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Effect of Magnetotellurics DataDensity in Geothermal Resource Exploration Mehran Gharibi Quantec Geoscience GeoPower, Turkey 2011
Outline:• Objectives• Introduction to Magnetotelluric (MT) Method• Titan-24 and Spartan MT measurements• MT in Geothermal Exploration• 3D Inversion and Interpretation• Summary
Objectives:Investigate resolution and reliability of the 3D MTresistivity models in terms of the data density usedin inversion; i.e. Resolution vs. Frequency density.
Magneto-Telluric (MT) Method• The MT is a frequency-domain natural-field EM geophysical method – frequency range 10000 Hz - 0.001 Hz• The method uses the earth’s EM natural time variations - Lower Frequencies ( < 1 Hz): Ionospheric; initiated by interaction of the solar wind with the earth’s magnetic field. - Higher Frequencies ( > 1 Hz): Atmospheric; initiated by global lightning activities.• Image the earths electrical resistivity structure – from nearly the surface to several tens of km.
MT measurementsSchematic of simultaneous measurement of plane-wave electromagneticfield fluctuations in five components: Ex, Ey, Hx, Hy and Hz Ground surface Electric field dipoles Ex Ey Hz Hy Hx Magnetic field sensors ⎛ E x ⎞ ⎛ Z xx Z xy ⎞⎛ H x ⎞ Apparent Resistivity of ⎜ ⎟=⎜ ⎟⎜ ⎟ Calculate Inversion ⎜E ⎟ ⎜Z Z yy ⎟⎜ H y ⎟ Resistivity Subsurface ⎝ y ⎠ ⎝ yx ⎠⎝ ⎠
Calibration Chamber for Magnetic Sensors3-layer passive shielded and active Active-Fieldfield cancellation calibration room Cancellation FrameSolenoid Magnetic Passive Active Frequency Sheilding Sheilding coil sensor (Hz) Factor Factor 4m 10K 100,000 100,000 1K 50,000 50,000 100 2,000 2,000 10 100 1,000 1 50 1,000 0.1 20 1,000 3-layer Passive Magnetically 0.01 10 1,000 Shielded Room
Spartan MT acquisition Acquisition system • Full tensor MT • 24-bit resolution • Automatic data acquisition • Remote reference processingField setup• Man portable• Coils buried• Flexible spacing• Low environmental footprint
Titan-24 MT Acquisition Typical 2 Channel Station AM Set-up • Full tensor MT • 24-bit resolution Battery • High spatial site density • DC-IP acquisition capability 2400m current electrode (mobile) cross-line potential electrode (fixed) cross- 50m in-line potential electrode (fixed) in- 100m Bx infinity current electrode (fixed) By 100m 2 channel AM >10 km Base magnetometer Site 1 channel AMLAN Link to Logging Truck Line Length - 2400 m 24 E x 100m dipoles 12 E y 100m dipoles 25 current stations 2 Bx/B y magnetometer sites >20 km Bx By Remote Magnetometer site 8
MT Method in Geothermal Explorations:• MT data are used to produce electrical resistivity distributionof the subsurface• Electrical resistivity is a function of; • solid matrix - geological formation and alteration • pore fluids - chemistry and salinity • porosity – geological fissure and fracture • temperature• A geothermal system or a hydrothermal reservoir is definedand controlled by a combination of the above factors.• Resistivity signature associated with the geothermal system isused to detect/map/characterize the reservoir
Interpretation of the MT Data ⎛ 0 Z⎞ ⎛ 0 Z xy ⎞ ⎛ Z xx Z xy ⎞• Data ⎜ ⎜− Z ⎟ ⎜ ⎟ ⎜ ⎟ ⎝ 0⎟ ⎠ ⎜Z ⎝ yx 0 ⎟ ⎠ ⎜Z ⎝ yx Z yy ⎟ ⎠ ρ ρxy, ρyx• Inversion 1-D 2-D 3-D• Resistivity Model
Spartan MT Field Survey• Large scale MT survey over a geothermal resource area• More than 160 MT sites• Site spacing between 500 m and 1000 m• Frequency range 250 Hz – 0.001 Hz• Objective is to identify an exploration drilling location• Geothermal reservoir target at >1500 m• Interpretation based on 3-D with different number of frequencies.
MT survey location Map 3D inversion area 2D cross-sections
3-D InversionsThe original frequency band is decimated for the 3-D inversions 5 Frequencies (1 per decade) Original Decimated
3-D Inversion – 5 Frequencies• # of frequency per site: 5• # of sites: 162• # of Mesh: 25 x 41 x 18• Inversion time: 602 hours• Hardware: 2 x CPU processor with 4 GB RAM 4 km
3-D Inversion – 5 Frequencies Surface = 0 m Conductive Cap Resistive Core Interpreted Geothermal System
3-D Inversion – 5 Frequencies Surface = -1300 m Conductive Cap Resistive Core Interpreted Geothermal System
3-D Inversion – 5 Frequencies Surface = -2000 m Anomaly or Artifact !? Interpreted Geothermal System
3-D Inversion – 5 Frequencies Surface = -3000 m Anomaly or Artifact !? Interpreted Geothermal System
3-D Inversion – 5 Frequencies Selected Cross-sections Conductive Resistive Cap Core Interpreted Geothermal System
3-D InversionsThe original frequency band is decimated for the 3-D inversions 18 Frequencies (3 per decade) Original Decimated
3-D Inversion – 18 Frequencies• # of frequency per site: 18• # of sites: 162• # of Mesh: 25 x 41 x 18• Inversion time: 552 hours• Hardware: 2 x 4 CPU processor with 24 GB RAM 4 km
3-D Inversion – 18 Frequencies Surface = 0 m Conductive Cap Resistive Core Interpreted Geothermal System
3-D Inversion – 18 Frequencies Surface = -1300 m Conductive Cap Resistive Core Interpreted Geothermal System
3-D Inversion – 18 Frequencies Surface = -2000 m No Artifact ! Geothermal reservoir!?
18 Frequencies Selected Cross-sections 5 Frequencies Interpreted Interpreted Geothermal Geothermal System System
Summary• Geothermal fields and resources can efficiently be mapped and characterized using MT measurements.• 3-D inversions are computationally expensive; - a subset of the dataset with 3 frequencies/decade would be sufficient to produce the resistivity distribution of the subsurface while minimizing the artifacts• Development in 3D inversion; - Parallel computing of the 3-D MT inversion using cluster of high performance CPUs (e.g. 48 CPUs and 64 GB RAM) - It speeds up the inversion process several times (5-10 times).
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