evidence for multiple boundary faults beneath the northwest moat of long valley caldera magnetotelluric results the boundaries of long valley caldera in eastern california are sharply delineated by steeply dipping normal faults which for the most part are buried and obscured by recent volcanic and sedimentary cover the only evidence for their presence is from their geophysical signatures on regional gravity seismic and electromagnetic surveys recently a surge of interest has focused studies in the western moat where the volcanism is most recent and where many workers feel that the geothermal potential is highest a detailed narrow band 20 80 s magnetotelluric profile in that area has delineated a parallel set of buried high angle normal faults which have virtually no expression at the surface the outermost fault has a throw of approximately 500 m while the innermost has a throw of approximately 2 km neither has an expression in the surface morphology moreover based on a relatively low resistivity contrast between the basement and the basin fill we infer a relatively high fracture porosity in the sierran basement which suggests that there may be considerable lateral and vertical migration of meteoric water a result which has considerable significance to understanding the regional hydrology

results of a magnetotelluric traverse across western oregon crustal resistivity structure and the subduction of the juan de fuca plate abstract as part of project emslab we have collected and analysed wideband magnetotelluric data along an east west transect in western oregon preliminary modelling of the data using one dimensional inversions based upon rotationally invariant earth response functions was followed by finite element two dimensional modelling the models produced indicate the presence of an electrical conductor beneath the oregon coast range dipping eastward at 12 18 from a depth of 23 32 km we believe that this conductor includes the thrust surface of the subducting juan de fuca plate and or adjacent water saturated rocks its high conductance about 200 s is thought to be due to one or more of the following mechanisms 1 sediments subducted atop and with the juan de fuca plate 2 saline fluids produced by dehydration of the former or 3 seawater contained within subducted oceanic basalts there is a distinct possibility that the high conductivity is due primarily to the presence of subducted sediments in contrast with the notion that the subduction of young buoyant lithosphere retards sediment subduction at this convergent margin the conductive layer is overlain by relatively resistive rocks presumed to be accreted oceanic lithosphere model determined resistivities for the upper part of the coast range section are in good agreement with deep well log data a strong electrical contrast appears in the determinant phase pseudosection between the coast range and the willamette valley suggesting a structural boundary between the two provinces a surficial conductor is present in the valley to depths of 1 2 km and is due to alluvial fill induction arrow data show the geomagnetic coast effect and a smaller effect by the willamette valley alluvial fill

rock physics transforms and scale of investigation abstractrock physics velocity porosity transforms are usually established on sets of laboratory and or well data with the latter data source being dominant in recent practice the purpose of establishing such transforms is to 1 conduct forward modeling of the seismic response for various geologically plausible what if scenarios in the subsurface and 2 interpret seismic data for petrophysical properties and conditions such as porosity clay content and pore fluid because the scale of investigation in the well is considerably smaller than that in reflection seismology an important question is whether the rock physics model established in the well can be used at the seismic scale we use synthetic examples and well data to show that a rock physics model established at the well approximately holds at the seismic scale suggest a reason for this scale independence and explore where it may be violated the same question can be addressed as an inverse problem assume that we have a rock physics tran

electromagnetic induction by ocean currents bempex abstract the theory of electromagnetic induction by ocean water currents is reviewed using a modal formulation with an emphasis on interactions of the induced fields with the conducting earth and on their association with distinct dynamically significant parts of the water velocity field it is shown that the dominant electromagnetic mode at low frequencies a recent major experiment to study the low frequency variability of the ocean using electromagnetic induction principles is described denoted by the acronym bempex for barotropic electromagnetic and pressure experiment this effort involves 41 seafloor magnetometers horizontal and vertical electrometers and pressure recorders covering an area 1100 km east west by 1000 km north south in the north pacific for 11 months the major oceanographic objectives of bempex are multifold and include a description of the wind forced barotropic variability of the deep ocean studies of the frequency wavenumber characteristics of certain oscillations of basin wide scale and tests of predictions from existing models of wind forcing

