Presented at Shrt Curse II n Surface Explratin fr Gethermal Resurces, rganized by UNU-GTP and KenGen, at Lake Naivasha, Kenya, 2-17 Nvember, 2007. GEOTHERMAL TRAINING PROGRAMME Kenya Electricity Generating C., Ltd. THE MAGNETIC METHOD N.O. Mariita Kenya Electricity Generating Cmpany Ltd. (KenGen) P.O. Bx 785, Naivasha KENYA nmariita@kengen.c.ke ABSTRACT The aim f a magnetic survey is t investigate subsurface gelgy n the basis f the anmalies in the earth's magnetic field resulting frm the magnetic prperties f the underlying rcks. In general, the magnetic cntent (susceptibility) f rcks is extremely variable depending n the type f rck and the envirnment it is in. Cmmn causes f magnetic anmalies include dykes, faults and lava flws. In a gethermal envirnment, due t high temperatures, the susceptibility decreases. It is nt usually pssible t identify with certainty the causitive lithlgy f any anmaly frm magnetic infrmatin alne. The magnetic methd invlves the measurement f the earth's magnetic field intensity. Typically the ttal magnetic field and/r vertical magnetic gradient is measured. Measurements f the hrizntal r vertical cmpnent r hrizntal gradient f the magnetic field may als be made. Magnetism is, just like gravity, a ptential field. Anmalies in the earth's magnetic field are caused by induced r remanent magnetism. Induced magnetic anmalies are the result f secndary magnetizatin induced in a ferrus bdy by the earth s magnetic field. The shape, dimensins, and amplitude f an induced magnetic anmaly is a functin f the rientatin, gemetry, size, depth, and magnetic susceptibility f the bdy as well as the intensity and inclinatin f the earth's magnetic field in the survey area. The magnetic methd is typically used t: Lcate abandned steel well casings, buried tanks, pipes and metallic debris Map ld waste sites and landfill bundaries Map basement faults and basic igneus intrusives Investigate archaelgical sites Fr the explratin f gethermal energy, bth grund and aer-magnetic data have been used t investigate the presence f a gethermal resurce in cmbinatin with gravity. Frm the aer-magnetic maps several f the anmalies can ften be crrelated with surface expressins f vlcanism such as craters, dmes r cnes, lcalised basaltics lavas r plugs. Frm these maps mst f the vlcanic centres tend t lie in areas with magnetic highs (psitives). Smetimes a superimpsed magnetic lw (negative) exist; but this is generally weak r zer.
The magnetic methd 2 Mariita 1. INTRODUCTION Gephysicists have been able t develp a mathematical mdel fr the earth s magnetic field, i.e., its shape and intensity acrss the surface f the earth, Magnetmeter surveys indicate that there are many unexpected variatins in this mdel, called magnetic anmalies. A magnetic high anmaly is where the measured field strength is higher than the value predicted by the glbal mdel, and a magnetic lw is where the measured field strength is lwer than the value predicted by the glbal mdel. Anmalies in the earth's magnetic field are caused by induced r remanent magnetism. Induced magnetic anmalies are the result f secndary magnetizatin induced in a ferrus bdy by the earth s magnetic field. Pssible causes fr magnetic highs include the presence f magnetically charged rcks in the subsurface. Magnetic prspecting lks fr variatins in the magnetic field f the earth that are caused by changes in the subsurface gelgic structure r by differences in the magnetic prperties f near-surface rcks. The inherent magnetism f rcks is called the magnetic susceptibility. Sedimentary rcks generally have a very small magnetic susceptibility cmpared with igneus r metamrphic rcks, which tend t have a much higher magnetite (a cmmn magnetic mineral) cntent. Mst magnetic surveys are designed t map the gelgic structure n r inside the basement rcks (the crystalline rcks that lie beneath the sedimentary layers) r t detect magnetic minerals directly. The aim f a magnetic survey is t investigate subsurface gelgy n the basis f the anmalies in the earth's magnetic field resulting frm the magnetic prperties f the underlying rcks. In general, the magnetic cntent (susceptibility) f rcks is extremely variable depending n the type f rck and the envirnment it is in. Cmmn causes f magnetic anmalies include dykes, faults and lava flws. Where the rcks have high magnetic susceptibility, the lcal magnetic field will be strng; where they have lw magnetic susceptibility, it will be weaker. Rck units with higher susceptibility will shw up as areas f high magnetic field strength. In a gethermal envirnment, due t high temperatures, the susceptibility decreases. Used with gravity, this methd can be used t infer heat surces as seen frm gethermal explratin at Olkaria. Grund magnetic measurements d prvide mre detailed infrmatin n sub-surface structures that culd act as heat surces in cmparisn t aermagnetic data. Magnetic gradient anmalies generally give a better definitin f shallw buried features such as buried tanks and drums, but are less useful fr investigating large gelgical features. Unlike EM surveys, the depth penetratin f magnetic surveys is nt impeded by high electrical grund cnductivities assciated with saline grundwater r high levels f cntaminatin. 