paleomagnetic secular variation at the azores during the last 3 ka 1 we report on 33 new paleomagnetic directions obtained from 16 lava flows emplaced in the last 3 ka on sao miguel the largest island of the azores the data provide 27 well dated directions from historical or 14c dated flows which together with 6 directions previously gathered from the same flows by johnson et al 1998 yield the first paleomagnetic directional record of the last 3 ka from the atlantic ocean within flow directions are consistent suggesting that inclination swings from 60 to 25 and declination changes between 10 to 20 reflect variations in the geomagnetic field over the last 3 ka to a first approximation the declination record is consistent with predictions from cals3k 4 and gufm1 global field models conversely inclination values are lower than model predictions at two different ages 1 four sites from the 1652 ad flow yield i 48 instead of i 63 predicted by gufm1 2 data from several flows nicely mimic the inclination minimum of 800 1400 ad but inclination values are lower by 10 than cals3k 4 model predictions by interpolating a cubic spline fit on declination inclination versus age data we tentatively infer the directional evolution of the geomagnetic field at the azores from 1000 bc to 1600 ad the obtained curve shows three tracks in virtual overlap during the 1000 800 bc 800 500 bc and 400 700 ad time spans

electromagnetic and geoelectric investigation of the gurinai structure inner mongolia nw china abstract in three field campaigns between the years 2000 and 2004 geophysical measurements were conducted in the ejina basin nw china research work in the year 2004 which is described in this paper was concentrated on the gurinai structure 101 25 e 41 n situated in the southeastern part of the ejina basin in transition to the dune fields of the badain jaran shamo on satellite images the gurinai structure can be identified by two almost 100 km long subparallel n s striking lineaments which may indicate tectonic deformations of late quaternary sediments to get a coherent picture of the structure a geophysical survey employing three electromagnetic methods magnetotellurics mt transient electromagnetics tem and geoelectrics dc has been conducted to map the subsurface resistivity at different depth scales the geophysical data interpretation for shallow and intermediate depth down to a few hundred meters links the subsurface distribution of electric resistivity to geomorphological units known from field work in reference with satellite images the westerly lineament of the gurinai structure coincides with a subvertical change in electric resistivity together with geomorphological indications from fieldwork and the analysis of elevation data srtm a tectonic deformation of unconsolidated sediments along a fault with an extensional component is interpreted in the central and eastern part of the gurinai structure a shallow resistive subsurface layer can be traced into the first dunes of the badain jaran shamo this resistive subsurface layer is linked to the presence of fresh water indicating infiltration from the dune field also in the eastern part of the gurinai structure a resistive approximately ene striking feature can be seen at intermediate depth which is interpreted as a crystalline basement ridge towards the southern margin of the gurinai structure a trough shaped unit with low resistivities and a thickness of about 1 km is identified and can be explained by a sediment package saturated with fluids of high salinity or substantial amounts of clay the strike direction of the structure can be connected to the regional pattern of tectonic faults and seismicity the interpretation of electromagnetic data at various depth scales contributes to the general understanding of the ejina basin s buildup and tectonic setting in the vicinity of the gurinai structure

assessment of near surface mapping capabilities by airborne transient electromagnetic data an extensive comparison to conventional borehole data abstracta newly developed helicopter transient electromagnetic tem system has the ability to measure very early times within just a few s after the turn off of the primary current for such a system careful calibration and accurate modeling of the electromagnetic em response is critical to get true resistivities of the very shallow geologic layers we discovered that this leads to resolution of the same level or in some cases even better than what can be obtained from airborne frequency em systems this allowed a range of important applications where high and accurate resolution is mandatory e g geotechnical applications such as urban planning railroad and road investigations landslides or distribution of raw materials and assessing aquifer vulnerability we evaluated the results of a pilot survey covering the norsminde catchment south of aarhus denmark where we found that near surface layers top 30 m can be mapped with an accuracy of a few meters in a complicated glacial sedimentary envir

