Magnetic anomalies associated with high temperature reservoirs in the taupo volcanic zone (New Zealand)

Geothermics Vol. 26, No. 1, pp. 1-24, 1997

Pergamon

© 1997 CNR Elsevier Science Ltd Printed in Great Britain. All rights reserved 0375-6505/97 $17.00 + [).(X) P I I : S0375-6505(96)00028--4

MAGNETIC ANOMALIES ASSOCIATED WITH HIGH TEMPERATURE RESERVOIRS IN THE TAUPO VOLCANIC ZONE (NEW ZEALAND) MANFRED P. HOCHSTEIN and SUPRIJADI SOENGKONO Geothermal Institute, The University of Auckland, Private Bag 92019, Auckland, New Zealand (Received May 1995;acceptedfor publication June 1996)

Abstract--Distinct magnetic total force anomalies are associated with many, but not all, liquid-dominated geothermal reservoirs in the Taupo Volcanic Zone, which stand in thick, young (<0.7 Ma) volcanic host rocks. The anomalies are caused by demagnetised rocks (a result of hydrothermal alteration) in the upper 0.5-1 km section of the reservoirs; the phenomenon has been confirmed by core studies. Four different patterns of magnetic anomalies have been recognised. The most common type can be found over hightemperature systems where demagnetisation is confined to the main reservoir. Different anomaly patterns are associated with prospects characterised by subsurface outflows and reservoirs hosted in both normally and reversely magnetised rocks. A rather subdued and almost featureless pattern occurs over a few prospects where rocks lying outside the reservoir have also lost their magnetisation by interaction with acidic, steam-heated water. © 1997 CNR. Published by Elsevier Science Ltd. All rights reserved. Key words: Geophysical prospecting, airborne magnetic survey, residual magnetic anomalies, hydrothermal alteration and demagnetisation, geothermal reservoirs, Taupo Volcanic Zone, New Zealand.

INTRODUCTION The phenomenon that distinct magnetic anomalies are often associated with geothermal reservoirs standing in Quaternary volcanic rocks in New Zealand was recognised 58 years ago by Watson-Munro (1938). Ground-magnetic surveys were used in the early 1950s to explore several geothermal prospects in the Taupo Volcanic Zone (TVZ) but were of limited use because of the high noise content of the data (Modriniak and Studt, 1959; Studt, 1959). The first regional airborne magnetic survey of the TVZ (at 1.5 km height) was made in 1951-52 and total force data were published by Gerard and Lawrie (1955); the data were reduced later in terms of total force anomalies (AF) by Whiteford (1976) and

2

M. 17. Hoctl.stein aml 5, S o e n g k o n o

reproduced as regional maps (for example: Hunt and Whiteford, 1979). Although the maps show that several geothermal prospects are close to areas with distinct negative anomalies, other prospects are not associated with such anomalies because spacing between adjacent flight lines of the Gerard and Lawrie survey was often more than 3 kin. The interest in using magnetic surveys as an exploration tool declined in New Zealand in the 1960s when electrical methods were found to be more effective in delineating the lateral extent of conductive reservoir rocks. A similar development occurred in Iceland, where distinct negative magnetic anomalies were observed over many geothermal prospects (Palmason, 1975: Layugan, 1981), but rather indistinct patterns also occur over others (Bj6rnsson and Hersir, 1981 ). There were several reasons why these surveys received little attention before 1980 and why no quantitative interpretation was attempted. There was some uncertainty as to the cause of these anomalies. Demagnetisation by hot fluids had already been suggested as a mechanism by Watson-Munro (1938). The first studies of the vertical component of magnetisation of cores from the Wairakei borefield (NZ) by Studt (1959) showed that most are nearly non-magnetic: Studt inferred that this was the result of hydrothermal alteration. The magnetic susceptibility of cores and cuttings of many geothermal bores in the T V Z was measured for more than a decade as suitable material became available (data compiled by Whiteford and Lumb, 1975). However, the other important magnetic parameter, namely magnetic remanence, was not studied and the phenomenon of demagnetisation by hydrothermal alteration received little attention. The data quality of the earlier airborne surveys of the T V Z was often poor: there was great uncertainty as to the actual "zero" level of the bipolar magnetic anomalies. Studies by Soengkono 11995) showed that magnetic data from the TVZ, reduced with respect to normal geomagnetic fields, contain a deep regional component (probably the magnetic effect of deep crustal intrusions). Quantitative interpretation of magnetic anomalies requires 3-D modelling; however, user-friendly algorithms only became available from about 1975 (see, for example, Barnett, 1976). When we first looked at the significance of magnetic anomalies over NZ geothermal prospects about 10 years ago, we found that the original data of the 1951-52 T V Z survey had been lost. New, low-level airborne magnetic surveys (flight level ca 400 m above ground: distance between adjacent flight lines ca 1 km) were begun in 1984, and one of the first surveys led to the discovery of a distinct magnetic anomaly over the Mokai field (Soengkono, 1985) that had been missed by the 1951-52 survey. Since then we have expanded the survey to cover an area of about 3000 km 2 (Fig. 1). Quantitative interpretation of 111 geothermal prospects within the area has been undertaken to date (Soengkono, 1985: Hochstein et al., 1987; Henrys and van Dijck, 1987; Henrys and Hochstein, 1991t: Soengkono et al., 1991: Soengkono and Hochstein, 1992; Soengkono, 1992; Soengkono, 1993: Soengkono and Hochstein, 1995; Soengkono and Hochstein, 1996). Our studies show that there is a large variation in the patterns of magnetic anomalies associated with active geothermal systems. However, some of the patterns are similar and allow grouping and, hence, comparison. Our interest in the interpretation of magnetic anomalies over high-temperature prospects in other settings was taken up by several graduate students of the Geothermal Institute (The University of Auckland). They interpreted anomaly patterns over the vapour-dominated field of Kamojang and the Gunung Wills project in Java (Soengkono et al., 1988; Rachman, 1990) and the natural two-phase systems of Olkaria and Eburru in

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand) I

~

I

~

I

~

NDvEXIlia P

i

TR

~

D

L. R O T O R U A

6340

ite

I i

. . . . . . . 6 HH '-'

6320

K'~D

830o -

WT []

-~C

I\

I ,,d 17"-'1

I if

I w I

I

[] "] RP [ [] ]. [~ | % .t.~Y'~ ~

NTq

I

.... 7--

2740

r ....

'

,

, ',l

I

t

[ IF

[]

/ W~_

2760

~

3(,.~ ~'O""

..... ~

,

[ ~[

[ ." .'"

J-" •

.-"

""

2780

_

N ~

2800

GeohtermaTlH=TauharaWK=Wfields: ariakeM i K=Mokai BR=Ohaaki-Broadlands OK=Orakeikorako TK=Te Ko0ia AT=Atiamuri WT=W=otapu

/

-

-

[]WI []Wlvl

AT

"~ ~

8280

. . . . . . . . . . . . . . . . . . .

