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

an estimate of the conductivity structure for the vancouver island region from geomagnetic results abstract the depth to the highly conductive layer of the mantle below the vancouver island region is estimated using the results from a scaled laboratory analogue model a two dimensional numerical model and field observations at the centre of the island the responses of numerical models with depths to the mantle of 40 and 80 km bracket the field results the subduction regime in the vancouver island region suggests a dipping interface for the conducting layer the numerical model based on this type of tectonic structure has a conducting layer within the mantle that varies in depth from 40 km at the continental margin increasing under the island to 80 km at the shallow channel and decreasing again to 40 km near the mainland coast the response of this model is compatible with the field observations

laboratory electromagnetic modelling of the subducting juan de fuca plate abstract the effect of the depth of the dipping juan de fuca plate subducting the west coast of the vancouver island washington oregon region is studied using a scaled laboratory analogue model the subducting plate is modelled horizontal offshore then dipping at 10 under vancouver island and dipping further at 30 as it subducts the continent for the british columbia region and 45 under the washington oregon region the strike of the bending plate follows the general strike of the continental coastline with an abrupt 42 change in direction in the puget sound area the model results show that the coast effect described by the vertical to horizontal magnetic field ratio is highly dependent upon the period and the depth of the dipping substructure while the conducting boundaries of the subducting plate significantly reduce the apparent resistivities for coastal locations enhanced induction arrows exist for a large range inland owing to the steeply dipping subducting plate

a laboratory electromagnetic model study of the juan de fuca plate region the behaviour of the magnetic field variations over the juan de fuca plate region is studied using a scaled laboratory analogue model the model includes a simulation of the complex juan de fuca plate subducting the vancouver island region the subducting plate is modelled with a profile of increasing inclination from east to west horizontal offshore dipping at 10 under vancouver island and bending further under georgia strait to subduct the continent at 30 for the b c region and 45 for the washington oregon region the strike of the bending plate follows the general strike of the continental coastline with an abrupt change in direction 42 in the puget sound area the model substructure simulates a subducting plate overplated by a sediment layer several kilometres thick and underlain by a 30 km thick highly conducting upper asthenosphere the model source frequencies used simulate periods 5 120 min in the geophysical scale in phase and quadrature hx hy and hz magnetic field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e and h polarizations electric field of the source approximately parallel and perpendicular respectively to the west coast of vancouver island the fields for three regions of the model over vancouver island over the olympic peninsula and over a linear portion of the u s coastline are examined in detail the general conclusion is that the effect of the dipping subducting plates is to significantly attenuate at short periods the maxima in the anomalies at the coastlines underlain by the 10 dipping plate while leading to anomalous vertical and horizontal fields over ranges as large as 500 km inland over a wide period range anomalous fields are observed over the offshore and inland knee bends of the subducting plates at all periods for both e and h polarizations for locations on land the in phase induction arrows point seaward and perpendicular to the strikes of the dipping plates for all periods while the quadrature arrows at short periods point landward and rotate to point seaward for periods greater than 20 min

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

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

electromagnetic induction in the british isles region analogue model and field station results the results of a laboratory electromagnetic analogue model study which employs a horizontal inducing field over a simple model of the british isles region delineate the location and frequency dependence of the major coast effect induction anomalies of the scotland region contours of amplitudes amplitude ratios and in phase and quadrature parts of the model field measurements are presented the model vertical magnetic fields for two orthogonal source field polarizations and field station values for two hypothetical events for corresponding polarizations are compared while major discrepancies occur between model and field hz amplitudes the hz gradients across scotland which can be attributed to the coast effect are comparable in value although sometimes reversed in sign superimposed on this coast effect the field data indicate the existence of current concentrations associated with the great glen and the southern uplands faults and possibly also of currents within the scottish mainland near the east and west coasts

