inversion of controlled source audio frequency magnetotellurics data for a horizontally layered earth we present a technique for inverting controlled source audio frequency magnetotelluric csamt data to recover a 1 d conductivity structure the earth is modeled as a set of horizontal layers with constant conductivity and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source the earth model has many layers compared to the number of data points and therefore the solution is nonunique among the possible solutions we seek a model with desired character by minimizing a particular model objective function traditionally csamt data are inverted either by using the far field data where magnetotelluric mt equations are valid or by correcting the near field data to an equivalent plane wave approximation here we invert both apparent resistivity and phase data from the near field transition zone and the far field regions in the full csamt inversion without any correction our inversion is compared with that obtained by inverting near field corrected data using an mt algorithm both synthetic and field data examples indicate that a full csamt inversion provides improved information about subsurface conductivity

high frequency ground motion scaling in the pacific northwest and in northern and central california abstract large data sets of vertical and horizontal seismograms from the pacific northwest seismic network northern california seismic network and berkeley digital seismic network are used to study the high frequency 0 25 16 hz ground motion scaling characteristics in washington oregon northern california 39 n to 42 n and 119 w to 124 w and central california 35 n to 39 n and 118 w to 123 w we used peak filtered ground velocities to characterize the propagation excitation and site terms the regression results for propagation were modeled using a geometrical spreading function g r and a frequency dependent attenuation q f q 0 f for the pacific northwest the best q model that fits the observation is expressed by q f 280 f 0 55 the eastern central california and western central california results are parameterized with q f 280 f 0 50 and q f 240 f 0 35 respectively the northern california results are not easy to model and require using a frequency dependent and a frequency dependent geometrical spreading the geometrical spreading effect for the frequencies higher than 5 hz is very strong in that region the excitation terms for the small events studied were modeled using a brune s source model an average stress drop of 30 bars was obtained for the pacific northwest the northern california average value for stress drop is 90 bars the observations of eastern central california were modeled with a stress drop of 49 bars while the best fit for the western part required 80 bars the range of the values obtained for q g r and indicates that the ground motion parameters for one region should not be used for another as an example our results show that the ground motion amplitude due to an m w 5 0 earthquake at a distance of 50 km is different by a factor of 1 5 between the pacific northwest and eastern central california

boat towed radio magnetotelluric and controlled source audio magnetotelluric study to resolve fracture zones at aspo hard rock laboratory site sweden boat towed radio magnetotelluric rmt measurements using signals between 14 and 250 khz have attracted increasing attention in the near surface applications for shallow water and archipelago areas

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

estimation of moment magnitude from ground motions at regional distances abstract we present a simple and robust approach to estimate moment magnitude m for events of m m can be estimated from 1 hz response spectral amplitudes for events in north america with an uncertainty standard deviation of residuals of m across regions and that it provides relationships that are consistent with the predictions of a stochastic point source model for typical regional attenuation models online material table of events for which the moment magnitude was estimated

quantitative spore colour spores exhibit changes in colour with increasing depth of burial these changes have found wide application for thermal maturity determination in hydrocarbon source rocks at present colours are determined visually here the application of the commission internationale de l eclairage colour system to the measurement of spores is described such that colour can be defined by the chromaticity co ordinates x y and the luminance l which define a colour solid the colour changes seen in spores with increasing burial define a maturation pathway in this colour solid which shows two distinct trends the first is a well defined progressive colour change to a cusp at which a broad spread of colours from yellow to brown occur in a single sample this is the interval over which hydrocarbons are generated and which is clearly defined by these colour differences the second trend is through a series of brown colours with an increasing lack of sensitivity of the spores to colour changes these changes are calibrated to vitrinite reflectivity the data and conclusions are amplified and supported through changes in colour chemistry and exine reflectivity in artificially matured lycopodium spores comparisons are made with existing qualitative spore colour scales the implications for maturation determination are that spore colour is a good thermal maturity index for the interval prior to oil generation and that it is suitable for quantitative modelling studies it demonstrates the interval of oil generation at the cusp but after this point it becomes inaccurate and inferior to other methods such as vitrinite reflectivity