-

WM=Waimangu HH=Horohoro RT=Rotorua TR=Tikitere _ RM=Rotoma-Tikorangi KW=Kawerau

2820

2840

Fig. 1. Location of geothermal fields in the Taupo Volcanic Zone (TVZ), Central North Island, New Zealand. The area covered by our low-level airborne magnetic surveys is outlined by broken lines. Detailed interpretation of the airborne magnetic data has been made inside the framed areas; those used as examples in this paper (Figs 3-7) are shown by shading. Grid coordinates are in terms of the New Zealand Map Grid (km). Kenya (Anderson et al., 1987; N g ' a n g ' a , 1989). It is likely that our attempt to classify magnetic anomalies observed over active systems in New Zealand has implications for recognising magnetic anomalies in similar settings elsewhere. BACKGROUND Several uncertainties have impeded useful interpretation of magnetic anomalies over geothermal prospects, namely: (1) uncertainties as to the cause of the anomalies, i.e. why they are significant over some prospects but not over others; (2) the problem as to what constitutes representative residual magnetic anomalies in large volcanic fields; and (3) uncertainties in ranking modelling constraints. These points are discussed here, as acceptance of magnetic modelling studies and classification of the types of magnetic anomalies over geothermal prospects require a clear understanding of these problems.

Magnetisation and natural demagnetisation of volcanic reservoir rocks All volcanic rocks in the T V Z were magnetic after their eruption, as a result of their induced and remanent magnetisation. Induced magnetisation is proportional to the

4

M.P. Hochstein attd S. Soengkono

magnetic susceptibility which, in turn, is related mainly to the volume fraction of two primary magnetic minerals, magnetite and titanomagnetite. In the western TVZ, the induced magnetisation (IM) of rhyolites and pyroclastics is of the order of 0.5 A/m (Hochstein and Soengkono, 1994), pointing to the presence of ca 0.8% (by volume) of primary magnetic minerals (Lawton and Hochstein, 1980). Petrological studies by Ewart (1966) indicate magnetite values between 0.3 and 0.8% from "point counting". The magnitude of the remanent magnetisation of these rocks is usually significantly greater and lies commonly within the range of 1-4 A/m (Hochstein and Soengkono, 1994), reflecting a magnetisation acquired during their cooling processes (thermal remanent magnetisation TRM). Hydrothermal alteration usually causes a "'de-magnetisation'" of initially magnetic reserw)ir rocks, a process which will bc discussed later. It can also produce, on a smaller scale, a "'re-magnetisation" when pyrrhotite is deposited, which occurs as a secondary mineral in some New Zealand geothermal reservoirs. Little is known about the formation of this hydrothermal, magnetic trace mineral, which often can be found near rocks containing organic matter. Its magnetic stability range is restricted by its low Curie temperature (ca 320°C according to Butler, 1992). A study by Browne and Ellis (1970) showed that small amounts of pyrrhotite occur in about one third of the OhaakiBroadlands wells. Pyrrhotite has also been found in a few wells at Wairakei and at Waiotapu (Steiner, 1977): it is usually confined to barren fissures, i.e. not dispersed. Our studies of cores from l 1 wells at Ohaaki-Broadlands (Fig. 2c) indicate that the magnetic effect of this alteration mineral (if present) must be small. However, two cores from well BR 5 (821 and 1135 m depth), which show up with significant magnetisation in Fig. 2c, contain no magnetite but some pyrrhotite (P. Browne, pers. comm.). In the following it will be assumed that "re-magnetisation'" of reservoir rocks caused by pyrrhotite deposition is small and can be neglected. Most of the volcanic rocks in the T V Z were deposited during the last 0.7 Myr (Hochstein, 1995) and exhibit a "normal" magnetisation where the direction of the IM is close to that of the TRM. However, there are also older volcanic rocks (0.7-1.5 Myr) in the T V Z where the direction of T R M is opposite to that of the present day earth's field (Soengkono et al., 1992: Grmdley et al., 1994). Vectorial addition gives the total magnetisation M, which is the magnetic rock parameter causing the magnetic anomalies described in this paper. The magnitude of total magnetisation IM] of unaltered, normally magnetised volcanic rocks in the T V Z lies between ca 0.5 A/m (tufts, pumice, and breccias) and ca 2.5 A/m (rhyolite lavas). The values cited are median values obtained from more than 200 rock samples (Hochstein and Soengkono, 1994); average values of similar volcanic rocks from the T V Z cited in Rogan (1982) and Hunt and Smith (1981) are between 2.5 and 3 A/re. The effect of an overprinted event (viscous remanent magnetisation or VRM) was found to be small and is neglected here since, for the interpretation of magnetic anomalies associated with high temperature geothermal prospects, only the average total magnetisation contrast between rocks inside and outside a geothermal reservoir is of importance. Stability c~/"magnetite in geothermal reservoir,s Petrological studies show that, in these reservoirs, primary (titano-) magnetite has been replaced by almost non-magnetic minerals such as pyrite, leucoxene, or hematite (Steiner, 1953). In New Zealand liquid-dominated systems (titano-) magnetite appears to be the

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand) (a) Mokai Total magnetJsatJon (A/m) 0.0 0.5 1.0 1.5 2.0 2.5 ,~,ll,,~l,,,Wl,~,,I,,,,l~l~t] ® ®

3.0

® ®

8 O O 1000

O o

g

£

Key:

a 1500 ,

I

O

inside the field

®

marginal (outltow) = MK1

2OOO

O

25OO

Fig. 2a. Plot of total magnetisation (magnitude) of cores from Mokai wells against depth (modified from Soengkono, 1985). The shaded vertical band covers the range (1.7-2.8 A/m) of values obtained from analyses of topographic effects in the western TVZ and includes the range of median values of unaltered surface samples.

first mineral replaced during thermal alteration; this also applies to many liquiddominated systems in the Philippines and Indonesia (Browne, 1994). However, in an oxidising environment, magnetite can be more stable, as is indicated by the magnetisation of two cores from above the shallow boiling level in the Mokai reservoir (Fig. 2a). Magnetite is also stable in the deeper oxidising environment of the natural two-phase system of Olkaria (Kenya) and, in that reservoir, it is less affected by thermal alteration than all other primary phases except quartz (Browne, 1994); the same applies to host rocks of the El Tatio outflow in Northern Chile. The stability of magnetite at shallow depths (<100 m) at Mokai, where vapour is dominant, suggests that demagnetisation by hydrothermal alteration should be insignificant in a vapour-dominated system. Core studies from the Kamojang field (Java),

6

M . P . Hochstein and S. Soengkono (b) Wairakei Total magnetisation (A/m) 0.0

0.5

1.0

1.5

2.0

2.5

3.0

®

O 5oo

®

®

O0 0 • •

q> ®

O • 1000

eO

• •

Key:

®

g 4)

o

O

inside the field

®

marginal

•

outside the field

1500

2o0o

O

!!!

