Topic 7: Geophysical and Remote Sensing Models

Presenter: Greg UssherOrganisation:SKMEmail: gussher@globalskm.com

Overview• Surface-based imaging of subsurface characteristics

– That indicate geothermal system location– That control system behaviour

• Multiple methods applied– Each provide different information– Relative value depends on geological environment and

hydrothermal system type

• Integration in Concept Models – Multi-method and multi-disciplinary integration– Develop and refine concept models – The basis for any 3D geological or numerical reservoir model

• Consider environments between systems

Objective• Locate subsurface extent of geothermal systems

– Mapping effects caused by the presence of thermal fluids and associated hydrothermal alteration

• Resistivity – fluids and alteration• Magnetics – alteration (demagnetization)• Gravity – densification from deposition

• Locate structures that control geothermal fluid movement– Faults, calderas, basement structures

• Gravity – offsets and infill structures• Resistivity –surface effects on fault traces and deep low permeable zones• MEQ – fault planes and active structure• Subsidence – map extent of pressure change

• Locate heat source• Resistivity – magma chamber• Gravity – major intrusives• Seismics – melt has significantly different structure• Magnetics – deep regional temperature

The toolboxMethod Detection of : Sensitivity / factor

Resistivity (MT) Hydrothermal alteration – clay capHydrothermal alteration – high TDirect temperatureSalinity

100 to 1000-10 to -10010 to 100Depends on porosity

Gravity StructuresMineralisation

HighLow

Magnetics Hydrothermal alterationGeology variationCurie point (thermal demagnetisation)

Surface – highSurface – high/low? V Low

Seismics / MEQ Fracture location (Acoustic noise) Fracture / structure orientationsVelocity model – structureTemperature ?

Low - ModerateModerateLow - Moderate?

Subsidence Reservoir pressure change - extent Moderate - High

Gravity change Fluid saturation changes Moderate - High

RESISTIVITY

DC methods –reliable mapping tool

DC Resistivity - Limitations• Limited depth penetration

– Very long arrays required to get good depth• AB/2=1000m probably gives depth of about 300m.

– Requires increasingly large power sources for greater depth

– Not so reliable in mountainous terrain• Survey logistics are complex in difficult terrain

– Limits survey extent

Magneto – telluric (MT) resistivity• Now the main resistivity exploration tool• Gives good depth penetration and reasonable

horizontal resolution• Complex data collection and processing

– Necessary that this is done well to get useful results– Equipment relatively expensive– Typically use specialist survey contractors

• Not “do-it-yourself”• Unless surveying very large areas for several years

2D Inversion Modelling

3D Model Inversion• Integrated modeling of

all data• 3D Visualisation and

vertical & horizontal slicing

• Model Topographic effects

• Model shallow features that cause static shifts– Avoiding need for

TDEM corrections now

Resistivity at 50 mModel cross -section

What we look for in the MT results

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A clear conductive layer (Low resistivity,

clay cap)

Top of conductor is probably 50-70 °C

Bottom of conductor is probably 180-200°C

Extent of probable reservoir lies under

“strongest” section of the conductive layer

Probable Reservoir

Shallow and Deep conductorsNW SESource: S. M. Sewell, W. B. Cumming, L. Azwar & C. Bardsley , 2012

Rotokawa 3D InterpretationN S

Source: S. M. Sewell, W. B. Cumming, L. Azwar & C. Bardsley , 2012

Orakeikorako - NgatamarikiSource: O’Brien, J.M., 2010 PhD thesis - Hydrogeochemical Characteristics of the Ngatamariki Geothermal Field and a Comparison with the Orakei Korako Thermal Area, Taupo Volcanic Zone, New Zealand.

Source: GNS NgatamarikiGeoscience Report, 2008 (AEE Appendices)

Regional MT survey of Sth TVZ

Source: Bertrand et al., 2012 (GNS)

Regional MT survey of Sth TVZ

Source: Bertrand et al., 2012 (GNS)

Resistivity - Conclusions• Shallow conductive alteration cap is a key

indicator of presence of geothermal system– Often indicating shallow reservoirs and outflows– Useful system located where vertical permeability

allows upflow to these shallower levels– Temperature effect means that a “strong conductor” is

not relict• Deeper conductors found in TVZ indicate mod-

high temperature but low permeability– These are probably conductive heat flow areas

• No conductor (deep nor shallow) – no temperature ???