comparison of induced polarization and controlled source audio magnetotellurics methods for massive chalcopyrite exploration in a volcanic area in this paper we compare and contrast the results of field experiments with the dipole dipole induced polarization ip and controlled source audio magnetotellurics csamt methods along the same survey profiles at a test area that was subject to extensive drilling and detailed geological investigation the ore bodies are interbedded between two series of dacitic tuff the depth and thickness of the massive chalcopyrite pyritesphalerite body vary between 25 and 100 m and 0 5 and 16 m respectively resistivity and ip phase measurements on the core samples collected from the test area provide some idea of the relative differences between the background rock units and the target the resistivity of the chalcopyrite samples varies between 0 6 to 2 ohm m and provides sufficient resistivity contrast with surrounding volcanic rock units for target detection the results of dipole dipole ip profiling with a 50 m dipole length conducted along two profiles are presented in the form of apparent resistivity and phase pseudosections csamt measurements were made at 13 frequencies from 2 hz to 8192 hz along three profiles the receiver dipole length was 25 m the csamt data are presented in the form of pseudosections using conventional and new definitions of apparent resistivity the elliptical contours of low apparent resistivity generated by the transition field notch can be misleading with respect to the real anomaly of the ore body these artificial anomaly patterns are suppressed by making use of an alternative apparent resistivity definition derived from the frequency normalized impedance the qualitative interpretation based on the ip and csamt pseudosections shows that the location and the extension of the ore body are indicated better in the csamt apparent resistivity data computed from the alternative definition the qualitative interpretation of the ip data is difficult because of the 3 d effect arising from the neighboring thicker parts of the ore body and pyrite particles within the basement at the final stage the far field range of the csamt apparent resistivity and phase data is identified by the visual inspection of the individual sounding diagrams the models derived from the 2 d occam inversion carried out on the far field data are compared with the drill hole information and are found to describe the actual geological situation

ground motions for earthquakes in southwestern british columbia and northwestern washington crustal in slab and offshore events regional ground motion generation and propagation must be charac terized to adequately assess seismic hazard in the cascadia region of southwestern british columbia and northwestern washington the ground motion issues are par ticularly complex because of the contributions to hazard from five distinct types of events all of which behave differently in terms of their ground motion propagation characteristics 1 shallow earthquakes occurring in the continental crust 2 shallow earthquakes occurring offshore in oceanic crust 3 earthquakes occurring within the subducting juan de fuca slab beneath the continent e g puget sound 4 earth quakes occurring within the subducting juan de fuca slab at the edge of the continent e g transitional events along the west coast of vancouver island and 5 great subduction earthquakes on the interface between the subducting juan de fuca plate and the overriding north american plate in this study empirical data recorded within the cascadia region are used to evaluate the source and attenuation characteristics of ground motion amplitudes from the first four of these event types crustal off shore in slab and transitional and examine their implications for regional ground motion relations for crustal earthquakes in cascadia a simple application of the hybrid empirical approach is used to suggest appropriate regional ground motion relations the ground motions are obtained by multiplying california ground motion relations by a frequency dependent factor to account for regional differences in crustal amplifica tion the developed ground motion relations for cascadia earthquakes are in rea sonable agreement with recorded ground motions in the region the proposed rela tions are the first region specific ground motion relations to be developed for the cascadia region and they provide a useful alternative to california relations for use in seismic hazard analysis

deep massive sulphide exploration using 2d and 3d geoelectrical and induced polarization data in skellefte mining district northern sweden geoelectrical and induced polarization data from measurements along three profiles and from one 3d survey are acquired and processed in the central skellefte district northern sweden the data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1 5 km the 2009 data were inverted previously and their joint interpretation with potential field data indicated several anomalous zones the 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area several high chargeability low resistivity zones interpreted as possible massive sulphide mineralization and associated hydrothermal alteration are revealed the 3d survey data provide a detailed high resolution image of the top 450 m of the upper crust around the maurliden east north and central deposits several anomalies are interpreted as new potential prospects in the maurliden area which are mainly concentrated in the central conductive zone in addition the contact relationship between themajor geological units e g the contact between the skellefte group and the j orn intrusive complex is better understood with the help of 2010 deep resistivity chargeability data the bottommost part of the vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to 1 km depth

tensor csamt studies at the buchans mine in central newfoundland a novel application of the tensor controlled source audio magnetotelluric csamt method was part of a multidisciplinary geophysical study of an existing mine site at buchans newfoundland the orthogonal components of the horizontal electromagnetic fields used for magnetotelluric and csamt interpretation of the earth s conductivity structure were found to be inappropriate at buchans because of strong scattering in the electric fields instead the length of the major axes of the electric and magnetic field polarization ellipses and the vertical magnetic field were used as data the data from two bipole sources demonstrate that the bulk response of the earth in the vicinity of buchans is predominantly one dimensional 1 d these data were inverted to layered earth models with a first order correction for electric field distortions the parameter space considered during the inversion was contracted substantially by incorporating the vertical magnetic field data and by using depths to interfaces as determi