palaeointensity determinations from historical and holocene basalt lavas in iceland summary palaeomagnetic measurements were made for 11 post glacial basalt lavas in iceland three pre 2 ka lavas seven historical lavas and one present day lavas palaeodirections were easy to determine at most sites but three of the sites seem to have suffered from some block movement at the lava front making determination of palaeodirections difficult reliable site mean palaeodirections were successfully obtained at two of these sites by analysing the remanence components with low blocking temperature but one site was rejected due to low convergence of remanence directions indicating a possible post cooling block movement a total of 45 samples 52 specimens from nine lava flows were selected for palaeointensity experiments using the thellier method based on the thermal stability of remanence and other rock magnetic features together with hysteresis parameters to show single domain to pseudo single domain natures 26 of the 45 samples 29 of the 52 specimens produced successful results subsidiary experiments using shaw s method were also carried out on one specimen from each of eight lava flows and all of these produced successful results combining these thellier and shaw results eight site mean palaeointensities were obtained including one 1984 ad lava giving a specimen level success rate of 56 per cent for the experiments using the thellier method and 100 per cent for those using shaw s method our results together with those of previous studies indicate that variations in palaeointensity and palaeodirection in iceland are basically similar to those in nw europe for the last 2000 yr variations in palaeointensity converted to virtual dipole moment or virtual axial dipole moment are in good agreement with both nw europe and global models some differences in inclination and declination are recognized between iceland and nw europe but overall variation curves are similar and good agreement with the global models is also observed

reconstructing holocene geomagnetic field variation new methods models and implications reconstructions of the holocene geomagnetic field and how it varies on millennial timescales are important for understanding processes in the core but may also be used to study long term solar terrestrial relationships and as relative dating tools for geological and archaeological archives here we present a new family of spherical harmonic geomagnetic field models spanning the past 9000 yr based on magnetic field directions and intensity stored in archaeological artefacts igneous rocks and sediment records a new modelling strategy introduces alternative data treatments with a focus on extracting more information from sedimentary data to reduce the influence of a few individual records all sedimentary data are resampled in 50 yr bins which also means that more weight is given to archaeomagnetic data during the inversion the sedimentary declination data are treated as relative values and adjusted iteratively based on prior information finally an alternative way of treating the sediment data chronologies has enabled us to both assess the likely range of age uncertainties often up to and possibly exceeding 500 yr and adjust the timescale of each record based on comparisons with predictions from a preliminary model as a result of the data adjustments power has been shifted from quadrupole and octupole to higher degrees compared with previous holocene geomagnetic field models we find evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 yr the new models also show intermittent occurrence of reversed flux at the edge of or inside the inner core tangent cylinder possibly originating from the equator less

fennostack and fennorpis varve dated holocene palaeomagnetic secular variation and relative palaeointensity stacks for fennoscandia a holocene palaeomagnetic secular variation master curve fennostack and a relative palaeointensity curve fennorpis for fennoscandia are presented these curves were produced by stacking palaeomagnetic data obtained from six annually laminated varved lake sediment sequences and one non laminated sediment sequence the six independent varve chronologies were combined to form a timescale which extends between the present year and 10 200 cal yr bp smoothed inclination and declination curves show trends that are similar to a palaeomagnetic secular variation curve for the united kingdom except that the ages of most of the statistically significant features are a few hundred years younger in the fennoscandian data set these differences are most likely due to excessively old ages of the united kingdom features produced by the radiocarbon dating of bulk sediments the standardized relative palaeointensity stack shows that the intensity of the geomagnetic field in fennoscandia was highest at 2300 cal yr bp and lowest at 7000 cal yr bp three palaeointensity peaks pips and three palaeointensity lows pils are identified in fennorpis and indicate that the intensity of geomagnetic field varied significantly on millennial scales in this region during the holocene but there is much scatter in the reconstruction prior to 8000 cal yr bp higher frequency palaeointensity features present in individual records are not significant in the stacked data set which implies that these are a result of local environmental bias significant discrepancies exist between the data and the output of the cals7k 2 geomagnetic field model for the last 7000 yr in particular the largest declination swing observed in the data at 2670 cal yr bp feature f is not produced by the model which is most likely due to the influence of an inaccurately dated icelandic secular variation curve on the model result short term trends in standardized relative palaeointensity do differ from the model output but it is difficult to make a comparison because of the relative nature of the sediment record better absolute palaeointensity data are needed to reduce uncertainties in reconstructions of past geomagnetic field strength