i

25o0

Fig. 2b. Total magnetisation of cores from Wairakci borcs measured by Lampoonsub (1987).

however, have shown that almost all rocks in its vapour reservoir are non-magnetic (Soengkono et al., 1988). The apparent inconsistency has been solved by petrological studies, which show that Kamojang was a liquid-dominated system when alteration occurred, as is indicated by the widespread occurrence of replacement and vein minerals that deposited from a liquid phase, including wairakite (P. Browne, pers. comm.). In several exposed rhyolite domes inside and near to the margins of several New Zealand systems, magnetite is also unstable at shallow levels, even in an oxidising environment. In the Rotokawa field, for example, one of these domes is completely demagnetised, whereas its twin just outside the resistivity boundary is still magnetic (Soengkono et aI., 1991). Another demagnetised rhyolite dome located at the margin of the Mokai geothermal reservoir is discussed later. At present, we believe that acidic condensates formed in shallow vapour zones can cause demagnetisation of volcanic rocks forming topographic highs. The solubility of magnetite increases rapidly in aqueous

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand) (c) Ohaald-Broadlands Total magne~satJon (A/m) 0.5

0.0 0

1.0

1.5

2.0

2.5

3.0

-

1000

E ®

Key:

® 0

1fi00

inside the field

®

marginal

•

outside the field

I

0 2000

0

25oo

Fig. 2c. Total magnetisation of cores from the Ohaaki-Broadlands borefield (data from Morales, 1988). solutions with decreasing pH values (for pH < 6) and at temperatures below 200°C, according to Bohnsack (1987). Magnetite is also unstable in volcanic rocks that are saturated with CO2-rich, steamheated waters with a pH value of <6. These occur, for example, near the top and margins of the Ohaaki-Broadlands reservoir (Hedenquist and Stewart, 1985). The waters are corrosive and have caused significant external corrosion to well casings. All cores from the cooler well BR-6, which discharged such fluids, were nearly non-magnetic. Siderite is an important replacement mineral at the depths from which the CO2-rich fluids derived (P. Browne, pets. comm.). Since cooler, CO2-rich waters at the margin of a gas-rich field like Ohaaki-Broadlands can also occur outside its boundary, it is possible that non-magnetic rocks can extend beyond the boundary of such a field delineated by resistivity surveys. Migration of geothermal activity can also produce a similar effect. To check whether demagnetisation of cores decreases in bores located near the margin

8

M . P . Hochstein and S. Soengkono

of, or outside, geothermal reservoirs, the data in Figs 2a, 2b, and 2c were ranked by using different symbols indicating whether a bore lies inside, at the margin, or outside a reservoir. A bore within 1 km of a low resistivity boundary was classified as marginal. One lying more than 1 km outside it was ranked as an "outside" bore. However, there are only a few bores in the T V Z that lie outside a geothermal field since almost all exploratory bores were sited on the results of resistivity surveys. The few bores drilled outside the OhaakiBroadlands prospect and outside the Wairakei resistivity anomaly, for example, all encountered temperatures greater than 120°C at 1 km depth. The data in Figs 2b and 2c, shown as points of "outside" bores, do not strictly pertain to "cold" wells. The data points in Figs 2a, 2b, and 2c show that there is no obvious difference in the scatter of the magnitude of magnetisation, although there is a tendency for most cores from the margins of the Mokai and the Wairakei fields to retain some of their magnetisation. The Wairakei data (Fig. 2b) indicate that demagnetisation of rocks also occurs outside the field, showing that magnetic anomalies are not necessarily restricted to the present reservoir area as defined by resistivity anomalies. An interesting phenomenon is indicated by the high (ca 1 A/m) magnetisation of a few cores from the Mokai and the Wairakei reservoirs taken from wells deeper than 1.6 km (Figs 2a and 2b). A similar observation was earlier made by Studt (1959). Unfortunately, only a few cores have ever been taken from deep exploratory bores in the TVZ, and the data shown in Figs 2a and 2b are insufficient to infer whether or not all deeper volcanic rocks in New Zealand reservoirs retain their magnetisation. However, magnetic modelling studies listed in the introduction indicate that deep rocks inside most New Zealand reservoirs are magnetised. Significant magnetisation of deep cores has also been reported for some bores in the Kamojang field in Java (Soengkono et al., 1988). Demagnetisation by hydrothermal alteration is an irreversible process if pyrrhotite deposition can be neglected. A distinct "'demagnetisation'" anomaly pattern can therefore be observed over many extinct geothermal reserw~irs located in volcanic terrain. Such an anomaly has been observed by Ignacio (1985) over the Ohakuri prospect in the T V Z (near to the Atiamuri prospect in Fig. 1): the prospect has been described in detail by Henneberger and Browne (1988). Studies of exposed, now cold reservoir rocks at Ohakuri showed that there is a correlation between the decrease in magnitude of their total magnetisation and the increase in rank of alteration (Allis, 1990); a similar finding has been reported for cores from the Kamojang wells (Soengkono et al., 1988). The phenomenon that subdued magnetic anomalies in ground surveys are a characteristic feature of some epithermal prospects in the volcanic terrain of the Coromandel Peninsula (New Zealand) had already been noticed by Modriniak and Marsden (1938). At that time there was still some doubt as to whether epithermal prospects in such a setting were associated with extinct high-temperature reservoirs. This association became more obvious as a result of overseas studies in the 1950s and it was confirmed by a thorough study of Criss et al. (1985). Their work showed that epithermal vein deposits in Idaho (U. S. A.) are hosted in hydrothermally altered volcanic rocks exhibiting a very low magnetisation with respect to that of their country rocks; the airborne magnetic anomaly over the main prospect (Yankee Fork District) has close affinity with those observed over many active geothermal systems described in this paper, Allis (1990) also commented on the close connection between geophysical anomalies in both settings. The finding that the volcanic cover of epithermal prospects is usually non-magnetic, thus causing rather subdued magnetic anomalies within volcanic terrain, is now used to locate

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

Table 1. Average magnetisation contrast of rocks in selected geothermal prospects Prospect

Host rocks

Northern Iceland Kamojang (Java) TVZ systems (NZ) Olkaria (Kenya)

basaltic andesitic rhyolitic trachytes pantellerites commendites

AIM I (A/m) - 4 to - 5" ca - 2 . 5 t - 1.5 to -2.55 -0.3 to -0.6§

*Values are from an analysis of the Theistareykjir prospect cited in Hochstein and Soengkono (1994). tBased on the study by Soengkono et al. (1988); in other parts of Java the magnetisation contrast can be as low as -0.5 A/m (Rachman, 1990). SThis paper. §From Anderson et al. (1987).

such prospects by magnetic reconnaissance surveys (Irvine and Smith, 1990). The same phenomenon probably explains the rather subdued magnetic anomalies over oceanic spreading centres (Levi and Riddihough, 1986). The "effective" magnetisation contrast

