Topic 7: Geophysical and Remote Sensing Models Presenter: Greg Ussher Organisation:SKM Email: 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 toolbox Method Detection of : Sensitivity / factor Resistivity (MT) Gravity Magnetics Seismics / MEQ Hydrothermal alteration clay cap Hydrothermal alteration high T Direct temperature Salinity Structures Mineralisation Hydrothermal alteration Geology variation Curie point (thermal demagnetisation) Fracture location (Acoustic noise) Fracture / structure orientations Velocity model structure Temperature? 100 to 1000-10 to -100 10 to 100 Depends on porosity High Low Surface high Surface high/low? V Low Low - Moderate Moderate Low - 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 Model cross -section Resistivity at 50 m
What we look for in the MT results A clear conductive layer (Low resistivity, clay cap) Top of conductor is probably 50-70 C Bottom of conductor is probably 180-200 C Probable Reservoir Extent of probable reservoir lies under strongest section of the conductive layer 11
Shallow and Deep conductors NW Source: S. M. Sewell, W. B. Cumming, L. Azwar & C. Bardsley, 2012 SE
Rotokawa 3D Interpretation N S Source: S. M. Sewell, W. B. Cumming, L. Azwar & C. Bardsley, 2012
Orakeikorako - Ngatamariki Source: 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 Ngatamariki Geoscience 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 modhigh 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 21
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 MEQ hypocentres can mark faults (often confirming surface mapping). In this case highlighted an offset in the active structure Basemap : GeothermEx 2005 Catherine Lewis Kenedi, Eylon Shalev, Alan Lucas, and Peter Malin, WGC 2010
MEQ Velocity models Velocity models can indicate variations of geology at depth. Reportedly temperature effect (Vp reduction at high T) Basemap : GeothermEx 2005 Catherine Lewis Kenedi, Eylon Shalev, Alan Lucas, and Peter Malin, WGC 2010
MEQ Darajat, Indonesia Base 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 changes Pressure change 1996-2002 Elevation change 1997-2003
Leyte Gravity change Gravity 1997-2003 Gravity change due to expansion of 2-phase zone
Wairakei - Tauhara Subsidence from levelling Subsidence from InSAR
InSAR: Svartsengi-Reykjanes, Iceland 1992-2000 Svartsengi Reykjanes Long term production at Svartsengi. Gradual pressure decline. Thóra Árnadóttir, Sigrún Hreinsdóttir, Marie Keiding, Sigurjón Jónsson, Judicael Decriem, Karolina Michalczewska, Martin Hensch, Halldór Geirsson, Andy Hooper, Benedikt G. Ófeigsson. GPS and InSAR observations on Reykjanes
InSAR: Svartsengi-Reykjanes, Iceland 2003-2009 Svartsengi Reykjanes 100 MW production at Reykjanes since 2006. Small production area, rapid pressure decline but wider subsidence effect Thóra Árnadóttir, Sigrún Hreinsdóttir, Marie Keiding, Sigurjón Jónsson, Judicael Decriem, Karolina Michalczewska, Martin Hensch, Halldór Geirsson, Andy Hooper, Benedikt G. Ófeigsson. GPS and InSAR observations on Reykjanes
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 multidisciplinary 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