2. INSTRUMENTATION A magnetmeter is a mre cmplex instrument which measures bth the rientatin and strength f a magnetic field. When the magnetic field f a rck sample is measured, the result is actually a measure f the field as it is being effected by the earth s magnetic field, as well as any ther large bdies f magnetic rck which are near by. Magnetmeter surveys measure small, lcalised variatins in the Earth's magnetic field. Magnetmeters are highly accurate instruments, allwing the lcal magnetic field t be measured t accuracies f 0.002%. There are several types f instruments n the market. The cmmn nes used fr cmmercial applicatins are the prtn precessin, fluxgate, caesium vapur and gradimeter magnetmeter systems. The systems perate n bradly similar principles utilising prtn rich fluids surrunded by an electric cil. A mmentary current is applied thrugh the cil, which prduces a crrespnding magnetic field that temprarily plarises the prtns. When the current is remved, the prtns realign r precess int the rientatin f the Earth's magnetic field. The precessin generates a small electrical current in the surrunding cil, at a frequency directly prprtinal t the lcal magnetic field intensity. Gradimeters measure the magnetic field gradient
Mariita 3 The magnetic methd rather than ttal field strength, which allws the remval f backgrund nise. Gradimeters measure the magnetic field gradient rather than ttal field strength, which allws the remval f backgrund nise. FIGURE 1: Prtn Precessin magnetmeters. (a) Gemetrics G-856 (b) Gemetrics G-858 3. DATA ACQUISITION Grund magnetic measurements are usually made with prtable instruments at regular intervals alng mre r less straight and parallel lines which cver the survey area. Often the interval between measurement lcatins (statins) alng the lines is less than the spacing between lines. The magnetmeter is perated by a single persn. Hwever, grid layut, surveying, r the buddy system may require the use f anther technician. If tw magnetmeters are available prductin is usually dubled as the rdinary peratin f the instrument itself is straightfrward. Intense fields frm man-made electrmagnetic surces can be a prblem in magnetic surveys. Steel and ther ferrus metals in the vicinity f a magnetmeter can distrt the data. Large belt buckles, etc., must be remved when perating the unit. A cmpass shuld be mre than 3 m away frm the magnetmeter when measuring the field. A final test is t immbilize the magnetmeter and take readings while the peratr mves arund the sensr. If the readings d nt change by mre than 1 r 2 nt,. On very precise surveys, the peratr effect must be held under 1 nt. Mst magnetmeters are designed t perate in fairly intense 60-Hz and radi frequency fields. Hwever extremely lw frequency fields caused by equipment using direct current r the switching f large alternating currents can be a prblem.
The magnetic methd 4 Mariita T btain a representative reading, the sensr shuld be perated well abve the grund. This prcedure is dne because f the prbability f cllectins f sil magnetite disturbing the reading near the grund. In rcky terrain where the rcks have sme percentage f magnetite, sensr heights f up t 4 m have been used t remve near-surface effects. Data recrding methds will vary with the purpse f the survey and the amunt f nise present. Methds include: taking three readings and averaging the results, taking three readings within a meter f the statin and either recrding each r recrding the average. Sme magnetmeters can apply either f these methds and even d the averaging internally. Items t be recrded in the field bk fr magnetics: (a) Statin lcatin, including lcatins f lines with respect t permanent landmarks r surveyed pints. (b) Nearby surces f ptential interference Fr mdern magnetmeters, the time f the reading and the actual magnetic field readings are stred internally by the meter. The experienced magnetics peratr will be alert fr the pssible ccurrence f the fllwing cnditins: (a) Excessive gradients may be beynd the magnetmeter s ability t make a stable measurement. Mdern magnetmeters give a quality factr fr the reading. Multiple measurements at a statin, minr adjustments f the statin lcatin and ther adjustments f technique may be necessary t prduce repeatable, representative data. (b) Nearby metal bjects may cause interference. Sme items, such as autmbiles, are bvius, but sme subtle interference will be recgnized nly by the imaginative and bservant magnetics peratr. Old buried curbs and fundatins, buried cans and bttles, pwer lines, fences, and ther hidden factrs can greatly affect magnetic readings. Airbrne surveys are nw rutinely flwn at a grund clearance and grid interval that allws the recrding, in great detail, ver a wide area f a greater range f frequencies f the magnetic field respnses frm tp t bttm f the subsurface. Aermagnetic surveys are usually made with magnetmeters brne by aircraft flying in parallel lines spaced tw t fur kilmetres apart at an elevatin f abut 500 metres when explring fr petrleum depsits r gethermal prspects and in lines 0.5 t ne kilmetre apart rughly 200 metres abve the grund when searching fr mineral cncentratins. Mst f the data nw being acquired frm the air has the capacity t allw the interpreter t als reslve anmalies related t structures, the very features that are hped t be fcused n when beginning a grund explratin prgram. 