geoelectrical measurements at the tag hydrothermal mound direct current electrical experiments with electrode arrays deployed by a submersible alvin were used to determine the resistivity structure of the seafloor below and vertical potential gradients in sea water above the hydrothermally active tag mound apparent resistivities of the sulfides at shallow depths 10 m below seafloor over the mound ranged between 0 18 0 21 ohm m compared to nearby basalt pillow apparent resistivities of 2 1 2 4 ohm m the potential differences between heights of about 18 to 27 m above the mound averaged 3 7 mv voltage increasing with height with variations between about 2 9 and 12 7 mv

an investigation of seawater and sediment depth using a prototype airborne electromagnetic instrumentation system a case study in broken bay australia previous studies using commercial airborne electromagnetic equipment that is not optimized for marine surveying have demonstrated the use of airborne electromagnetic methods for measuring water depth and estimating sediment thickness a new prototype helicopter time domain airborne electromagnetic system seatem 0 is now under development for bathymetric surveying the first sea trial of the seatem 0 system took place over broken bay new south wales australia in shallow water up to 30 m in depth broken bay was chosen because the separate paleodrainage systems for the hawkesbury river brisbane water and pittwater which join in broken bay give rise to paleovalleys infilled with unconsolidated sediments ranging in thickness between 0 m bedrock outcrop and 200 m the survey area also included a tombolo with a beach either side which provided the opportunity to measure water depth through a surf zone sediment thickness and water depth is predicted from stitched layered earth inversion of data based on a simplified two layer model that represents seawater and sediment overlying a resistive half space basement bedrock the resulting bathymetric profiles show agreement typically to within 1 m and 0 5 m with known water depths in areas less than 20 and 6 m deep respectively the inverted depth profile of the second sediment layer is noisy however the profiles reveal coarse topographic features of paleovalleys to depth limits of 60 to 80 m below sea level in 20 to 30 m water depth as well as resolving bedrock ridges and exposed reefs in shallow waters

the middle to late pleistocene geomagnetic field recorded in fine grained sediments from summer lake oregon and double hot springs nevada u s a abstract a 400 000 year record of the paleomagnetic field has been acquired from 22 meters of middle to late pleistocene fine grained sediments from summer lake in south central oregon and double hot springs in northwestern nevada the stratigraphy is based on 55 tephra layers nine of which have been correlated with tephra layers from other localities on the basis of their distinct major and trace element geochemistry and their distinct petrography the paleomagnetic samples carry a strong and stable magnetization that does not appear to have been affected by the inclination error commonly associated with the magnetization of sediments the samples have accurately recorded the declination and inclination of the geomagnetic field at or near the time of deposition except for errors arising from rotations of discrete blocks of sediment predominantly about vertical axes errors introduced by this type of rotation were corrected by using paleomagnetic directions associated with correlated tephra layers the summer lake paleomagnetic record suggests that secular variations occurred throughout the middle and late pleistocene often maintaining the same waveform through several oscillations the amplitudes of these variations were similar to those of holocene variations and the periods ranged from 15 000 years to greater than 100 000 years

first results of the moses experiment sea sediment conductivity and thickness determination bute inlet british columbia by magnetometric offshore electrical sounding a static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea the transmitter is a vertical long wire bipole extending from the sea surface to the sea floor a commutated current generated on the ship is fed to two large electrodes one near the sea surface the other at the end of a long insulated wire the return path for the current is through the sea and the subjacent crust the receiver is a self contained remote microprocessor controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered the data are measurements of the azimuthal component of the magnetic field as a function of transmitter receiver horizontal separation the acronym moses has been coined for the method the choice of the name moses is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water in particular accurate estimates of sea floor resistivity ar

late holocene paleoenvironments of northwest iceland from lake sediments abstract frozen sediment cores from two lakes in northwest iceland vatnsdalsvatn and thi riksvallavatn provide a nearly 1000 year decadal scale record of past environmental changes based on environmental magnetism inductively coupled plasma optical emission spectroscopy geochemistry carbon content and c n analyses accelerator mass spectrometry 14 c and 210 pb results provide chronologies for both sites covering the late 20th century to 960 bp at vatnsdalsvatn and to 480 bp at thi riksvallavatn these analyses reveal higher levels of sedimentary carbon during the little ice age between 600 and 50 bp at vatnsdalsvatn c n analyses suggest that some of this carbon enrichment came from terrestrial plants higher erosion rates during the little ice age may also have increased the influx of limiting nutrients enhancing algal productivity erosion is the dominant mechanism for sedimentary changes at both sites and seems to occur at predictable intervals geochemical indicators of erosion oscillate with a distinct 80 100 yr periodicity for nine centuries at vatnsdalsvatn and for four centuries at thi riksvallavatn these erosion cycles are consistent with long term stability in the positive mode of the north atlantic oscillation but may also be attributed to local climatic effects associated with great salinity anomalies

