Malcolm Ingham

Spectral Induced Polarization Measurements on New Zealand Sands - Dependence on Fluid Conductivity

Spectral induced polarization (SIP) measurements explore the variation of the complex conductivity (σ*) of a material with frequency. Much of this variation results from polarization effects associated with the electric double layer on the surfaces of pore spaces. The consequent dependence of the SIP signature on pore structure thus has the potential to provide a link to the hydraulic properties of the material, which have a similar dependence. We report here on the variation of the SIP signature of unconsolidated sands, typical of those found in coastal aquifers in New Zealand, with the conductivity of the pore fluid. The SIP parameters of the measurements are modelled in terms of a Cole-Cole model and demonstrate the independence of relaxation time on fluid conductivity. The contribution of surface conductivity to the overall conductivity is calculated and the variation of the imaginary part of the surface conductivity with fluid conductivity is tested against two models for the origin of surface conductivity. The measured hydraulic conductivity is also compared with estimates provided by three proposed equations relating hydraulic properties to structural and electric properties.

Electrical conductivity structure of the Broadlands-Ohaaki geothermal field, New Zealand

Abstract Audiomagnetotelluric and magnetotelluric soundings have been obtained at 11 sites on a south-north line across the Broadlands-Ohaaki geothermal field. The orientation of the line is approximately parallel to the eastern boundary of the Central Volcanic Region of the North Island of New Zealand and makes anything other than one-dimensional quantitative modelling difficult. As the data quality is generally good and the measured apparent resistivities are isotropic at high frequencies it is believed that one-dimensional modelling gives, in this instance, an accurate picture of the structure of the geothermal field. This is supported by the excellent agreement between the structure obtained and the available borehole, geological and geophysical information. The modelling results clearly show a very low resistivity layer which may be identified with the upper aquifer of the geothermal reservoir. Beneath the field the depth to this layer varies from 100 m to ∼ 450 m. This low-resistivity region associated with the reservoir appears to extend well to the south of the field at a depth of ∼ 1.5 km. In contrast, the northern boundary of the field is relatively abrupt. Beneath the field the very low resistivity zone is underlain by a resistivity of ∼ 20 ω m which extends to several kilometres deep and which in at least the upper 1 km can be identified with the circulation of geothermal fluids. There is also some tentative evidence of a second conducting region, possibly marking the base of the crust, at a depth of ∼ 20 km.

Deep electrical structure of the Central Volcanic Region and Taupo Volcanic Zone, New Zealand

Magnetotelluric measurements have been made along a transect crossing the Central Volcanic Region (CVR) and Taupo Volcanic Zone (TVZ) of the North Island of New Zealand. Analysis of the observed data using the phase tensor method shows that at the south-eastern end of the transect, where it crosses the TVZ, the data are largely 2-dimensional in character and respond to electrical structure which has a strike orientation of N45°E. There is however, evidence of some degree of 3-dimensionality in data from the central part of the TVZ. 2-dimensional inversion and forward modelling of data indicate the existence of low resistivity beneath about 10 km depth centred below the Taupo Fault Belt. The existence of low resistivity beneath this depth is consistent with a low velocity zone identified from receiver function analysis and with measurements of the transition from brittle to ductile behaviour and the depth penetration of the convective systems associated with the geothermal fields within the TVZ. The low resistivity provides support for the existence of igneous intrusion to shallow depths beneath the TVZ. Data from the north-western end of the transect, where it crosses the western boundary of the CVR, are 2-dimensional and may respond to electrical structure associated with the Hauraki Rift, an active rift containing Tertiary sediments, which cuts the boundary of the CVR in this region.

Electromagnetic induction in New Zealand: analogue model and field results

Abstract The behaviour of electric and magnetic variations over North Island (New Zealand) is studied with the aid of a laboratory analogue model. The source field frequencies used in the analogue modelling simulate naturally occurring geomagnetic variations of 5–120 min periods. In-phase and quadrature magnetic and electric fields for a selection of traverses for the modelled region of North Island are presented. Since North Island is of a relatively narrow cross-section, the field responses, even for inland locations, are expected to show strongly the effects of the surrounding ocean. The irregular coastlines, as well as the strait between North and South Islands, lead to coastal and inland field anomalies due to induced currents being deflected and channelled to produce localized current densities. The comparison of model results with field station measurements obtained earlier individually by Ingham and by Midha for sites in the northeastern, central, and southern (near Cook Strait) regions of North Island demonstrates the large role the ocean has in the observed field responses. Differences in the model and field results at some sites are expected and should reflect the effects of the local geology and the conductive substructure related to the complex tectonics of the region not simulated in the model.