4. DATA PROCESSING T make accurate magnetic anmaly maps, tempral changes in the earth s field during the perid f the survey must be cnsidered. Nrmal changes during a day, smetimes called diurnal drift, are a few tens f nt but changes f hundreds r thusands f nt may ccur ver a few hurs during magnetic strms. During severe magnetic strms, which ccur infrequently, magnetic surveys shuld nt be made. The crrectin fr diurnal drift can be made by repeat measurements f a base statin at frequent intervals. The measurements at field statins are then crrected fr tempral variatins by
Mariita 5 The magnetic methd assuming a linear change f the field between repeat base statin readings. Cntinuusly recrding magnetmeters can als be used at fixed base sites t mnitr the tempral changes. If time is accurately recrded at bth base site and field lcatin, the field data can be crrected by subtractin f the variatins at the base site. After all crrectins have been made, magnetic survey data are usually displayed as individual prfiles (Figure 2) r as cntur maps (Figure 3). Identificatin f anmalies caused by cultural features, such as railrads, pipelines, and bridges is cmmnly made using field bservatins and maps shwing such features. FIGURE 2: A magnetic reading prfile acrss a dyke 5. DATA INTERPRETATION Ttal magnetic disturbances r anmalies are highly variable in shape and amplitude; they are almst always asymmetrical, smetimes appear cmplex even frm simple surces, and usually prtray the cmbined effects f several surces. An infinite number f pssible surces can prduce a given anmaly, giving rise t the term ambiguity. Individual magnetic anmalies - magnetic signatures different frm the backgrund- cnsist f a high and a lw (diple) cmpared t the average field. In the Suthern Hemisphere the high is lcated t the nrth and the lw t the suth f the magnetic bdy. The psitin and size f the anmaly depend n the psitin and size f the magnetic bdy. A change in latitude will als affect the psitining f anmalies ver the magnetic bdy. This allws the gescientists t interpret the psitin f the bdy which has caused the anmalus reading. Often hwever the reading is cmplicated because f the psitin f the bdy in relatin t ther rcks, its size, and what happens t the bdy at depth. Data are usually displayed in the frm f a cntur map f the magnetic field, but interpretatin is ften made n prfiles. Frm these maps and prfiles gescientists can lcate magnetic bdies (even if they are nt utcrpping at the surface), interpret the nature f gelgical bundaries at depth, find faults etc. Like all cntured maps, when the lines are clse tgether they represent a steep gradient r change in values. When lines are widely spaced they represent shallw gradient r slw change in value. A mdern technique is t plt the magnetic data as a clur image (red=high, blue=lw and all the shades in between representing the values in between). This gives an image which is easy t read such as that shwn in Figure 3.
The magnetic methd 6 Mariita When interpreting the aermagnetic image it is useful t knw that magnetite is fund in greater cncentratins in igneus and metamrphic rcks. Magnetite can als be weathered r leached frm rcks and re-depsited in ther lcatins, such as faults. In a gethermal envirnment, this is a very useful feature as it may indicate the presence f faults, target fr drilling. FIGURE 3: Magnetic Survey cntur map t lcate pits cntaining buried Metallic Cntainers 6. RESULTS FROM AEROMAGNETIC SURVEYS OF OLKARIA GEOTHERMAL FIELD, KENYA Residual aermagnetic data acquired within the Rift Valley shws that the Greater Olkaria Gethermal Area has a psitive anmaly that has a NW-SE trend. The psitive magnetic anmaly separates tw negative anmalies t the suth and the nrth. The negative anmalies crrespnd t nrmally magnetized rcks. The psitive anmaly ccurs in a demagnetized zne crrespnding t the heat surce that is silicic rigin. This prvides sme evidence fr heat surce at a temperature abve the Curie pint f Magnetite (abve 575 C) clse t the surface. The ccurrence f magnetic and gravity anmalies at the intersectins f NE and NW rift faults, is an indicatin f distinct near surface heat surces cntrlling the reservir characteristics f the gethermal systems.