lake bottom magnetotellurics the study of the electrical conductivity structure of the lower crust and the upper mantle and midmantle promises to place important constraints on the thermobaric and compositional state of the earth s interior such work particularly for depths above approximately 400 km is best attempted by magnetotelluric sounding an effort frustrated by the almost complete absence of stable long period electric field measurements in this paper we propose that proven techniques developed for seafloor magnetotellurics be applied also to deep freshwater lakes the establishment of such long period electric and magnetic observatories in the thermally stable low noise lake bottom environment will yield new insights on the deep electrical structure beneath the continents a lake bottom long span electric field array was deployed in a deep freshwater lake adjacent to seattle washington a vector flux gate magnetometer was buried on the shore of the lake and magnetotelluric data of high quality were collected the analysis of the data described in this work clearly demonstrates the feasibility of establishing such lake bottom continental magnetotelluric observatories inversion of the data provides a view of structure beneath the site consistent with regional seismic and tectonic models

the inversion of time domain airborne electromagnetic data using the plate model airborne electromagnetic em methods were developed in the early 1950s mostly by canadian mining exploration companies as a means of economically carrying out prospecting for sulfide deposits associated with volcanics in resistive shield areas present interpretation techniques are based on the use of nomograms but the approach is easily amenable to digital processing for highly accurate interpretation however it is necessary to develop quantitative interpretation techniques that can make full use of all the data available inverse theory has been used for interpretation with great success in most geophysical disciplines however in airborne em surveying inversion has only been used for the interpretation of airborne em data using half space and one layer models by introducing some approximations to the rectangular thin plate model it is now possible to apply inverse theory to the interpretation of time domain em data this approach provides estimates of the parameter errors the correlation matr

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

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

an evaluation of electromagnetic methods in the presence of geologic noise an important element of electromagnetic em prospecting is survey design numerical modeling algorithms may be used to calculate signal to geologic noise s n ratios to compare different survey configurations and measured responses quantitatively our models consist of a prismatic three dimensional 3 d target in a conductive half space which may contain an overburden conductor the models are energized by a time varying current transmitted in a loop of wire the signal is the scattered or anomalous response of the target while the geologic noise is either the response of the half space or the anomalous response of the overburden conductor for typical loop sizes in exploration the coincident loop configuration has a relatively high s n ratio and thus a relatively high capability to resolve the target in the case of half space noise measurements made with the horizontal loop moving coil configuration can be just as effective if the coil separation is one and one half to two times the depth of buria

metamorphic fluids and anomalous porosities in the lower crust abstract because of the plastic behavior of rocks at high temperature and confining pressure the porosity of lower crustal rocks is a variable dependent on the amount of free fluid present on the basis of hydrologic theory it is predicted that the permeability k of such rocks can be expressed by a relationship of the form k n where and n are material specific constants the exponent n is likely to vary between 3 and 6 whereas which is ultimately related to grain size and shape probably varies over several orders of magnitude consequently substantial contrasts between the k relationships of lower crustal rocks are to be expected the case in which thin crustal layers aquitards are characterized by relatively low values of or n with respect to the crust in general is of particular interest such layers would acquire high porosities in the presence of miniscule steady state crustal fluid fluxes and are a possible explanation for anomalous seismic brightspots and high electrical conductivities in the lower crust modelling of collision belt metamorphism suggests that even long after the cessation of active metamorphism and isostatic reequilibration relict metamorphic fluid fluxes could generate aquitard porosities between 0 1 and 0 4 adequate to explain such geophysical anomalies

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

electrical resistivity structure of the flathead basin in southeastern british columbia canada in june 1985 wide band magnetotelluric data were acquired at 12 equally spaced sites along a 30 km profile crossing the flathead kishenehn basin in southeastern british columbia canada these d

electrical resistivity of cajon pass cores at elevated pressures the electrical resistivity of rock cores recovered from the cajon pass hole ranged from 5 to 46 k m as determined at pressures to 200 mpa and frequencies of 10 30 000 hz the resistivities of cores recovered from depths less than 1 6 km are about one order of magnitude greater than the in situ resistivity as measured by the deep laterolog however at greater depths the laboratory resistivities and those from the deep laterolog are in fairly good agreement the results suggest that at these depths the fractures whether mineralized or fluid filled are no longer well interconnected

should the electric line be straight in magnetotelluric surveys the usual description of electric field measurements in terms of potential differences is not entirely adequate at high frequencies in general the telluric electric field is non conservative and voltage measurements depend on the contour described by the cable a simple error analysis helps to recognize those situations where systematic errors may be significant for present day standards

a numerical characterization workflow for assessing the strength and failure modes of heterogeneous oil sands understanding the strength and failure modes of overburden and reservoir are critical components in safety assessments for oil sands surface mining and in situ thermal recovery operations currentl