Because of the large scatter in the value of IMI of unaltered rocks and the lack of cold wells outside the prospects, it is not possible to assess directly the magnetisation contrast, A IMI = (I M I inside - IMI outside), from core studies. Instead, this parameter has to be inferred from modelling studies since it controls the magnitude of the residual magnetic anomaly over most prospects. Table 1 shows that the best fit values of A IM I can vary by one order of magnitude. This range in AIM I reflects, in part, the range of magnetic properties of unaltered host rocks but also differences in intensity of alteration as a function of fluid properties and elapsed time. The parameter A IMI also decreases if a sequence of normally and reversely magnetised rocks coexist in the same reservoir, i.e. if, for example, younger ( < 0.7 Ma) and older ( > 0.7 Ma) volcanic rocks occur together. This setting occurs in the western T V Z (Soengkono et al., 1992) and also in the eastern T V Z near Waiotapu (Grindley et al., 1994). Although no interpretation of fossil prospects in this setting has yet been made, we predict that A IM I values will be small. THE REGIONAL FIELD PROBLEM

The parameter measured during the airborne magnetic surveys described here is the magnitude of the total magnetic force IF]. Residual A IF I anomalies are obtained by subtracting the effect of the "normal" geomagnetic field (usually an international reference field) from the measured data. We found that these "first order" residuals over large volcanic fields mostly contain signatures of deep-seated magnetic structures (crustal intrusions), causing a "static shift" due to a long wavelength regional anomaly (Soengkono, 1995). Such shifts can distort the bipolar anomalies. Interpretation of residual

10

M. P. Hochstein and S. Soengkono

anomalies affected by "regional shifts" always results in unsatisfactory models unless the phenomenon is recognised and appropriate reductions are made. Two procedures can be used to obtain a representative "zero level" for residual anomalies in volcanic terrain: (a) the level can be defined from analysis of the topographic magnetic effect of an isolated topographic feature lying outside the prospect (not disturbed by the effects of concealed shallow magnetic bodies); (b) the effect of the regional field can be assessed from a simultaneous analysis of "first order" anomalies recorded at two flight levels if the surveys are also extended over non-magnetic basement rocks. For reducing data over prospects in the T V Z we now use the second method (Soengkono et al., 1991); however, for one prospect at its northern margin (Kawerau) the first method has been used.

RANKING OF CONSTRAINTS AND PROCEDURES USED FOR MAGNETIC

MODELLING Whether interpretation of magnetic data can provide information to support existing exploration models or to improve our understanding of the reservoir structure depends upon the emphasis given to various constraints during interpretation. We found the following. (1) A simple numerical interpretation model that reproduces the main features of an observed anomaly pattern is usually better than any detailed qualitative interpretation, which often induces bias. (2) Numerical modelling should start using 3-D structures, since preliminary 2-D modelling can also induce bias. (3) Modelling of terrain effects should be done before attempting to model any effects associated with a demagnetised reservoir. It is usually sufficient to assume a homogeneous magnetisation for the terrain. Terrain modelling provides useful information as to which parts of the terrain lying close to the prospect have been partly or completely demagnetised by earlier geothermal events. Terrain effects should not be subtracted from the residual anomalies because this can introduce a bias towards the resulting second-order residual anomalies. We usually retain the magnetic terrain as part of the final interpretation model. (4) To model the effect of demagnetised reservoir rocks it is usually sufficient to assume vertical boundaries for the demagnetised part of the reservoir; the thickness of the demagnetised body is constrained by the appropriate AIM I value. The lateral extent of the demagnetised body should be constrained initially by the extent of the lowresistivity anomaly. Resistivity boundaries, however, do not always coincide with the boundary of the demagnetised body. We have found several cases where the demagnetised part of a reservoir is smaller than that indicated by its low-resistivity structure. There are also prospects, for example the Wairakei field, where concealed demagnetised rocks extend outside the resistivity boundary of a liquid-dominated reservoir (see Soengkono et al., 1991). (5) Other concealed magnetic bodies producing significant effects near the boundaries of the reservoirs should only be included in the modelling process if their magnetic effect conceals important parts of the main bipolar anomaly caused by the demagnetised reservoir. Wairakei is an example of this, where the magnetic effect of a concealed

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

11

andesitic strato-volcano had to be included in the latest model (Soengkono and Hochstein, 1992; Soengkono and Hochstein, 1995). CLASSIFICATION OF MAGNETIC ANOMALIES OVER HIGH-TEMPERATURE GEOTHERMAL PROSPECTS

Our interpretations of magnetic data over high-temperature prospects in the Taupo Volcanic Zone show that four types of magnetic anomaly patterns exist in this setting. Similar patterns can also be recognised for overseas prospects. The residual anomalies presented in this chapter can be compared directly since most surveys were made using similar flight elevations (350-400 m above mean ground level), except for the BroadlandsOhaaki survey, where the ground clearance was only ca 150 m, and the Kawerau survey, where the mean flight elevation was ca 650 m above ground. The distance between adjacent flight lines was ca 1 km except at Broadlands-Ohaaki, where the spacing was reduced to ca 0.5 km. Target anomalies of Type A (example: Mokai) This anomaly type can be recognised by its significant residual, negative total force anomaly if the reservoir is hosted in normally-magnetised country rocks. The wavelength of the anomaly is similar to that of the hydrothermally altered, electrically conductive part of the upper reservoir which, in turn, is often outlined by a characteristic low-resistivity anomaly. The anomaly pattern is impressive if it occurs over a prospect in fiat terrain, i.e. if the effect of the surrounding magnetic topography is small. In such a setting, the magnetic anomaly can exhibit rotational symmetry. Such an anomaly occurs over Mokai (Soengkono, 1985), Ngatamariki (Soengkono, 1992), and Rotokawa (Soengkono et al., 1991); outside New Zealand it was found over the Namafjall prospect in Iceland (Palmason, 1975) and over the Kamojang field in Indonesia (Soengkono et al., 1988). Sometimes the magnetisation contrast can vary across a prospect causing, for example, several coalescing negative anomalies such as occur at Rotokawa (Soengkono et al., 1991). Distinct, larger negative and positive anomalies can also occur together over the same reservoir. This pattern occurs at Wairakei, where the positive magnetic anomaly of a concealed deeper, unaltered andesite body conceals, in part, the negative anomaly located on the southern side of the field (Soengkono and Hochstein, 1992; Soengkono and Hochstein, 1995). The magnetic anomaly over the epithermal deposit in the Yankee Fork District in Idaho (U.S.A.) is a good example of this type of anomaly over an extinct geothermal system (Criss et al., 1985). Observed residual and computed best-fit magnetic anomalies over the Mokai field (New Zealand) are shown in Figs 3a and 3b (from Soengkono, 1985). This was our first study to assess the extent of demagnetised rocks associated with a New Zealand geothermal prospect. Analysis of the topographic effect shows that the nearby Pukemoremore rhyolite dome is also demagnetised; other domes outside the area (not shown in Fig. 3a) were found to be magnetised (av. IMI - 2.5 A/m). The demagnetised dome (in Fig. 3a) indicates that some thermal fluids from the Mokai reservoir ascended through this extrusion in the past. The extent of hydrothermally altered rocks beneath the main reservoir was taken from resistivity surveys (Bibby et al., 1984). The first magnetic model already provided a good fit (see Fig. 3b). The boundaries of altered, electrically conductive rocks and of demagnetised rocks almost coincide at Mokai.