Audiomagnetotelluric soundings on White Island volcano

Abstract Audiomagnetotelluric soundings in the frequency range 100-1 Hz have been made at three locations within the crater region of White Island volcano, New Zealand. Exceptionally low apparent resistivities are prevalent. Despite the possibility of terrain effects and static shift, and the three-dimensional nature of the island, the measured values appear to be indicative of the true electrical structure. One-dimensional modelling of the E polarisation responses has been used to obtain approximations to the electrical structure beneath each site. The results suggest a variable depth to the acid hydrothermal system which is believed to underly the crater region, with low resistivities coming closest to the surface near the active fumaroles of Donald Mound.

Lower crustal and upper mantle electrical conductivity contrasts in the central North Island of New Zealand

Abstract Magnetotelluric soundings made in the central North Island of New Zealand over the last 20 years have been reanalysed, together with more recent soundings, in an attempt to detect any electrical conductivity contrast associated with a known boundary in other geophysical properties. ‘Invariant’ apparent resistivity curves from 11 sites in all have been compared and curves which appear to be representative of the different regions have been selected. One-dimensional modelling has led to the identification of a good conductor at lower crustal or upper mantle depths in the northwest of the North Island. No such conductor is identified in the south and east. Modelling of a single site in the Taupo Volcanic Zone has led to the first indication yet obtained of a deeper conductor beneath the geothermal region.

Magnetovariational measurements in the Cook Strait region of New Zealand

Abstract Magnetovariational measurements have been made at 10 sites on the northern side of the Cook Strait, New Zealand. Single-station transfer functions have been calculated for the sites and indicate that the effect of induction in the shallow water of the Cook Strait is most important at around 1000 s period. At longer periods the effect of induced currents in the Pacific Ocean predominates. A two-dimensional electrical conductivity model including local conductivity structure has been shown to satisfy the measured responses at sites about 60–80 km distance from the strait. Closer to the strait the inductive process is strongly three-dimensional. A simple d.c. line current model of current flow has been shown to reproduce some of the features of the observed responses. Induction arrows indicate the existence of conductivity anomalies associated with a known lateral seismic boundary and with one of the two principal faults in the region.

Electrical structure along a transect of the central South Island, New Zealand

Abstract Regional magnetotelluric responses from a transect across the central South Island, New Zealand, have been inverted to produce a smooth two‐dimensional model of electrical resistivity structure associated with the Southern Alps orogen. The resulting structure is consistent with proposed models for the orogen and shows a zone of enhanced conductivity at a depth of only 10 km beneath the Southern Alps. This can be interpreted as being caused by the presence of “wet” greenschist facies rocks upthrust from lower crustal depth. There is some indication of reduced resistivity at 25 km depth beneath the Canterbury Plains, which may be associated with the existence of a lower crustal detachment surface. A large resistivity contrast occurs near the Alpine Fault, and this may be an indication of a lack of lateral fluid movement across the fault zone.

The low frequency electrical properties of sea ice

Impedance measurements were performed across the frequency range 40 Hz-1MHz on laboratory grown sea ice using a custom four-electrode capacitive cell. The measurements were performed in-situ, throughout the ice growth in horizontal and vertical orientations. Two-electrode measurements were susceptible to electrode polarization and a four-electrode measurement was required to measure the true electrical properties of the ice. A broadband mathematical model is presented which distinguishes polarization and conduction in the bulk material from space charge polarization at interfaces. The fit of this model to the impedance data gives insight into the relative contribution of these phenomena. The bulk material undergoes a dielectric relaxation and we have examined the temperature dependence of the relaxation time and susceptibility. Aspects of the susceptibility and DC conductivity can be reconciled with a conductive dielectric model in which brine is confined to regular ellipsoids.

Electrical resistivity structure of the Canterbury Plains, New Zealand

Abstract Magnetotelluric (MT) soundings from nine locations across the Canterbury Plains, South Island, New Zealand, are used to deduce the electrical resistivity structure of the Plains down to c. 3 km depth. The Quaternary gravels covering the Plains represent a resistive layer which is inferred to be c. 600 m thick and are underlain by more conductive Tertiary sediments. Over most of the Plains, a rise in resistivity at greater depth cannot be unambiguously identified as greywacke basement, but correlation of the electrical structure with earlier seismic and gravity work suggests that the gravels and Tertiary sediments thin towards the western edge of the Plains where greywacke is very close to the surface, dc resistivity soundings are used as a check on the correction of static‐shift in the MT data and are shown to be compatible with the MT results. Joint interpretation of the dc and MT data allows hydrological implications to be drawn on the porosity and pore fluid resistivity of the layers.