electrical conductivity of molten basalt and andesite to 25 kilobars pressure geophysical significance and implications for charge transport and melt structure electrical conductivities of molten hawaiian tholeiite and crater lake andesite were measured between 1200 c and 1400 c at atmospheric pressure and at pressures up to 17and 25 kbar respectively isobaric plots of log versus 1 t is electrical conductivity are linear with the exception of the zero pressure tholeiite melt data conductivities decrease with increasing pressure in both melts with the andesitic melt exhibiting a greater pressure dependence between 5 and 10 kbar abrupt decreases in the slopes of isothermal log versus p plots i e decreases in activation volume are observed for both rock melts this discontinuity probably reflects changes in melt structure as opposed to changes in conduction mechanism in each pressure range the data for each rock melt can be described reasonably well by an equation of the form 0 exp ea p v kt where 0 is a preexponential constant ea is the activation energy and v is the activation volume a qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers minimum melt fraction estimates of 5 10 are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis

simulated electrical conductivity response of clogging mechanisms for managed aquifer recharge abstractmotivated by the need for improved understanding and monitoring of clogging during managed aquifer recharge we use numerical experiments to evaluate the effect of three different clogging mechanisms on electrical conductivity ec porosity specific surface area and electrical tortuosity of a simulated sediment pack the clogging experiments are designed to simulate effect of clogging due to a addition of finer grains b addition of nonconductive films and c addition of conductive films the simulations involved starting with a random grain pack of 43 porosity and subsequently reducing the porosity as would occur during clogging for each of the experiments we compute the ec response specific surface area and electrical tortuosity across the range of porosities the differences in ec response between a and b is minor however the sediment parameters measuring pore space configuration show very different responses i e specific surface area and tortuosity indicating ec is l

dc resistivity measurements of model saline ice sheets the dc resistivity structure of an artificially grown saline ice sheet at various stages of growth thermal modification and desalination has been determined by finding layered models to best fit schlumberger array sounding data values between 2 and 90 were obtained for model layers varying from 2 to 200 o brine volume after a strong warming and cooling cycle following 32 cm growth the average resistivity had decreased by a factor of about 2 despite the brine volume having also decreased by nearly a factor of 2 this change with age plus the variability in the resistivity values implies that no simple correlation exists between brine volume and resistivity because permeability which is related to age and temperature is also a strong factor the low resistivity values measured show that resistivity will strongly affect microwave radiometric emissions from young or warm sea ice

the north american central plains conductivity anomaly and its correlation with gravity magnetic seismic and heat flow data in saskatchewan canada abstract the north american central plains conductivity anomaly nacp lies virtually in its entirety within the trans hudson orogen accordingly should these two features prove to be contemporaneous then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the superior and churchill provinces in the hudsonian and a model that does not include a mechanism for the generation of this anomaly is obviously untenable in order to map better the trend of the nacp in the province of saskatchewan canada two magnetotelluric mt profiles were conducted over the nacp previously defined by magnetometer arrays and profiles data from these two profiles along with an earlier mt profile just north of the u s canadian border suggest that the nacp is not a continuous feature but rather that it exhibits a definite break at latitude 51 n other geophysical evidence examined herein is concordant with this characteristic if a second mt anomaly mapped to the northwest of this break is in fact a manifestation of the same geological structure then one possible interpretation is of a major nw se trending sinistral fault in the deep crust previously undetected with a movement of some 100 150 km along strike the mt data from the southernmost profile after correction for static shift are modelled in a 2d manner and it is shown that the nacp is consistent with an arcuate structure in vertical section of high conductivity 2 s m 1 beginning at a depth of 10 km centred on 103 w dipping down to the west reaching possibly the base of the crust it is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal such high conductivities cannot be explained in terms of connected fluids as it would require implausibly high porosities 12 20 a comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region which are high electrical conductivity positive density contrast no magnetization and high heat flow in the basement in terms of a single causative body one possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low density differentia from a mantle derived body an alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite however difficulties exist with both of these explanations and further data are required

electromagnetic expression of axial magma chambers the electrical conductivity of sea floor materials is a strong function of porosity pore geometry fluid content temperature and percentage of partial melt these material properties are particularly volatile near the midocean ridge tracking electromagnetic eddy currents generated on the sea floor as they diffuse across the ridge can provide images of the subaxial electrical and through interpretation the geological structure we have computed numerically in 2d the response of a crustal magma chamber located beneath the east pacific rise to excitation in two fundamental orthogonal modes rise normal te and rise parallel tm electric field a finite element representation of the electromagnetic em diffusion equations is solved in the laplace domain and the results inverted into the time domain via the gaver stehfest algorithm a sequence of snapshots for each mode for the time range 0 1 to 15 s shows the complete progression of the eddy currents into through and around the magma chamber the te and tm signals at a receiver site above the conductive magma chamber are delayed and attenuated in comparison with those expected for normal layered oceanic crust