12

M. P. Hochstein and S. S o e n g k o n o

2761

2763

2765

2767

2769

Fig. 3a. Observed residual total force magnetic anomaly (solid contours, values in nT) at ca 400 m above ground over the Mokai geothermal field. The topography is shown by broken contours (contour interval: 100 m). The resistivity boundary is outlined by a darker band (taken from Bibby et al., 1984). Grid coordinates are in terms of the New Zealand Map Grid (km).

There is, however, an important difference between the magnetic and resistivity models shown in Figs 3a and 3b. The Mokai field is associated with a concealed outflow that transfers hot water from the main reservoir towards the Waikato River. The effect of this outflow can just be recognised in the resistivity maps of Bibby et al. (1984), but it is not apparent in the magnetic map. The outflow is significant, transferring heat at a rate of up to 400 MW over a distance of ca 8 kin, although only 1% of that heat is discharged by steam over the central part of the field. The absence of clear demagnetisation anomalies over the inferred outflow regime of the Mokai field indicates that the outflow is confined to a thin shallow layer, or that it is a deep outflow, or that the outflow is recent. In several other prospects that occur in a similar setting, shallow outflows are often associated with distinct magnetic anomalies that can be interpreted in terms of the presence of demagnetised rocks confined to the outflow (see below).

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

-

- ]2"/61

2763

2765

2767

13

2769

Fig. 3b. Theoretical (computed) total force magnetic anomaly over the Mokai geothermal field (solid contours, values in nT), showing the effects of topography and demagnetised bodies. The nonmagnetic rocks are outlined by shaded polygons at Mokai (sea-level to -500 m) and at Pukemoremore (surface to +400 m); modified from Soengkono (1985).

Elongated anomalies of Type B (example: Rotoma-Tikorangi) Concealed outflows from high-temperature reservoirs hosted in volcanic rocks can also be associated with demagnetised rocks, giving rise to distinct elongate-shaped magnetic anomalies. The characteristics of outflows in steeper terrain have been summarised by Hochstein (1988); one important feature that allows their recognition is their temperature inversions in exploratory wells that penetrate them. A good example for the magnetic signature of such outflows is the residual anomaly over the Rotoma-Tikorangi prospect shown in Fig. 4. This anomaly was discovered by a regional magnetic survey conducted by Salt (1985). Geophysical follow-up studies confirmed the existence of hot ground beyond a small area with active surface manifestations that appears to be located over an upflow (Hochstein et al., 1987). A resistivity reconnaissance survey by Bromley et al. (1988) showed that the elongated demagnetised structure here is associated with a low-resistivity area that reflects the extent of hydrother-

14

M . P . Hochstein and S. Soengkono

2818

2820

2822

2824

2826

2828

Fig. 4. Residual total force magnetic anomalies (solid contours, values in nT) at ca 350 m above ground over the Rotoma-Tikorangi geothermal prospect (modified from Hochstein et al., 1987). The Tikorangi magnetic low (residual anomalies ~< -200 nT) is shaded. The resistivity boundary is shown as a darker band (taken from Bromley et al., 1988). Grid coordinates are in terms of the New Zealand Map Grid (km).

mally altered rocks (see Fig. 4). This magnetic anomaly cannot be interpreted in terms of topographic effects. Geochemical data confirm that thermal fluids are transferred by concealed outflows to Lake Rotoehu and Lake R o t o m a in the north and to the Tarawera River in the south (Nairn and Finlayson, 1981); the chloride flux data point to a heat transfer of at least 300 MW (Hochstein, 1995). Only a few airborne magnetic surveys have been made over other prospects with outflows. One of these surveys covers the O r a k e i k o r a k o prospect (Fig. 1), which is probably located at the toe of an elongated, SW-trending outflow. A characteristic negative anomaly over O r a k e i k o r a k o has been interpreted by Soengkono (1993) in terms of a shallow, demagnetised reservoir. The anomaly can also be recognised in an apparent magnetisation map based on a low-level (ca 100 m ground clearance) airborne magnetic survey undertaken by a mineral exploration group (AIlis, 1990). In both maps the

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

15

magnetic anomaly over Orakeikorako connects with another elongated, NE-trending negative anomaly that probably represents a non-magnetic (i.e. demagnetised) feeder system within a major fault zone (Paeroa Fault). Most exploratory holes in the Orakeikorako prospect show significant temperature inversions, supporting the outflow hydrology suggested by Sheppard and Lyon (1984). Elongate-shaped magnetic anomalies have been observed over the Krafla prospect in Iceland (Palmason, 1975). A pronounced negative anomaly that extends from the central bore field to the south-southwest encloses at its southern end the Hvitholar prospect (Armannsson et al., 1989). Magnetic anomalies over the outflow of the G. Wilis prospect in Java are also elongated (Rachman, 1990). The phenomenon of concealed outflows of thermal fluids has not received much attention in the literature although it was already described 23 years ago (Healy and Hochstein, 1973). These flows exist and recognition of any magnetic anomaly pattern similar to that described here as Type B pattern can elucidate such hydrological features. However, not all outflows are necessarily associated with Type B anomalies, as shown by the absence of a clear magnetic anomaly over the outflow from the Mokai reservoir (Fig. 3a).

Complex patterns of Type C (example: Waiotapu-Reporoa) Recently we compiled a magnetic anomaly map of the Waimangu-Waiotapu-Reporoa area (Fig. 5a). These anomalies exhibit a rather complex pattern that differs significantly from those discussed so far (i.e. Type A and Type B anomalies). The resistivity data for the area are shown in Fig. 5b and outline a coherent, almost 20 km-long band of low-resistivity rocks that extends from the Reporoa prospect in the south to the Waimangu field in the north (Bibby et al., 1994). Boundary segments can be recognised on each side of the lowresistivity areas. The complex magnetic pattern can be recognised if one compares Fig. 5a with Fig. 5b. Although there are some negative magnetic anomalies inside the lowresistivity structure, there are also several negative anomalies that lie outside it. Two of the "outside" anomalies occur over ground where reversely magnetised rocks have been reported by Grindley et al. (1994) and Wood (1994); these areas are shown by dark shading in Fig. 5b. The anomalies labelled by "R" in Fig. 5a can be modelled by a sequence of reversely magnetised rocks with an apparent magnetisation contrast of - 0 . 5 to - 1 . 5 A/m (Soengkono and Hochstein, 1996). Reversely magnetised rocks are also associated with the rim of the ill-defined Reporoa Caldera (Wood, 1994), as shown by the band of negative anomalies along the northern and eastern margin of the Reporoa resistivity structure. The complex magnetic anomaly pattern over the area shown in Fig. 5a represents, therefore, the combined magnetic effect of at least three different rock types, namely: (1) older, reversely magnetised rocks; (2) hydrothermally altered and demagnetised rocks occurring in parts of the thermal reservoirs; and (3) younger, normally magnetised country rocks. Most of the anomalies in Fig. 5a cannot be recognised in the magnetic map of Bibby et al. (1994), which covers the same area and which is based on data originally collected in 1951-52 by Gerard and Lawrie. The subdued anomalies of Type D (examples: Ohaaki-Broadlands and Kawerau) Subdued or insignificant magnetic anomalies over active high-temperature systems are called here Type D anomalies. We first observed this type of anomaly over the OhaakiBroadlands field (Henrys and van Dijck, 1987). Older magnetic surveys had already shown