Nrthings Mariita 7 The magnetic methd 9910000 nt 34000 9905000 Olkaria N-E 33875 9900000 Olkaria West Olkaria East Olkaria Dmes 33750 33650 33500 9895000 33350 185000 190000 195000 200000 205000 Eastings FIGURE 4: Ttal magnetic intensity ver the Greater Olkaria area 7. COSTS OF MAGNETIC SURVEYS Magnetic surveying is ideal fr bth recnnaissance and fcused surveys. It is expedient and cst effective, cvers mre grund in less time, and requires a minimum f field supprt. The prtability f the instruments makes magnetic surveying well suited t sites with tpgraphic variatins. The csts are much lwer if the equipment belngs t the client. Estimate f the cst f a grund magnetic survey is derived frm the parameters designed fr the survey such as the size f the area, tpgraphy and statin spacing. These can be mdified t reduce cst withut causing significant degradatin f data quality. Ecnmic factrs gverning a grund survey include: It takes 30 secnds t take a reading Mbilizatin and dembilizatin will require 1/2 day each Ttal persn-hurs required fr prcessing, interpretatin and reprt preparatin is the same as ttal persn-hurs in the field Estimate the diurnal cmpnent f the field by cntinuusly mnitring the field at a base statin. Thus, ne will need t rent tw instruments and supply field crews fr bth. Field hands make $10/hur, and tw are required at all times in the field with the survey instrument. One is required at all times t mnitr the base statin instrument. Field hands will nly wrk 8 hurs per day. Data prcessing, interpretatin and reprt writing make $20/hur
The magnetic methd 8 Mariita Subsistence and travel expenses are $100/persn/8-hur day while ding the field wrk The magnetmeters depreciated at the rate f 1%/day (riginal cst = $7,500) Vehicle depreciatin is $50/day Fringe benefits fr emplyees are 25% f salary Overhead is 100% f ttal direct cst excluding equipment depreciatin Cnsulting fee is $200/hur Aermagnetic surveys are mre expensive than grund investigatins but they cver a wide area ver a shrt time. Further, they are less affected by cultural nise. 8. CONCLUSIONS The Earth's magnetic field is a cmpsite f anmalies f varying frequencies. The highest frequency events f interest are thse created by gelgical cnditins in the shallw subsurface and the lwest frequency events are caused by magnetic prperty cntrasts at r beneath the basement surface. Intermediate frequency events are created within a sedimentary sectin. There are a number f gelgical causes fr lcal distrtins in the Earth's magnetic field. Causes especially imprtant t an explratinist are thse that put near-surface frmatins with cntrasting magnetic prperties in cntact with ne anther. The result is minr but measurable anmalies. Imprtant lateral variatins and cntrasts in magnetic prperties f the shallw frmatins are brught abut singularly r by sme cmbinatin f faulting, depsitin and mineralizatin assciated with structural displacement. Airbrne surveys are nw rutinely flwn at a grund clearance and grid interval that allws the recrding, in great detail, f a greater range f frequencies f the magnetic field, respnses created thrughut the near surface structures, tp t bttm. These types f surveys are expensive, as cmpared t grund investigatins but they d cver a wide area in a cmparatively shrt time. Structuring the near-surface frmatins gives rise t a characteristic high wave-number magnetic expressin. The symmetry f the varius elements f such an anmaly, imprtant t their accurate interpretatin and definitin, is best bserved in residual prfile frm. With the prper prcessing and analysis f an area's ttal spectrum f magnetic respnses an interpreter can prduce nt nly a picture f basement structure and tpgraphy but als an verlay f the areal distributin f frmatin magnetic anmalies - linear anmalies thught t be the expressin f fracture znes and nn-linear anmalies attributable t lcal structural relief. Mdern magnetic surveys have the capacity t let us view the distributin and trending f shallw structure relative t deep structure fr an expanded perceptin f the structural setting. It's still a recnnaissance methd, but nw, ne with a greater fcusing pwer and infrmatin ptential than cnventinal magnetic surveys f years past. In a gethermal envirnment, due t high temperatures, used alng with gravity, this methd can be used t infer heat surces and faults as seen frm gethermal explratin at Olkaria. Grund magnetic measurements d prvide mre detailed infrmatin n sub-surface structures that culd act as heat surces in cmparisn t aermagnetic data.