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

two dimensional topographic responses in magnetotellurics modeled using finite elements we simulate the magnetotelluric response to two dimensional earth topography using finite elements linear interpolation of the secondary field parallel to strike over triangular elements allows accurate modeling of inclined resistivity boundaries including topographic surfaces to avoid discontinuities in field derivatives or resistivity care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media the nodal locations may be shifted but the derivatives still are evaluated at the field points of interest correct values may be returned at gentle breaks in slope as well as along straight surfaces the finite element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical depression and with rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization agreement with the other methods generally is excellent with t

observations of motional electromagnetic fields during emslab the long period 1 day behavior of the seafloor electromagnetic fields during emslab is considered in detail with an emphasis on interpretation in terms of oceanic motions the study begins with a summary of the physics of motional electromagnetic induction in which the seawater conductivity weighted vertically integrated velocity measured by the horizontal electric field is emphasized using frequency domain methods it is shown that seafloor and terrestrial magnetic variations have similar spectral shapes indicating a common origin but the seafloor electric field is not consistent with either at periods longer than 4 days suggesting an oceanic source the magnetic field variations are highly coherent across the emslab array at periods shorter than 9 days but the similarity decreases at longer periods probably due to long term instrumental drift the seafloor horizontal electric field data exhibit the broadband coherence characteristic of ionospheric sources only at periods shorter than 1 2 days and are essentially incoherent between 4 days and a week except in association with certain propagating wave phenomena the distinction between the depth averaged velocity inferred from the horizontal electric field and its point measurement by the vertical electric field is demonstrated with data some specific properties of the seafloor electric fields are then considered in detail the first of these is a 4 day wave observed to propagate from north to south along the east flank of the juan de fuca ridge the wavelength propagation sense and association with the ridge are all consistent with topographically trapped rossby wave behavior at a zero in the group velocity a second north to south propagating wave is seen at periods of 8 12 days in the middle of the emslab area with a probable wavelength of order 1000 km unresolved low frequency structures in the electric field are also mentioned these observations clearly demonstrate the power of electromagnetic array methods for the study of long period oceanic behavior

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

scale of experiment and rock physics trends because rock is heterogeneous at all scales it is often invalid to assign a single porosity permeability resistivity or velocity to a volume in the subsurface well data demonstrate that these properties can vary appreciably between two points just a foot apart the same happens at the pore scale consider for example a ct scan of a 1 3 mm berea sandstone cube figure 1 the porosity of this sample as calculated by counting the blue pore space pixels is 0 164

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

electromagnetic sounding in the columbia basin yakima washington a controlled source electromagnetic csem survey was conducted near yakima washington to map the thickness and resistivity of a thick volcanic sequence overlying a sedimentary section and to infer structure in the sediments the survey was conducted with a frequency domain system employing loop transmitters 400 500 m on a side and three component squid magnetometer receivers separated from the loop by 1 to 5 km data collected along a 30 km profile orthogonal to regional strike were interpreted initially with 1 d layered models which were then pieced together to make a geoelectric section induction logs in a 5000 m exploration hole at one end of the profile agree very well with the csem soundings made around the hole the geoelectric section reveals a smoothly varying thickness of volcanics with a pronounced anticlinal structure approximately concordant with a surface topographic ridge to assess the validity of inferring lateral structure from 1 d interpretations we made scale models of an anticlinal structure and of a surface inhomogeneity and conducted csem measurements over the scale models layered model inversions of these data show that the anticlinal structure and its location are very well determined by 1 d inversion but its height and width are not accurately determined csem sounding over the surface inhomogeneity model shows that this feature does not significantly degrade the interpretation of a deep target layer in this setting a geoelectric section made up from 1 d interpretations provides good qualitative measures of subsurface structure and also provides excellent starting models for detailed 2 d or 3 d modeling