airborne electromagnetic bathymetry airborne electromagnetic techniques provide an efficient means for determining the bathymetry of shallow seas and coastal regions offshore time domain data collected with the input system may be interpreted quickly and inexpensively using the simple table look up algorithm described here this interpretation algorithm is based on two dimensionless parameters which depend on the six channel amplitudes of the recorded input data these amplitudes are computed and tabulated for a suite of simple two layer models in which seawater depth is the only variable quantity the experimental data can then be interpreted by comparison with the tables of theoretical values to obtain the bathymetry beneath the flight path the inversion algorithm has proven accurate and stable in the range of 0 40 m of seawater the representative difference between the interpreted depths and the charted depths is about 2 m bottom gradients lower than 5 percent were resolved accurately beneath 0 25 m of water independent interpreta

palaeosecular variation of the earth s magnetic field during the last 10000 years based on the annually laminated sediment of lake nautajarvi central finland a palaeomagnetic study of the lake nautajarvi sediment sequence in central finland demonstrates that varved lake sediments can provideregionally applicable secular variations curves for holocene stratigraphic correlation and age control based on x ray radiography and digital image analysis techniques we constructed an inherent and continuous varve chronology for the lake nautajarvi varve sequence which covers nearly 10000 years with an estimated counting error of less than 1 and a mean sedimentation rate of 0 66 mm yr 1 we reconstructed a record of the palaeomagnetic secular variation psv of the earth s magnetic field in this nearly 7 m long section of clastic organic varves mineral magnetic measurements indicated that pseudosingle domain magnetite is the major carrier of the remanence psv shows many of the familiar features of declination and inclination that have previously been recorded in northern europe in addition the lake nautajarvi record contains a continuous record of relative palaeoi

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

holocene geocentric dipole tilt model constrained by sedimentary paleomagnetic data paleomagnetic data although undeniably limited have been used to constrain complex time varying geomagnetic field models here we present four different versions of a time varying geocentric dipole model for the past approximately 9000 years based on a limited selection of sedimentary paleomagnetic data with good spatial coverage the robustness of the modeling approach is evaluated using modern magnetic observation station data provided by the world data centre for geomagnetism in edinburgh and we discuss the magnitude of the error introduced by insufficient data coverage the prediction in terms of directional variations is tested against paleomagnetic data from 63 different sedimentary sites and compared to results from recently published spherical harmonic geomagnetic field models we find that the dipole estimate can explain the paleomagnetic data almost as well as the more complicated spherical harmonic models specific areas and records are identified where there are significant differences between the performance of the dipole estimate and the spherical harmonic models our results suggest that many of the differences between the dipole model predictions and the paleomagnetic data can be related to poor paleomagnetic quality and or the lack of accurate and precise time control inherent to some of the records these results agree with valet et al 2008 who suggested that the cals7k 2 model probably underestimates the dipole contribution in order to reduce the misfit between the inversion and the data we conclude that better paleomagnetic data and associated time control are needed to improve global geomagnetic field models less

icelandic holocene palaeolimnomagnetism abstract sediments spanning the last 9000 y from two sites in lake vatnsdalsvatn lat 66 n long 23 e in northwest iceland hold repeatable palaeomagnetic direction records the vatnsdalsvatn sediments have mean palaeomagnetic inclinations of 76 close to that expected for a geocentric axial dipole field and direction fluctuations of around 20 from the mean the palaeomagnetic directions are stable under alternating field partial demagnetization experiments a time scale for the vatnsdalsvatn sediments has been estimated from 14 c dating the pattern of palaeomagnetic secular change shows few similarities with british records 2000 km distant and a central north american record 5000 km distant