the effects of source polarization in csamt data over two massive sulfide deposits in australia since the advent of the controlled source audio magnetotelluric method it has been recognized that the location and orientation of the bipole source is important in determining the response of the earth at the receiver in this study two dimensional 2 d far field modeling has been used to illustrate the frequency domain electromagnetic response of a simple conductive dike for two orthogonal polarizations of the source field the current gathered from the surrounding media by the dike when the electric field is parallel to the strike direction e polarization produces a strong anomaly not seen in the perpendicular h polarization this model response has been identified in real data sets over the rosebery and flying doctor orebodies of eastern australia in the case of rosebery the e polarization data yielded little structural information because the penetration of the signal was reduced by strong current channeling in the orebody and adjacent black shales at the flying doctor prospect the model pred

empirical attenuation of ground motion spectral amplitudes in southeastern canada and the northeastern united states a database of 1700 digital seismograms from 186 earthquakes of mag nitude mn 2 5 5 6 that occurred in southeastern canada and the northeastern united states from 1990 to 2003 was compiled maximum likelihood regression analysis of the database was performed to determine a model for the attenuation of fourier spectral amplitudes for the shear window for the vertical and horizontal component of motion for frequencies from 0 2 to 20 hz fourier amplitudes follow a hinged trilinear attenuation model fourier spectral amplitudes decay as r 1 3 where r is hypocentral distance within 70 km of the source there is a transition zone from 70 to 140 km as the direct waves are joined by strong postcritical reflections where the attenuation is described as r 0 2 spectral amplitudes actually increase with distance in this range for low frequencies beyond 140 km the attenuation is well described by r 0 5 corresponding to geometric spreading in two dimensions the associated model for the regional quality factor for frequencies greater than 1 hz can be ex pressed as q 893f 032 q can be better modeled over a wider frequency range 0 2 20 hz by a polynomial expression log q 3 052 0 393 log f 0 945 log f 2 0 327 log f 3 the polynomial expression accommodates the observation that q values are at a minimum about 1000 near 1 hz and rise at both lower and higher frequencies correction factors for the spectral amplitude model that describe the effects of focal depth on the amplitudes and their attenuation are developed using the subset of events with known focal depth the attenuation model is similar to that determined from an earlier study with more limited data atkinson and mereu 1992 but the enlarged database indicates more rapid near source amplitude decay and higher q the attenuation model is used to play back attenuation effects to determine the apparent source spectrum for each earthquake in the database and hence determine moment magnitude m and brune stress drop the events have moment magnitude in the range from 2 5 to 5 stress drop increases with moment magnitude for events of m 4 3 then appears to attain a relatively constant level in the range from 100 to 200 bars for the larger events as previously noted in atkinson 1993b the results of this study provide a useful framework for improving regional ground motion relations in eastern north america they further our understanding of attenuation in the region through analysis of an enlarged ground motion database in particular the inclusion of the three component broadband data gathered over the last decade allows extension of attenuation models to both horizontal and vertical components over a broad frequency range 0 2 20 hz

regional differences in ground motion amplitudes of small to moderate earthquakes across north america abstract we studied the response spectral amplitudes and peak ground acceleration pga of earthquakes with moment magnitude m from 3 0 to 6 0 to investigate regional differences in ground motion amplitudes across north america on a nonparametric basis for a common national earthquake hazards reduction program b c site condition e g shear wave velocity of 760 m s in the top 30 m of the soil column we use southern california as the reference region because it has the most plentiful data against which to compare other regions amplitudes of response spectra in eastern canada northeastern united states become increasingly larger than those in southern california as distance frequency and magnitude increases in the central united states ground motion amplitudes are larger than those in southern california at distances more than 100 km and lower at distances less than 100 km over all frequency and magnitude ranges studied amplitudes for crustal events in the pacific northwest british columbia are lower than those in southern california at frequencies 1 hz over all magnitude and distance ranges but have similar amplitudes below 1 hz amplitudes in northern california are similar to those in southern california at low frequencies below 3 3 hz but have lower amplitudes at high frequencies pga over all distances and magnitudes we conclude that regional amplitude differences are largely attributable to complex frequency dependent differences in regional attenuation for example the slope of apparent geometric spreading at distances less than 100 km is significantly steeper more negative in eastern canada northeastern united states than other regions and may be frequency dependent the regional attributes of the attenuation comparisons appear to be the same for both vertical and horizontal components at least for b c site conditions online material figures of region dependent ground motion amplitudes apparent geometric spreading statistical tests of the ground motions and figures and table of ratios of ground motions relative to those from southern california

definitions of apparent resistivity for the presentation of magnetotelluric sounding data1 a new definition of apparent resistivity for the presentation of magnetotelluric sounding data is proposed the new definition is based on the frequency normalized impedance function both the existing and proposed definitions of apparent resistivity are analysed theoretically and are compared using model curves computed for a 1d earth model apparent resistivity curves computed using the proposed definition are a better approximation to the true resistivity values of the subsurface layers in addition the layers are more noticeable on the apparent resistivity curves which is an advantage especially for the ascending and descending type of apparent resistivity curve