16

M. P. Hochstein and S. S o e n g k o n o

2795

2800

2805

2810

2815

Fig. 5a. Residual total force magnetic anomalies (solid contours, values in nT) at ca 400 m above ground over the Waimangu-Waiotapu-Reporoa geothermal fields. Residual anomalies with < -200 nT are shaded. Segments of the inferred resistivity boundary are shown by darker shading. The letter "R" over some anomalies refers to anomalies over areas where reversely magnetised rocks have been found; the letter "D'" is attached to anomalies most likely caused by hydrothermally altered, demagnetised rocks occurring inside low-resistivity areas (modified from Soengkono and Hochstein, 1996). Grid coordinates are in terms of the New Zealand Map Grid (km). that the magnetic anomalies over this prospect were subdued (Hochstein and Hunt, 1970). We used, therefore, a flight elevation of only ca 150 m above ground when the magnetic survey was repeated in 1984. The survey produced the anomaly pattern shown in Fig. 6a. This figure shows that the geothermal reservoir, as outlined by its resistivity boundary, is not associated with any obvious magnetic anomaly. A positive anomaly lies just outside the eastern boundary; it is caused by normally magnetised rocks of the unaltered part of a concealed, young dacite v o l c a n o - - t h e Broadlands strato-volcano. The western and major portion of this body lies inside the reservoir; it is demagnetised by intense hydrothermal alteration. An interpretation model across this structure is shown in Fig. 6b (from Henrys and Hochstein, 1990). Other concealed rhyolite domes lie to the northwest and southwest of the O h a a k i -

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

--2795

2800

2805

2810

17

2815

Fig. 5b. Resistivity structure of the Waimangu-Waiotapu-Reporoa geothermal fields. The contours show the apparent resistivity (in ohm-m) for a nominal Schlumberger array spacing of 1 km (taken from Bibby et al., 1994). Segments of the inferred resistivity boundary are shown by darker shading. The extent of active surface manifestations is shown by a light shading.

Broadlands reservoir; they are not associated with any characteristic magnetic anomaly. It has been inferred by Henrys and van Dijck (1987) that most volcanic rocks outside the field have also lost their magnetisation. The overall, rather uniform demagnetisation, as shown in Fig. 2c, points to an alteration mechanism that is not restricted to the thermal reservoir only. At present we believe that the acidic, CO2-rich, cooler condensates that surround the reservoir have altered rocks lying outside, causing a quasi-regional demagnetisation. Since the anomalies shown in Fig. 6a would decrease by half an order of magnitude if the survey had been made at the usual ground clearance of ca 4 0 0 m , it is inferred that the magnetic anomaly at that height would have been similar to the magnetic anomaly observed over the Kawerau reservoir (Fig. 7). The magnetic data here were recorded at an elevation of ca 650 m above ground. This height was required to clear the young (/> 3000 yr) andesite strato-volcano of Mt. Edgecumbe, which produces the topographicallycontrolled anomaly shown at the bottom of Fig. 7. The modelling of the topographic effect

18

M. P. Hochs~in and S. Soengkono

Fig. 6a. Residual total force magnetic anomaly (solid contours, values in nT) at 160 m above ground over the Ohaaki-Broadlands geothermal field (modified from Henrys and Hochstein, 1990). Grid coordinates are in terms of the New Zealand Map Grid (km). Average spacing between flight lines was ca 0.5 kin.

~.~150

n Observed AF anomalies ]~ /

Computed A F anomalies

[]

/~..

50

.... ~E'-O. 6 ~"

Surface ~L

I

I

Broaclends Dacite

AM=2.5-3.0 A/m

...j

0

2

4 6 8 Distance (km) Fig. 6b. Observed and computed magnetic anomalies of profile A - A ' (see Fig. 6a) together with interpreted cross-section (taken from Henrys and Hochstein, 1990). The computed magnetic anomalies (solid line in the upper figure) are the magnetic effect of a 3-D magnetic dacite body outlined by a thick line in the lower section. The resistivity boundary is shown by a shaded band.

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

19

Fig. 7. Residual total force magnetic anomalies (solid contours, values in nT) at ca 650 m above ground over the Kawerau geothermal field. The topography is shown by broken contours (contour interval 100 m). Grid coordinates are in terms of the New Zealand Map Grid (km).

of the extinct volcano defined the zero level of the residual anomalies shown in Fig. 7, which have not been published previously. The upper section of the Kawerau reservoir consists of young volcanic rocks (down to ca 1-1.5 km depth), which are underlain by non-magnetic Mesozoic sediments. The absence of any significant magnetic anomaly over the Kawerau prospect indicates that the volcanic rocks inside and outside this reservoir have lost their magnetisation. The absence of magnetic anomalies over rhyolite domes (Onepu domes) exposed near the western margin, as defined by the resistivity boundary shown in Fig. 7, shows that these domes have also been demagnetised by alteration. Since the magnetic properties of cores from Kawerau wells have not been measured yet, we cannot give a satisfactory explanation for the absence of a clear magnetic anomaly pattern over this prospect.

SUMMARY AND DISCUSSION

The interpretation of magnetic anomalies associated with high-temperature systems in the Taupo Volcanic Zone (New Zealand) has shown that, in most cases, these anomalies

