Magnetic and Gravity Methods for Geothermal Exploration Dr. Hendra Grandis Geophysics - ITB method and survey procedure Aero- or ground magnetic (covers a large area) Schlumberger resistivity mapping and sounding (concentrated in the area between broad magnetic low and high) self-potential (across high and low resistivity areas) gravity (covers low and high magnetic areas) expected anomaly low anomaly low anomaly high or low anomaly high or low anomaly interpretation Ql : can be associated with thermally altered zones Qt : geometry (?) Ql : can be associated with thermal fluids upflow and outflow zones Qt : shallow resistivity structure Ql : ascending thermal fluid (and / or descending cold water) Qt :? Ql : existence of deep structure, i.e. intrusive body or caldera structures Qt : geometry of those above (the upper structure must be closely defined) 1
method and survey procedure Thermal gradient and anomalous temperature (to figure out the cause of low resistivity layer) Magnetotelluric sounding micro-seismics (M< 3) expected anomaly high anomaly low anomaly high anomaly Ql : uprising or horisontal thermal fluid movement, if depth to resevoir is relatively shallow Qt : defined the upper structure Ql : can be associated with thermal fluids upflow and outflow zones Qt : deeper resistivity structure Ql : permeable zones Qt :? interpretation Probable sequence of geophysical exploration methods used to investigate young volcanic geothermal prospect (revised from Sudarman,, 1983) Magnetic Method Covers large area as regional reconnaissance (aero-mag mag,, + remote sensing) More local coverage (ground-mag mag) ) to search for demagnetized bodies associated with thermally altered zones Precaution for dipolar nature of magnetic anomalies Advanced processing for anomaly enhancement and modeling 2
Aero-magnetic Survey aero-magnetic data and interpretation 3
Ground-magnetic Survey Vectors of the Earth s s magnetic field 4
Dipolar anomaly related to inclined inducing main magnetic field Typical magnetic anomaly at low latitudes 5
Anomaly detected in N-S N S traverse over E-W E W anomalous body Reduction to Equator and Pole of Magnetic data A' B' C' Magnetic Anomaly Reduced to Equator Reduced to Pole A B C Frequency domain filtering process C A B C' A' B' prism position 6
Spectral analysis Depth and spatial extent of the anomalous sources are related to frequency or wavelength of the data low frequency / long wavelength ~ deep and regional anomalies high frequency / short wavelength ~ shallow and local anomalies information on depth of anomalous sources can be obtained from spectral analysis of potential filed data Wavelength or frequency and anomaly s s depth 7
Spectral analysis Calculate 2-D 2 D FFT and radially averaged spectra relationship between slope of power spectrum with depth filtering (low-pass, high-pass, band-pass) regional residual anomaly separation pseudo-depth depth slicing 10 deep 5 log power 0-5 intermediate shallow -10 noise -15 0.0 0.2 0.4 0.6 0.8 1.0 1.2 wavenumber (cycle/km) Radially averaged spectra and line-fitted segments 8
Softwares for magnetic data processing and modeling Kamojang (Sungkono & Hochstein, 1995) 9
Wairakei,, NZ (Sungkono( & Hochstein, 1995) Gravity method g gravity data g 1 g 2 g 3 distance 10
Gravity Measurement We try to obtain exess or deficiency of gravity relative to normal gravity gravity anomaly difference between observed gravity and normal or theoretical gravity g ANOMALY = g OBSERVED - g THEORY must be corrected from factors affecting them Observed Gravity Use of relative gravimeter only measure m the difference of gravity values between two places measure gravity values at gravity stations relative to gravity base station (BS) BS St-1 R = R2 R1 g 1 = g BS + R R1 R2 known 11
Gravity Base Station Local Base Station all stations gravity values are relative to this local Base Station tied or referenced to a higher order gravity Base Station (regional, national) used for drift correction 12
Factors Affecting Observed Gravity Instrumental drift correction determined by looping procedure, i.e base station measurement at the beginning and at the end of a survey Gravity Anomaly g ANOMALY = g OBSERVED g THEORY g OBSERVED gravity value relative to a known base station g THEORY Gravity Reference Field (GRF) corrected to meet the observation condition (elevation MSL) corrections: Free-Air, Bouguer,, Terrain 13
Gravity Data Processing Data Reduction Field gravity measurements must be corrected to account for several factors which effect the readings, also known as reducing the data Solid earth s s tide Instrumental drift Field readings Station s s Gravity (g obs ) (relative to a known Base) 14
Gravity Data Processing Gravity Correction Station s s Gravity must be corrected such that the values represent a perfect homogenous sphere, called a geoid.. If there are still differences in the readings after the corrections are made, then they may truly represent a gravity anomaly. Corrections Latitude, Free-air, Bouguer,, Terrain Gravity Correction Latitude correction Normal Gravity from International Gravity Formula 1967 g N (φ)) = 9.78031846 (1 + 0.0053024 sin 2 φ 0.0000058 sin 2 2φ) g N (φ)) represents theoretical gravity at sea-level (reference), φ = latitude 15
Gravity Correction Free-air correction To compensate the height of gravity stations above sea-level (level of reference) g FA = 0.3086 h A h MSL Gravity Correction Bouguer correction To compensate the rock mass between sea-level to station s s elevation (level of reference). g B = + 0.04193 ρ h Bouguer slab formula A h MSL 16
Gravity Correction Terrain correction To compensate the topography: existence of rock mass of hills (M1) and the inexistence of rock-mass in valleys (M2) g TC are obtained from table or calculated A M1 M2 MSL Gravity Correction Terrain correction M1 will decrease gravity value at A (negative vertical attraction), Bouguer correction considers that there is M2 so mass attraction equivalent to M2 must be extracted from values at A A M1 M2 MSL 17
Bouguer Anomaly Gravity corrections are applied to g N (φ), i.e. theoretical gravity at reference-level, to obtain theoretical gravity values at measurement stations g theor = g N (φ) g FA + g B g TC Bouguer Anomaly is the difference between observed Gravity and theoretical gravity g BA = g obs g theor = g obs g N (φ)) + g FA g B + g TC Bouguer Anomaly Gravity corrections do not bring (reduce) gravity values from stations elevation to the reference / sea-level Bouguer anomalies are values at stations elevation (A), not values at reference / sea- level A h MSL 18
Regional-Residual Residual Anomaly Separation Free-hand smoothing Grid (weighted) moving average Second Vertical Derivative (SVD) Polynomial surface fitting Spectral analysis based filtering Qualitative vs. Quantitative Anomaly enhancement Regional-Residual Residual Anomaly Separation Free-hand smoothing Profile Map 19
Regional-Residual Residual Anomaly Separation Grid (weighted) moving average related to SVD Regional-Residual Residual Anomaly Separation Second Vertical Derivative (SVD) 20
Regional-Residual Residual Anomaly Separation Grid (weighted) moving average 21
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