GRAVITY & MAGNETICS

Gravity – caldera mapping

Supri Soengkono 2012. Gravity Modelling Of Reporoa Basin, Eastern Taupo Volcanic Zone (Tvz), New Zealand. NZGW

Gravity mapping a pull-apart basin

Gravity models can identify basement structure that can be critical for understanding deeper controls on permeability

Gravity 2D Model

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Densification due to Geothermal Activity

This is a subtle effect and only identifiable when geology is consistent and terrain flat

Airborne Gravity - Gradiometry

• Enables accurate measurement from a moving vehicle / aircraft

• Can cover large areas

Aeromagnetic anomaly – maps near surface alteration

Low magnetic anomaly (Reduced to pole) where hydrothermal alteration is exposed near surface.

Areas of fresh rock cover have magnetic high even though underlain by hydrothermal alteration

Gravity & Magnetics - Conclusions• Gravity

– Has value in determining major structures– Basement and sediment layers can be a critical

control on permeability at depth in systems– Good density of data is required to resolve structures– An undervalued and neglected tool

• Magnetics– Shallow effects strongest– Probably lower value for deeper investigation

SEISMICS / MEQ

MEQ – Locating stucture

Basemap : GeothermEx 2005 Catherine Lewis Kenedi, Eylon Shalev, Alan Lucas, and Peter Malin, WGC 2010

MEQ hypocentres can mark faults (often confirming surface mapping).

In this case highlighted an offset in the active structure

MEQ – Velocity models

Basemap : GeothermEx 2005 Catherine Lewis Kenedi, Eylon Shalev, Alan Lucas, and Peter Malin, WGC 2010

Velocity models can indicate variations of geology at depth.

Reportedly temperature effect (Vp reduction at high T)

MEQ – Darajat, IndonesiaBase level activity Activity during injection

MEQ induced by injection - Darajat

MEQ induced by injection can be effective in identifying active permeable zones.

This can work even where natural activity is very low

SUBSIDENCE AND GRAVITY CHANGE

Reservoir change

Leyte Geothermal Field, Philippines

Nilo A. Apuada, Rhoel Enrico R. Olivar and Noel D. Salonga (PNOC - 2005)

Leyte – Reservoir changesPressure change1996-2002

Elevation change1997-2003

Leyte – Gravity change

Gravity change due to expansion of 2-phase zone

Gravity 1997-2003

Wairakei - Tauhara

Subsidence from levelling Subsidence from InSAR

InSAR: Svartsengi-Reykjanes, Iceland1992-2000

Thóra Árnadóttir, Sigrún Hreinsdóttir, Marie Keiding, SigurjónJónsson, Judicael Decriem, Karolina Michalczewska, Martin Hensch, Halldór Geirsson, Andy Hooper, Benedikt G. Ófeigsson. GPS and InSAR observations on Reykjanes

Svartsengi

Reykjanes

Long term production at Svartsengi. Gradual pressure decline.

InSAR: Svartsengi-Reykjanes, Iceland2003-2009

Thóra Árnadóttir, Sigrún Hreinsdóttir, Marie Keiding, SigurjónJónsson, Judicael Decriem, Karolina Michalczewska, Martin Hensch, Halldór Geirsson, Andy Hooper, Benedikt G. Ófeigsson. GPS and InSAR observations on Reykjanes

Svartsengi

Reykjanes

100 MW production at Reykjanes since 2006. Small production area, rapid pressure decline but wider subsidence effect

Conclusions• Geophysics exploration methods are:

– Describing the presence and extent of hydrothermal systems

– Defining areas of high conductive heatflow (and low!)– Defining major geological features including

• Upflow controlling structures• Basement and lateral permeability controls

• Geophysics monitoring is indicating :– Extent of pressure change (lateral recharge areas)– Areas of mass change– Fluid flow paths at depth

• Consider a suite of exploration methods, regardless of any pre-conceived model for a target area

Implications for modelling systems• Geophysics interpretations are part of a sound multi-

disciplinary approach to system modelling• Geophysical methods provide key constraints on

concept models– Indicating depths and extent of key characteristics that can

only otherwise be speculated upon beyond wells– Provide indications of deeper parameters (beyond drilling

depth) that may affect long term behaviour– Indicating conditions between adjacent hydrothermal

systems• Considering wider constraints on hydrothermal

systems may have value in modelling more localised reservoir performance