optimized fast hankel transform filters in the linear digital filter theory for calculation of hankel transforms it is possible to find explicit series expansions for the filter coefficients a method is presented for optimizing the hankel filters calculated in this way for a certain desired accuracy of computation the sampling density and filter length are minimized by choosing the parameters determining the filter characteristics according to the analytical properties of the input function a new approach to the calculation of the filter coefficients has been developed for these optimized filters the length of the filters may be further reduced by introducing a shift in the sampling scheme

airborne electromagnetic footprints in 1d earths existing estimates of footprint size for airborne electromagnetic aem systems have been based largely on the inductive limit of the response we present calculations of frequency domain aem footprint sizes in infinitehorizontal thin sheet and half space models for the case of finite frequency and conductivity in a half space the original definition of the footprint is extended to be the side length of the cube with its top centered below the transmitter that contains the induced currents responsible for 90 of the secondary field measured at the receiver for a horizontal coplanar helicopter frequency domain system the in phase footprint for induction numbers less than 0 4 thin sheet or less than 0 6 half space increases from around 3 7 times the flight height at the inductive limit to more than 10 times the flight height for a vertical coaxial system the half space footprint exceeds nine times the flight height for induction numbers less than 0 09 for all models geometries and frequencies the quadrature footprint is approximately half to two thirds that of the in phase footprint these footprint estimates are supported by 3d model calculations that suggest resistive targets must be separated by the footprint dimension for their individual anomalies to be resolved completely analysis of frequency domain aem field data acquired for antarctic sea ice thickness measurements supports the existence of a smaller footprint for the quadrature component in comparison with the in phase but the effect is relatively weak in phase and quadrature footprints estimated by comparing aem to drillhole data are considerably smaller than footprints from 1d and 3d calculations however we consider the footprints estimated directly from field data unreliable since they are based on a drillhole data set that did not adequately define the true 3d sea ice thickness distribution around the aem flight line

characterization of alluvial formation by stochastic modelling of paleo fluvial processes the concept and method summary modelling fluvial processes is an effective way to reproduce basin evolution and to recreate riverbed morphology however due to the complexity of alluvial environments deterministic modelling of fluvial processes is often impossible to address the related uncertainties we derive a stochastic fluvial process model on the basis of the convective exner equation that uses the statistics mean and variance of river velocity as input parameters these statistics allow for quantifying the uncertainty in riverbed topography river discharge and position of the river channel in order to couple the velocity statistics and the fluvial process model the perturbation method is employed with a non stationary spectral approach to develop the exner equation as two separate equations the first one is the mean equation which yields the mean sediment thickness and the second one is the perturbation equation which yields the variance of sediment thickness the resulting solutions offer an effective tool to characterize alluvial aquifers resulting from fluvial processes which allows incorporating the stochasticity of the paleoflow velocity

on mapping seafloor mineral deposits with central loop transient electromagnetics electromagnetic methods are commonly employed in exploration for land based mineral deposits a suite of airborne land and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose in contrast although the commercial value of marine mineral deposits has been recognized for decades the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy one particularly interesting electromagnetic method which could be used to image a mineral deposit on the ocean floor is the central loop configuration central loop systems consist of concentric transmitting and receiving loops of wire while these types of systems are frequently used in land based or airborne surveys to our knowledge neither system has been used for marine mineral exploration the advantages of using central loop systems at sea are twofold 1 simplified navigation because the transmitter and receiver are concentric and 2 simplified operation because only one compact unit must be deployed we produced layered seafloor type curves for two particular types of central loop methods the in loop and coincident loop configurations in particular we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick modeling and resolution analyses showed that using a 50 m 2 transmitting loop with 20 a of current these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness in the most extreme case absolute voltage errors on the order of 10 nv are required to resolve the base of a 30 m thick ore deposit whether such noise floors can be achieved in real marine environments remains to be seen

differentiating tidal and groundwater dynamics from barrier island framework geology testing the utility of portable multifrequency electromagnetic induction profilers abstractelectromagnetic induction emi techniques are becoming increasingly popular for near surface coastal geophysical applications however few studies have explored the capabilities and limitations of portable multifrequency emi profilers for mapping large scale 101 102 km barrier island hydrogeology the purpose of this study is to investigate the influence of groundwater dynamics on apparent conductivity a to separate the effects of hydrology and geology from the a signal shore normal and alongshore surveys were performed within a highly conductive barrier island wind tidal flat system at padre island national seashore texas usa assessments of instrument calibration and signal drift suggest that a measurements are stable but vary with height and location across the beach repeatability tests confirm a values using different boom orientations collected during the same day are reproducible measurements over a 12 h tidal cycle suggest that there is a tide dependent step response in a c