earthquake ground motion prediction equations for eastern north america new earthquake ground motion relations for hard rock and soil sites in eastern north america ena including estimates of their aleatory uncertainty vari ability have been developed based on a stochastic finite fault model the model incorporates new information obtained from ena seismographic data gathered over the past 10 years including three component broadband data that provide new in formation on ena source and path effects our new prediction equations are similar to the previous ground motion prediction equations of atkinson and boore 1995 which were based on a stochastic point source model the main difference is that high frequency amplitudes f 5 hz are less than previously predicted by about a factor of 1 6 within 100 km because of a slightly lower average stress parameter 140 bars versus 180 bars and a steeper near source attenuation at frequencies less than 5 hz the predicted ground motions from the new equations are generally within 25 of those predicted by atkinson and boore 1995 the prediction equations agree well with available ena ground motion data as evidenced by near zero average residuals within a factor of 1 2 for all frequencies and the lack of any significant residual trends with distance however there is a tendency to positive residuals for moderate events at high frequencies in the distance range from 30 to 100 km by as much as a factor of 2 this indicates epistemic uncertainty in the prediction model the positive residuals for moderate events at 100 km could be eliminated by an increased stress parameter at the cost of producing negative residuals in other magnitude distance ranges adjustment factors to the equations are provided that may be used to model this effect

observations on regional variability in ground motion amplitudes for small to moderate earthquakes in north america abstract the regional variability in earthquake ground motion amplitudes for a given magnitude and distance in western north american environments was examined using shakemap data from small to moderate events the abundance of data for small to moderate events in california allows average ground motion levels as a function of magnitude and distance to be resolved with a high level of confidence ground motion amplitudes in northern california are lower on average than those for southern california for events of the same magnitude at distances in the range from 120 to 250 km over all frequencies the observed regional variations could be indicative of regional differences in attenuation effects or site effects an unexpected result of the study is the finding that ground motions for events of m m m m

use of magnetotelluric signals from 50 hz power lines for resistivity mapping of geothermal fields in new zealand electromagnetic em fields radiated from the transmission lines of the new zealand electricity grid have been digitally recorded at test sites near the tokaanu geothermal field amplitudes and phases of the 50 hz signals and the odd harmonics up to 450 hz were determined using a software implementation of a phase locked filter these data were then analyzed to determine the components of the magnetotelluric impedance tensor and the corresponding apparent resistivities and phases at most sites there was sufficient variation in the elliptical polarization of the em fields to enable the impedance tensors to be determined in full sites where the em data had been affected by near source effects were identified by having large vertical magnetic field components and by being closer to a power line source than about 3 5 skin depths with the test measurements the north eastern part of the tokaanu geothermal field was successfully delineated giving low resistivities gt 5 omega m on the inside and higher resistivities on the outside in agreement with the schlumberger array dc apparent resistivities the small size of the 50 hz magnetotelluric equipment and its portable nature make this method of resistivity measurement suitable for reconnaissance resistivity mapping in places with difficult access

out of plane effects in 2d borehole to surface resistivity tomography and applications in mineral exploration in this paper we discuss the effects of anomalous out of plane bodies in two dimensional 2d borehole to surface electrical resistivity tomography with numerical resistivity modelling and synthetic inversion tests the results of the two groups of synthetic resistivity model tests illustrate that anomalous bodies out of the plane of interest have an effect on two dimensional inversion and that the degree of influence of out of plane body on inverted images varies the different influences are derived from two cases one case is different resistivity models with the same electrode array and the other case is the same resistivity model with different electrode arrays qualitative interpretation based on the inversion tests shows that we cannot find a reasonable electrode array to determine the best inverse solution and reveal the subsurface resistivity distribution for all types of geoelectrical models because of the three dimensional effect arising from neighbouring anomalous bodies the qualitative interpretation of inverted images from the two dimensional inversion of electrical resistivity tomography data without prior information can be misleading two dimensional inversion with drilling data can decrease the three dimensional effect we employed two and three dimensional borehole to surface electrical resistivity tomography methods with a pole pole array and a bipole bipole array for mineral exploration at abag banner and hexigten banner in inner mongolia china different inverse schemes were carried out for different cases the subsurface resistivity distribution obtained from the two dimensional inversion of the field electrical resistivity tomography data with sufficient prior information such as drilling data and other non electrical data can better describe the actual geological situation when there is not enough prior information to carry out constrained two dimensional inversion the three dimensional electrical resistivity tomography survey is the better choice

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station