20

M. P. Hochstein and S. Soengkono

are controlled by the lateral extent of demagnetised rocks in the upper (0.5-1 km) sections. Some of these anomalies are distorted by the magnetic effects of topography and deepseated crustal masses, effects that can be reduced. The lateral extent of the demagnetised rocks, which are the product of hydrothermal alteration, is usually confined to the extent of altered rocks containing conductive alteration minerals that can be located by resistivity surveys. Although hydrothermal alteration removes primary magnetic minerals (titanomagnetite), new secondary magnetic minerals, such as pyrrhotite, can also be deposited in specific and Iocalised settings. The associated "secondary" magnetisation, however, is very small. For Type A anomalies there is often good agreement between the demagnetisation and resistivity boundaries. There are, however, prospects where the magnetisation contrast between the hot reservoir and surrounding cold rocks is small. Here, rather indistinct magnetic anomaly patterns (classified as Type D anomaly) have been observed. A low magnetisation contrast can be caused by the widespread demagnetisation of rocks lying outside the reservoir that have probably reacted with cool, mildly acidic, CO2-rich steam condensates. Demagnetisation of exposed domes and smaller volcanoes lying over or close to a high-temperature reservoir has also been observed. It is an interesting phenomenon since hydrothermal alteration of these features only involves vapour/condensate interaction, which should not affect the stability of primary magnetic minerals. However, demagnetisation of high-standing topographic features does occur, presumably involving dissolution of magnetite by acidic condensates. Demagnetisation of magnetic host rocks is also significant where hot fluids are discharged from a liquid-dominated system as "concealed outflows", which are common where a reservoir occurs beneath steeper volcanic terrain. If these outflows occur at shallow-to-intermediate depth, they can be recognised in airborne magnetic surveys by their often elongate-shaped (Type B) anomalies. Mapping of the magnetic anomalies associated with two larger, concealed outflows led to the discovery of a large geothermal prospect, the Rotoma-Tikorangi prospect, in the Taupo Volcanic Zone. However, not every concealed outflow hosted in volcanic rocks is associated with a Type B anomaly. Characteristic demagnetisation anomalies can also be found over extinct geothermal reservoirs and over certain areas lying outside the resistivity boundary of still active reservoirs. Migration of thermal activity is indicated in the latter case. A complex magnetic anomaly pattern can occur where geothermal reservoirs are surrounded by reversely and normally magnetised country rocks. So far, we have found only one example with such a pattern, classified here as a Type C anomaly. Its pattern is distinctly different from that of all other magnetic anomaly patterns observed elsewhere. Our experience with modelling the magnetic effects of 10 geothermal prospects allows us to distinguish between four types of magnetic anomalies, which have been described here. The examples presented cover the whole spectrum of magnetic anomaly patterns over geothermal prospects as seen in the Taupo Volcanic Zone. The classification is still incomplete since other possible patterns, for example, anomalies over reservoirs hosted in reversely magnetised rocks, have not been included because they have not yet been observed in New Zealand.

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

21

Acknowledgements--The authors would like to thank Patrick Browne (Geothermal Institute) and Chris Bromley (Wairakei Research Centre) for their valuable comments on the first draft of this paper.

REFERENCES Allis, R. G. (1990) Geophysical anomalies over epithermal systems. Journal of Geochemical Exploration 36, 339-374. Anderson, E. B., Mwangi, M. N. and van Dijck, M. F. (1987) Magnetic modelling of the Olkaria geothermal field, Kenya. Proceedings 9th NZ Geothermal Workshop, pp. 37-41. Armannsson, H., Benjaminsson, J. and Jeffrey, A. W. A. (1989) Gas changes in the Krafla geothermal system, Iceland. Chemical Geology 76, 175-196. Barnett, C. T. (1976) Theoretical modelling of the magnetic and gravitational fields of an arbitrarily shaped three-dimensional body. Geophysics 41(6), 1353-1364. Bibby, H. M., Bennie, S. L., Stagpoole, V. M. and Caldwell, T. G. (1994) Resistivity structure of the Waimangu, Waiotapu, Waikite and Reporoa geothermal areas, New Zealand. Geothermics 23,445-471. Bibby, H. M., Dawson, G. B., Rayner, H. H., Stagpoole, V. M. and Graham, D. J. (1984) The structure of the Mokai geothermal field based on geophysical observations. Journal of Volcanology and Geothermal Research 20, 1-20. Bj6rnsson, A. and Hersir, G. P. (1981) Geophysical reconnaissance over the Hengill high-temperature geothermal area, SW-Iceland. Geothermal Resources Council Transactions 5, 55-58. Bohnsack, G. (1987) The Solubility of Magnetite in Water and in Aqueous Solutions of Acid and Alkali. Hemisphere, Washington, D.C., 161 pp. Bromley, C. J., Bottomley, J. J. and Pearson, C. F. (1988) Geophysical exploration for prospective geothermal resources in the Tarawera forest. Proceedings lOth NZ Geothermal Workshop, pp. 123-128. Browne, P. R. L. (1994) 86.101 Geology Lecture Notes. Geothermal Institute, The University of Auckland. Browne, P. R. L. and Ellis, J. A. (1970) The Ohaaki-Broadlands hydrothermal area, New Zealand: mineralogy and related geochemistry. American Journal of Science 269, 97-131. Butler, R. F. (1992) Paleomagnetism: Magnetic Domains to Geologic Terranes. Blackwell Scientific, Boston, M.A., 319 pp. Criss, R. E., Champion, D. E. and Mclntyre, D. H. (1985) Oxygen isotope, aeromagnetic, and gravity anomalies associated with hydrothermally altered zones in the Yankee Fork Mining District, Custer County, Idaho. Economic Geology 80, 12771296. Ewart, A. (1966) Review of mineralogy and chemistry of the acidic rocks of Taupo Volcanic Zone, New Zealand. Bulletin Volcanologique 29, 147-172. Gerard, V. B. and Lawrie, J. A. (1955) Aeromagnetic surveys in New Zealand, 19491952. Geophysical Memoir 3, Department of Scientific and Industrial Research, Wellington. Grindley, G. W., Mumme, T. C. and Kohn, B. P. (1994) Stratigraphy, paleomagnetism, geochronology and structure of silicic volcanic rocks, Waiotapu/Paeroa Range area, New Zealand. Geothermics 23,473-499. Healy, J. and Hochstein, M. P. (1973) Horizontal flow in hydrothermal systems. Journal of Hydrology (NZ) 12, 71-82.