on results of a controlled source audiofrequency magnetotelluric survey at the puhimau thermal area kilauea volcano hawaii by l c bartel and r d jacobson geophysics 52 665 677 may 1987 bartel and jacobson try to correct apparent resistivity values from csamt measurements where the plane wave approximation is not valid they want to convert csamt curves to passive mt curves cagniard curves which can be interpreted by 2 d modeling the measured csamt a value is projected onto the a axis in parallel with the half space curves of a horizontal electrical dipole in the given distance between receiver and transmitter the example looks convincing and it may be supposed to be a simple near field correction

schlumberger soundings near medicine lake california the use of direct current resistivity soundings to explore the geothermal potential of the medicine lake area in northern california proved to be challenging because of high contact resistances and winding roads deep schlumberger soundings were made by expanding current electrode spacings along the winding roads measured apparent resistivities were corrected using the geometric factor for the exact array geometry instead of a linear array geometry for horizontally stratified laterally homogeneous media the apparent resistivities measured with a nonlinear schlumberger array are equal to those measured with a linear schlumberger array provided that a distances from the current electrodes to the center of the array are equal and b the proper geometric factor is used to calculate the apparent resistivity corrected sounding data were interpreted using an automatic interpretation method forty two maps of interpreted resistivity were calculated for depths extending from 20 to 1000 m computer animati

etudes in computational rock physics alterations and benchmarking abstractwe tested computational benchmarking data for the absolute permeability electrical formation factor and elastic moduli based on the finney pack a physical dense random pack of identical spheres digitally rendered into a 3d rectangular coordinate system as the starting digital object it is altered by a changing the radius of each sphere and b geometrically inverting these new packs by swapping grains and pores porosity the absolute permeability electrical formation factor and elastic moduli are computed for all these alterations the direct grain based objects are relevant to clastic rock and the inverse objects are proxies for carbonates with moldic pores to corroborate these computational results we matched the permeability versus porosity formation factor versus porosity and elastic moduli versus porosity trends they form by established theoretical rock physics models these trends persisted when we reduced the scale of investigation by subsampling some of the digital objects

a magnetotelluric profile of the wind river thrust wyoming abstract data from ten magnetotelluric mt stations over the wind river uplift and adjacent basins are interpreted with constraints from the consortium for continental reflection profiling cocorp seismic reflection data and from gravity data the mt data reveal the general configuration of the conductive basins and resistive uplifts low resistivity zones are interpreted as faults which correspond to those visible in the cocorp sections the wind river thrust fault is modelled as a conductive zone that can be traced to a depth of at least 20 km and the crust beneath the green river basin is about 40 km thick the modelled constant dip of the wind river thrust is consistent with a tectonic model of lateral compressive stress

an integrated approach to quantify groundwater transport of phosphorus to narrow lake alberta an integrated approach was used to quantify groundwater phosphorus flux to narrow lake a smallglacial terrain lake in central alberta data from a drilling program major ion concentrations environmental isotopes and computer simulations indicated that the lake gains water through the nearshore region from a small shallow groundwater flow system at deep offshore regions water moves from the lake to the groundwater flow system seepage flux was quantified by water budget darcy s equation with data from wells near the lake darcy s equation with data from minipiezometers in the lake and seepage meters whole lake seepage flux determined from minipiezometer data 30 mm yr i was only lo 25 of the other estimates mean 221 mm yr i range 133 332 mm yr l from seepage meter and water budget data respectively groundwater contributed 30 of the annual water load to the lake the p concentration pi in pore water from lake sediments mean 175 mg m was 8 times higher than groundwater from wells near the lake mean 2 1 mg m 3 thus if well water was used to estimate the p of the seepage water the rate of groundwater p loading to the lake would be underestimated the rate of groundwater p loading to the lake computed from average seepage flux and average pore water p was 39 mg m 2 yr i and groundwater may be the largest single source of p to epilimnetic water in the lake

incorporation of topography into two dimensional resistivity inversion direct current resistivity data acquired on rough terrain can be interpreted by using an appropriate inversion technique after a topographic correction in order to avoid the influence of a possibly incomplete topographic correction an improved two dimensional resistivity inversion algorithm has been developed to estimate the subsurface resistivity distribution in the presence of topography in this paper fully discretized modeling is based on the finite element method and the iterative inversion scheme is derived from the second order marquardt damped least squares method the algorithm has been tested on both synthetic and field resistivity data with topography incorporated explicitly into the inversion model both theoretical and field studies indicate that the technique is computationally efficient and provides an improved geologic interpretation for complex subsurface structures a four electrode configuration is used in the algorithm so that the inversion can represent most field measurements

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

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