The structure of the Mokai geothermal field based on geophysical observations

Bibby, H. M.; Dawson, G. B.; Rayner, H. H.; Stagpoole, Vaughan; Graham, Duncan

doi:10.1016/0377-0273(84)90063-5

Cited by 31 publications

(10 citation statements)

References 7 publications

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“…Such low-resistivity zones have also been observed at other geothermal fields of the TVZ (e.g. Mokai: Bibby et al, 1984;Ohaaki: Bibby, 1986). This feature is a characteristic response to the very sharp, near-surface resistivity discontinuity and can be reproduced using numerical models that simulate this measurement technique.…”

Section: The Tensor Bipole-dipole Methods At Wairakeimentioning

confidence: 86%

Investigations of deep resistivity structures at the Wairakei geothermal field

et al. 2009

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Section: The Tensor Bipole-dipole Methods At Wairakeimentioning

confidence: 86%

Investigations of deep resistivity structures at the Wairakei geothermal field

et al. 2009

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“…a single geothermal system (Mokai) lies within the area of Fig. 3 and stands out as a region of low resistivity (<20 m) within a generally much more resistive, non-geothermal background (Bibby et al 1984). Such low resistivity values reflect the presence of highly saline, hot geothermal fluids that saturate the layers of young volcanics, and the alteration of the volcanic rocks which produces conductive clay species (Risk 1983).…”

Section: Electrical Resistivity Surveysmentioning

confidence: 99%

Resistivity structure of western Taupo Volcanic Zone, New Zealand

Bibby

Risk

Caldwell

et al. 2008

New Zealand Journal of Geology and Geophysics

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data from two deep-penetration electrical resistivity surveys crossing the western margin of the Taupo Volcanic Zone (TVZ) near Tihoi, northwest of lake Taupo, North island, New Zealand, have been used to identify the elements of the margin. in this vicinity the volcanologically defined boundary of the TVZ includes the andesitic cones of Pureora and Titiraupenga. The faulted margin of the TVZ lies east of these cones and is marked in the resistivity data by a dipping interface (faulted margin) between the resistive greywacke rocks to the west and the relatively conductive volcanic rocks that fill the upper few kilometres of the TVZ. The resistivity and gravity signatures of the margin are coincident in this vicinity. To the immediate east of the margin is a low-resistivity zone c. 10 km wide and 2.5 km deep. The low-resistivity material within this region is identified as old (>1 Ma) volcaniclastics that contain clays produced by low-temperature alteration and devitrification. This material is reversely magnetised. we suggest that this zone was formed following the onset of active extension of the TVZ and has since remained relatively undisturbed while the focus of volcanism and deformation has moved to the east. The eastern boundary of this low-resistivity zone coincides with the western edge of the whakamaru caldera (the boundary of the "young TVZ"). although the resistivity data can be matched using an interface with a dip to either the east or west, an eastward dip would be consistent with the boundary having been formed during the collapse associated with the formation of the whakamaru caldera. The resistivity signature within the whakamaru caldera is consistent with >2 km thickness of younger, more resistive volcanic sequences, suggesting the low-resistivity layers found to the west have been disrupted and downfaulted to be replaced with young, resistive caldera infill.

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“…In the model, we have extended the nearsurface conductor laterally to simulate the presence of a subsurface out-flow of geothermal fluid that occurs in geothermal systems both in New Zealand and elsewhere (e.g. Bibby et al 1984). Although the near-surface high-conductivity region can be easily delineated by many different electrical exploration techniques, identifying the deeper conductor (which represents the 'target' high-temperature fluid reservoir useful for electric power production) is much more difficult.…”

Section: Static Shift Modelmentioning

confidence: 99%

“…This led Bibby (1977Bibby ( , 1986 to introduce the concept of an 'apparent resistivity tensor' to represent the electric field data produced in 'multiple-source bipole-dipole' surveys (e.g. Bibby et al 1984;Risk et al 1993). The DC apparent resistivity tensor provides a compact representation for the data produced by collocated grounded sources and a simple way of visualizing the surface electric field in 3-D situations (Bibby & Hohmann 1993).…”

Section: Introductionmentioning

confidence: 99%

Controlled source apparent resistivity tensors and their relationship to the magnetotelluric impedance tensor

Caldwell

Bibby

Brown

2002

Geophysical Journal International

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S U M M A R YThe time-domain surface electric fields produced by a step current in collocated grounded sources can be represented by an apparent resistivity tensor defined by the relationship between the measured (time-varying) electric field and a reference field equal to the steady-state current density in a uniform half-space. A magnetic field response tensor is similarly defined for the horizontal components of the magnetic field. The 'magnetic field apparent resistivity tensor' is derived from a linear combination of the contractions of the outer product of the magnetic field response tensor. Frequency-domain apparent resistivity tensors are derived from the Laplace transforms of the corresponding time-domain electric and magnetic field response tensors. Both the frequency and time-domain tensors are independent of the source orientation where the sources can be approximated as (infinitesimal) dipoles. A simple combination of the frequencydomain (impulse response) tensors can be used to derive the controlled source magnetotelluric (CSMT) impedance tensor.The magnetic field apparent resistivity tensor is a useful representation of the conductivity structure only where the source-receiver offset is much less than the diffusion or skin depth and behaves in a similar way to the 'early-time' apparent resistivity traditionally used to represent time-domain electromagnetic data. Numerical modelling results demonstrate that the (horizontal) magnetic field apparent resistivity is insensitive to the localized 3-D conductivity structures that are typically the target of exploration surveys. In contrast, the electric field apparent resistivity tensor depends sensitively upon the conductivity structure and is well behaved over the entire time or frequency range used in long-offset transient electromagnetic or CSMT measurements. By using the electric field apparent resistivity tensor, 'source effects' that hinder a conventional impedance tensor analysis of CSMT data can be avoided. Images of simple 3-D structures created from the invariants of the electric field apparent resistivity tensor (in either the time or frequency domain) provide a useful representation of the subsurface conductivity structure despite the simplicity of this imaging procedure. These images are independent of the coordinate system used to express the data and are, to a good approximation, independent of the current source orientations.

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The structure of the Mokai geothermal field based on geophysical observations

Cited by 31 publications

References 7 publications

Investigations of deep resistivity structures at the Wairakei geothermal field

Investigations of deep resistivity structures at the Wairakei geothermal field

Resistivity structure of western Taupo Volcanic Zone, New Zealand

Controlled source apparent resistivity tensors and their relationship to the magnetotelluric impedance tensor

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