Overview of the Magnetic Method

MAGNETIC METHOD IN GEOTHERMAL EXPLORATION

Basic principle of magnetic method

The magnetic force between 2 poles of strength m1 and m2

Magnetic field due to a pole of strength m at distance r

The magnetic potential v at distance r from the pole

Origin of the Earths magnetic field. Displacement of the force lines of the Earths magnetic field, equivalent to the ones of the magnet

OVERVIEW OF THE MAGNETIC METHODThe magnetic method measures the intensity of natural magnetic field. This includes contribution from the earth core and crust as well as secondary magnetic field induced in magnetic geological bodies, which locally creates positive and negative anomaly Magnetic anomalies are the targets for magnetic surveys for geological purposesThe magnetic susceptibility of rocks is extremely variable, depending on the type of rock and the environment it is in and it is an extremely important property of rocks in magnetic exploration method

Common causes of magnetic anomalies include:DykesFaultsLava flow

Magnetic instrumentsFluxgate magnetometerProton precession magnetometerTorsion balance

Examples of a field measurement with a magnetometerMAGNETIC METHOD IN GEOTHERMAL EXPLORATIONThe magnetic method has come into use for identifying and locating masses of igneous rocks that have relatively high concentrations of magnetite. Strongly magnetic rocks include basalt and gabbro, while rocks such as granite, granodiorite and rhyolite have only moderately high magnetic susceptibilities. The magnetic method is useful in mapping near -surface volcanic rocks that are often of interest in geothermal exploration, but the greatest potential for the method lies in its ability to detect the depth at which the Curie temperature is reachedFerromagnetic materials exhibit a phenomenon characterised by a loss of nearly all magnetic susceptibility at a critical temperature called the Curie temperature. Various ferromagnetic minerals have differing Curie temperatures, but the Curie temperature of titano-magnetite, the most common magnetic minera1 in igneous rocks, is in the range of a few hundred to 570C. The ability to determine the depth to the Curie point would be an ability to determine the depth to the Curie point isotherm as well. For magnetic field observations made at or above the surface of the earth, the magnetisation at the top of the magnetic part of the crust is characterised by relatively short spatial wavelengths, while the magnetic field from the demagnetisation at the Curie point in depth will be characterised by longer wavelength and lower amplitude magnetic anomalies. This difference in frequency characteristics between the magnetic effects from the top and bottom of the magnetised layer in the crust can be used to separate magnetic effects at the two depths and to determine the Curie point depth. Case studyCase study 1:Magnetic Method Used In Geothermal Exploration in Ie-Seu Um, Aceh Besar (Indonesia)Case study 2:Applications Of Potential Field Methods For Geothermal Exploration A Case For Olkaria And Menengai Geothermal Fields, Kenya

magnetic anomalies occur in local field from magnetic rock below surface (similar to gravity anomalies)Earths magnetic fieldmagnetic materialbelow addsmagnetismand createspositive anomalymagnetic rocksincludeiron ore,gabbro,graniteEARTHS MAGNETIC FIELDremoval of magnetic material from near surface causes negative anomaly (example is normal faulting)