tomography section showing velocity variation in color
Pullammanappallil et al and Unruh et al. 2001 6

53. Reflection Seismic Rye Patch
Majer and Gritto, 2003

54. Reflection Seismic Rye Patch
LBNL Modeling of Rye Patch
P-wave
Majer and Gritto, 2003

55. 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 over a decade or so.
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 MEQ’s are detected, the episodic data and high cost make it a risky exploration tool.
Cumming Geoscience 2

56. 3D MT Resolution Research
Cumming and Mackie, 2003
Cumming Geoscience 11

57. New Geophysical Technology
Question:
What investment in geophysical technology is likely to pay out in geothermal exploration?
Past answers and successes:
Leverage petroleum and mineral technology
MT, TDEM, VES, CSMT, Gravity etc but adaptation crucial
University / Lab basic science and method validation
TDEM - MT static correction validated for geothermal
MT - 1D and 2D smooth inversion imaging for geothermal
MT - remote-reference processing for noise leveraged
dGPS - leveraged to make all surveys cheaper and better
Audit geophysics with geology, geochemistry, etc
Illite-smectite clay model for resistivity interpretation
Top down modeling for gravity and magnetic interpretation
Integrate using case histories and risk assessment
Integrate geophysics into resource risk assessment by interpreting in context of likelihood of conceptual model cases
Cumming Geoscience 2

58. Audit Geophysics with Geology
e.g. Joe Moore pointed out lithologic permeability at Bulalo
Litho/Structural Facies Model of Bulalo Reservoir
Moore, 2006
Cumming Geoscience 2

59. Geophysical Exploration for Geothermal Resourcesby
William Cumming
Cumming Geoscience, Santa Rosa CA
Cumming Geoscience 1

60. Discussion Session
JAXA L-band satellite interferometry for deformation
Re-do geological ground truthing of geophysics aided by remote sensing
Gas seep and spring surveys should never be assumed to be complete
Exploring for injection can be different …
Cumming Geoscience 2