an analogue model study of electromagnetic induction in the tasmania region abstract laboratory analogue model magnetic measurements are carried out for a model of the region including tasmania bass strait with its highly conductive deep sedimentary basins and the south coast of mainland australia the model source frequencies used simulate naturally occurring geomagnetic variations of periods 5 120 min in phase and quadrature magnetic hx hy and hz field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e polarization electric field of the source in the n s direction and for h polarization electric field of the source in the e w direction large anomalous in phase and quadrature model magnetic fields are observed over bass strait and the coastal regions at short periods for both e and h polarization but with increasing period the field anomalies decrease more rapidly for e polarization than for h polarization the difference in response with polarization for the bass strait region is attributed to current induced in the deep ocean for all periods being channelled through bass strait for h polarization but not for e polarization the persistent large coastal field anomalies elsewhere for h polarization can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea floor and channelled through bass strait and around the southern coast of tasmania the phenomena of current deflection and channelling for h polarization for the geometry of the southern australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range the coastal horizontal hx and hy field anomalies present for e polarization at short periods and for h polarization at all periods do not extend far inland and thus for inland station sites somewhat removed from the coast should not present serious problems for magnetic soundings in field work the sharp vertical field hz gradient over tasmania at short periods which is predominantly in the e w direction for e polarization and the n s direction for h polarization is strongly frequency dependent becoming almost undetectable at 60 min the behaviour of the hz field gradients however are very similar from traverse to traverse over inland tasmania and thus the effects of the ocean should not present too serious a problem in the interpretation of field station studies the discrepancies between model and field station results should be useful in mapping geological boundaries in the region

analogue model study of electromagnetic induction in the newfoundland region abstract the behaviour of magnetic field variations over the newfoundland canada region is studied using a scaled laboratory analogue model the model source frequencies used simulate periods of 100 s 300 s 900 s and 1800 s in the geophysical scale contour views of in phase and quadrature phase parts of the three magnetic field components for two orthogonal source field polarizations for 100 s and 1800 s period variations are shown large anomalous in phase and quadrature model magnetic fields are observed over the strait of belle isle and cabot strait for all four periods studied with the anomalous fields reaching maxima when the skin depth is of the order of six times the depth of the strait the cape effect that is the large field enhancements due to deflection of induced current at capes is observed for all four periods enhanced fields are observed at the continental shelf edge for 1800 s variations but not for 100 s variations a general shift in the anomalous field from the in phase to the quadrature part for all three magnetic components is found for increasing period the magnitudes and directions of inter station induction arrows and single station induction arrows are studied the two types of induction arrows differ when an anomalous horizontal magnetic field is present this is especially evident near the coastline for a period of 100 s for all periods studied the in phase induction arrows point towards the nearest and largest current concentrations in traversing the coastline the single station quadrature arrows reverse direction for the 100 s period but not for the 1800 s period the model results indicate that there is no preference for using either single station or inter station induction arrows with respect to minimizing the effects due to coastal variations for inland locations further the model results show that for periods studied 100 s 1800 s the coastal effects would not produce serious perturbations to the field measured over crustal anomalies within the central regions of newfoundland

near inertial and tidal currents detected with a vessel mounted acoustic doppler current profiler in the western mediterranean sea 1 the objective of this study is to separate the subinertial inertial and tidal fields from vessel mounted acoustic doppler current profiler vm adcp observations acquired during six cruises in the catalan and alboran seas western mediterranean sea to fulfill this objective we use a method that successfully detects and filters out the main high frequencies that are present in the vm adcp data the resulting near inertial component has frequencies below the coriolis frequency f in the catalan sea and above f in the alboran sea the tidal component is consistent with observed and modeled tides in the catalan and alboran seas the steady component is highly correlated with the geostrophic field there is also a nonfitted or residual component which represents the misfit between the vm adcp data and the model proposed

electromagnetic induction in the san juan bay region of vancouver island abstract the response of the electric and magnetic field variations over the san juan bay region of vancouver island is studied using a scaled laboratory analogue model the laboratory frequencies simulate periods of 10 and 100 s in the geophysical problem the model results indicate that for both e and h polarization of the source field induced current in the ocean is deflected around cape flattery and channelled into the juan de fuca strait with increasing period the proportion of current channelled into the strait decreases sharply induced current in the strait is also funnelled into san juan bay a finger shaped bay ca 4 km wide and 7 5 km long for both polarizations of the source field the effect of the bay on the field response is confined to the local region within approximately 6 km of the centre of the bay good agreement between field station and analogue model hz results was obtained for the san juan bay the behaviour of the parkinson arrow for these two stations is examined with the aid of the analogue model results