22

M. P. Hochstein and S. Soengkono

Hedenquist, J. W. and Stewart, M. K. (1985) Natural CO2-rich steam-heated waters in the Broadlands-Ohaaki geothermal system, New Zealand. Geothermal Resources Council Transactions 9, 245-250. Henneberger, R. C. and Browne, P. R. L. (1988) Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research 34, 211-231. Henrys, S. A. and van Dijck, M. F. (1987) Structure of concealed rhyolites and dacites in the Broadlands-Ohaaki geothermal field. Proceedings 9th NZ Geothermal Workshop, pp. 43-48. Henrys, S. A. and Hochstein, M. P. (1990) Geophysical structure of BroadlandsOhaaki geothermal field (New Zealand). Geothermics 19, 129-150. Hochstein, M. P. (1988) Assessment and modelling of geothermal reservoirs (small utilization schemes). Geothermics 17, 15--49. Hochstein, M. P. (1995) Crustal heat transfer in the Taupo Volcanic Zone (New Zealand): comparison with other volcanic arcs and explanatory heat source models. Journal of Volcanology and Geothermal Research 68, 117-151. Hochstein, M. P. and Hunt, T. M. (1970) Seismic, gravity and magnetic studies, Broadlands geothermal field, New Zealand. Geothermics, Special Issue 2, 2,333-346. Hochstein, M. P. and Soengkono, S. (1994) Geophysical rock parameters (1). 86.101 Lecture Notes, 3rd edition. Geothermal Institute, The University of Auckland. Hochstein, M. P., Yamada, Y., Kohpina, P, and Doens, E. F. (1987) Reconnaissance of the Tikorangi geothermal prospect (Haroharo-Okataina caldera), New Zealand. Proceedings 9th NZ Geothermal Workshop, pp. 31-36. Hunt, T. M. and Smith, E. G. C. (1981) Magnetisation of some New Zealand igneous rocks. Journal of the Royal Society of New Zealand 12, 159-180. Hunt, T. M. and Whiteford, C. M. (1979) Sheet 5, Rotorua. Magnetic map of New Zealand 1:250000, Total Force Anomalies. DSIR, Wellington, New Zealand. Ignacio, C. P. (1985) Geophysical study of the Atiamuri and Ohakuri geothermal systems. M.Sc. thesis, The University of Auckland, 166 pp. Irvine, R. J. and Smith, M. J. (1990) Geophysical exploration for epithermal gold deposits. Journal of Geochemical Exploration 36,375-412. Lampoonsub, K. (1987) Magnetic properties from Wairakei, Orakeikorako, Te Kopia and Reporoa geothermal fields. Geothermal Institute Report 87.13, The University of Auckland. Lawton, D. C. and Hochstein, M. P. (1980) Physical properties of titanomagnetite sands. Geophysics 45,394-402. Layugan, D. B. (1981) Geoelectrical sounding and its application in the Theistareykir high-temperature area, NE-Iceland. UNU Geothermal Training Programme Report 1981-5, 101 pp. Levi, S. and Riddihough, R. (1986) Why are marine magnetic anomalies suppressed over sedimented spreading centres? Geology 14, 651~554. Modriniak, N. and Marsden, E. (1938) Experiments in geophysical survey in New Zealand. Geological Memoirs 4, Department of Scientific and Industrial Research (DSIR), Wellington, 92 pp. Modriniak, N. and Studt, F. E. (1959) Geological structure and volcanism of the Taupo-Tarawera district. NZ Journal of" Geology and Geophysics 2,654-684. Morales, Z. J. (1988) Magnetic properties of selected cores from the Broadlands, Rotokawa, Tauhara and Mokai geothermal fields. Geothermal Institute Report 88.15, The University of Auckland. Nairn, I. A. and Finlayson, B. F. (1981) Chloride flux measurements in the Tarawera River system. Unpublished Geothermal circular, IGNS, Wairakei.

Magnetic Anomalies in Taupo Volcanic Zone (New Zealand)

23

Ng'ang'a, M. (1989) Assessment of topographic and demagnetisation effects of airborne magnetic anomalies in the Eburru Prospect, Kenya. Geothermal Institute Report 89.16, The University of Auckland. Palmason, G. (1975) Geophysical methods in geothermal exploration. Proceedings 2nd UN Symposium on the Development and Use of Geothermal Resources, Volume 2, pp. 1175-1184. U.S. Government Printing Office, Washington, D.C. Rachman, A. (1990) Analysis of airborne magnetic anomalies over the G. Wilis geothermal prospect (Java). Geothermal Institute Report 90.22, The University of Auckland. Rogan, M. (1982) A geophysical study of the Taupo Volcanic Zone, New Zealand. Journal of Geophysical Research 82, 4073--4088. Salt, D. (1985) Aeromagnetic survey of the northern part of the Okataina volcanic centre. M.Sc. thesis, The University of Auckland, 115 pp. Sheppard, D. S. and Lyon, G. L. (1984) Geothermal fluid chemistry of the Orakeikorako field, New Zealand. Journal of Volcanology and Geothermal Research 22, 329-349. Soengkono, S. (1985) Magnetic study of the Mokai geothermal field. Proceedings 7th NZ Geothermal Workshop, pp. 25-30. Soengkono, S. (1992) Magnetic anomalies over the Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand. Proceedings 14th NZ Geothermal Workshop, pp. 241246. Soengkono, S. (1993) Interpretation of aeromagnetic data over the Orakeikorako geothermal field, Central North Island, New Zealand. Proceedings 15th NZ Geothermal Workshop, pp. 207-212. Soengkono, S. (1995) A magnetic model for deep plutonic bodies beneath the central Taupo Volcanic Zone, North Island, New Zealand. Journal of Volcanology and Geothermal Research 68, 193-207. Soengkono, S. and Hochstein, M. P. (1992) Magnetic anomalies over the Wairakei geothermal field, Central North Island, New Zealand. Geothermal Resources Council Transactions 16,273-278. Soengkono, S. and Hochstein, M. P. (1995) Application of magnetic method to assess the extent of high temperature geothermal reservoirs. Proceedings 20th Workshop Geothermal Reservoir Engineering, Stanford University, CA, pp. 71-78. Soengkono, S. and Hochstein, M. P. (1996) Preliminary interpretation of magnetic anomalies over the Waimangu, Waiotapu, Waikite and Reporoa geothermal areas, New Zealand. Proceedings 17th Annual PNOC-EDC Geothermal Conference, PNOC Energy Development Corporation, Manila, pp. 197-203. Soengkono, S., Hochstein, M. P. and van Dijck, M. F. (1991) Magnetic anomalies of the Rotokawa geothermal field. Proceedings 13th NZ Geothermal Workshop, pp. 3338. Soengkono, S., Hochstein, M. P. and Suranto (1988) Magnetic anomalies over the Kamojang geothermal field (West Java, Indonesia). Proceedings lOth NZ Geothermal Workshop, pp. 139-142. Soengkono, S., Hochstein, M. P., Smith, I. E. M. and Itaya, T. (1992) Geophysical evidence for widespread reversely magnetised pyroclastics in the western Taupo Volcanic Zone (New Zealand). NZ Journal of Geology and Geophysics 35, 47-55. Steiner, A. (1953) Hydrothermal rock alteration at Wairakei, New Zealand. Economic Geology 48, 1-13. Steiner, A. (1977) The Wairakei Geothermal Area, North Island, New Zealand. NZ Geological Survey Bulletin 90, DSIR, Wellington. Studt, F. E. (1959) Magnetic survey of the Wairakei hydrothermal field. NZ Journal of Geology and Geophysics 2,746-754.

24

M. P. Hochstein and S. Soengkono

Watson-Munro, C. N. (1938) Reconnaissance survey of the variation of magnetic force in the New Zealand thermal regions. N Z Journal of Science and Technology B20, 99-115. Whiteford, C. M. (1976) Magnetic anomaly map of Central Volcanic region. Geophysics Division Report 101, DSIR, Wellington. Whiteford, C. M. and Lumb, J. T. (1975) A catalogue of physical properties of rocks. Vol. 1, Geophysics Division Report 105, DSIR, Wellington. Wood, C. P. (1994) Aspects of the geology of Waimangu, Waiotapu, Waikite and Reporoa geothermal systems, Taupo Volcanic Zone, New Zealand. Geotherrnics 23, 401-421.