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

an analogue model study of electromagnetic induction for cape and bay coastlines abstract the electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models the effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions diameter of the bay or cape of the irregularity for the e polarization of the source field the perturbations in hz and hy are more confined mainly to the immediate region of the bay or cape the coast effect hz hy for this polarization and variations with a 4 2 min period is approximately a factor of two larger for the cape than for the bay while the straight coastline hz hy is roughly mid way between the cape and bay values for h polarization the coast effect is again approximately a factor of two larger for the cape than for the bay but both values are larger than the value for a straight coastline the response of the various field components to the bay and cape features is decreased at longer periods and as a measure the coast effect is approximately a factor of 1 2 greater for the cape than for the bay for a 42 min period as compared with the factor of 2 for a 4 2 min period the bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land this study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source field variations

laboratory electromagnetic model results for the emslab region apparent resistivities and vertical to horizontal magnetic field ratios for the lincoln line traverse in the emslab project region are studied using a scaled laboratory analogue model the model includes a simulation of the juan de fuca plate subducting the canadian u s pacific northwest region in the model the subducting plate is taken to be horizontal offshore dipping at 10 beneath the coast then inland bending farther to dip at 45 in the washington oregon region and at 30 in the british columbia region the model results show that for inland sites the apparent resistivity for the telluric field component parallel to the coast and thus also parallel to the strike of the dipping conductive substructure depends strongly on period while the resistivity for the telluric field perpendicular to the coast is largely a galvanic effect and changes less with period the amplitude of the vertical to horizontal magnetic field ratio along the lincoln line shows a saddle shaped response with the coastal maximum followed by a second maximum inland due to the dipping substructure further rather than decreasing rapidly with distance inland as expected for a simple coast effect the field ratios retain significant values large distances inland for periods of 3 120 min model results for 120 min period are compared with one example of field results for the emslab project region

subduction and back arc activity at the hikurangi convergent margin new zealand the hikurangi margin is a region of oblique subduction with northwest dipping intermediate depth seismicity extending southwest from the kermadec system to about 42 s the current episode of subduction is at least 16 20 ma old the plate convergence rate varies along the margin from about 60 mm a at the south end of the kermadec trench to about 45 mm a at 42 s the age of the pacific lithosphere adjacent to the hikurangi trench is not known

the tamar conductivity anomaly magneto variational measurements in eastern tasmania have shown the presence of a conductivity anomaly coinciding with the tamar lineament magneto telluric observations indicate a shallow body 2 5 km deep with a high conductivity 0 2 1 s m 1 analogue model measurements made at the university of victoria have been used to correct for the effect of the oceans surrounding tasmania rather than model the form of the conducting body the results have been interpreted in terms of current flowing in a nnw sse direction in a channel 40 km wide with a sharp eastern edge and a more gradual tapering to the west the most likely explanation of the high conductivity is fractured rock saturated with conducting pore fluids

an analogue model study of em induction in the north china korea coastal region abstract the behaviour of the magnetic field variations over the north china korea coastal region is studied with the aid of a scaled laboratory analogue model the model source frequencies simulate naturally occurring geomagnetic variations of 3 60 min periods in phase and quadrature magnetic bx by and bz field measurements for the modelled region are presented for e and b polarizations large anomalous in phase and quadrature model magnetic fields are observed over the korea japan strait for e polarization and over the bohai strait for b polarization due to current channelling through the straits large responses of the peninsulas in the shallow coastal areas occur at short periods but decrease abruptly with increasing period in general the effects of peninsulas straits bays and irregularities in the coastlines play an important role in the magnetic field responses both on shore and off shore for this complex north china korea coastal region model and field site induction arrows are compared for three sites west of bohai bay

thermoluminescence dating studies of rapidly deposited silts from south central british columbia to determine whether quaternary sediments from small lake and fan deposits can be dated by thermoluminescence tl methods i have applied recently proposed techniques for tl dating to known age rapidly deposited silts and have examined the sensitivity of the tl of fine grain feldspars and quartz to light these feldspar dominated silts exhibited little or no detectable anomalous fading the tl of quartz was observed to be very resistant to light with wavelengths above 400 nm whereas that of feldspars was sensitive to all visible wavelengths no significant resetting of the tl of the 11 ka old glaciolacustrine silt deposition rate could be detected with these techniques implying that silts deposited at such rates into small glacier bordered lakes cannot be dated by these methods however accurate equivalent doses were measured for the derivative 7 5 ka old mudflow silts 1 mm year but only with the use of the r technique applied to the feldspars

comparison of analogue model and field station em responses on southern vancouver island abstract measurements of geomagnetic induction responses for a laboratory analogue model that includes a simulation of the subducting juan de fuca plate are compared with those obtained at three sites on vancouver island good agreement between model and field responses at the central vancouver island site is observed over the period range 3 60 min while at the east and west coastal sites good agreement is achieved only for periods greater than 20 min at shorter periods departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model nevertheless the analogue model results are consistent with the premise of a juan de fuca plate underlain by the conductive asthenosphere subducting at a shallow depth beneath vancouver island