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

spline analysis of holocene sediment magnetic records uncertainty estimates for field modeling 1 sediment and archeomagnetic data spanning the holocene enable us to reconstruct the evolution of the geomagnetic field on time scales of centuries to millennia in global field modeling the reliability of data is taken into account by weighting according to uncertainty estimates uncertainties in sediment magnetic records arise from 1 imperfections in the paleomagnetic recording processes 2 coring and sub sampling methods 3 adopted averaging procedures and 4 uncertainties in the age depth models we take a step toward improved uncertainty estimates by performing a comprehensive statistical analysis of the available global database of holocene magnetic records smoothing spline models that capture the robust aspects of individual records are derived this involves a cross validation approach based on an absolute deviation measure of misfit to determine the smoothing parameter for each spline model together with the use of a minimum smoothing time derived from the sedimentation rate and assumed lock in depth departures from the spline models provide information concerning the random variability in each record temporal resolution analysis reveals that 50 of the records have smoothing times between 80 and 250 years we also perform comparisons among the sediment magnetic records and archeomagnetic data as well as with predictions from the global historical and archeomagnetic field models combining these approaches we arrive at individual uncertainty estimates for each sediment record these range from 2 5 to 11 2 median 5 9 interquartile range 5 4 to 7 2 for inclination 4 1 to 46 9 median 13 4 interquartile range 11 4 to 18 9 for relative declination and 0 59 to 1 32 median 0 93 interquartile range 0 86 to 1 01 for standardized relative paleointensity these values suggest that uncertainties may have been underestimated in previous studies no compelling evidence for systematic inclination shallowing is obtained from the analysis of the available database of holocene sediment magnetic records the analysis highlights the importance of collecting oriented cores publishing and archiving unprocessed raw paleosecular variation determinations and presenting a detailed chronology so that changes in the sedimentation rate can be assessed with regard to future field models workers should consider rejection of anomalous cores through comparisons to other sources and ensure that realistically large uncertainties are allocated to high latitude declination records

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

testing the limits of aem bathymetry with a floating tem system a floating transient electromagnetic tem system sea ring simulating a low altitude helicopter airborne electromagnetic aem system was constructed to test the accuracy of the aem method for measuring water depth and estimating sediment thickness in shallow coastal waters a square transmitter loop 10 10 m plus concentric inner and outer receiver loops was strung from masts supported by the circular sea ring base data were stacked over periods from 1 to approximately 60 s and with loop heights ranging between approximately 5 5 and 12 m above sea level the towed sea ring provides a stable platform at a known fixed altitude in calm waters we have undertaken modeling to investigate the effect of vertical and horizontal displacements of the loops and to compare circular and square loopgeometries in proximity to the sea surface with relatively long stacking times as long as approximately 60 s the uncertainty in altitude can be reduced to very low levels the sea ring has been deployed near

improved seawater depth determination using corrected helicopter time domain electromagnetic data abstract two areas within sydney harbour were surveyed in 2002 with a helicopter borne time domain electromagnetic system to test its potential for bathymetric mapping in shallow seawater as delivered the data were improperly calibrated therefore a re calibration was performed to reconcile the measured data with ground truth synthetic electromagnetic transients were computed for two layer models representing the seawater and the sediment overlying bedrock at a number of locations within the survey area the seawater depth in the models varied between 3 m and 70 m the measured and calculated data were compared at each delay time and were found to be linearly related the slope and intercept of the line of best fit were used to correct all the measured data inversion of the corrected time domain electromagnetic data generally resolved the bathymetry to submetre accuracy down to depths of about 55 m

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

geomagnetic evolution 400 years of change on planet earth abstract spherical harmonic coefficients of the geomagnetic field calculated from historical observations of declination inclination and intensity and from archaeomagnetic inclination results have been used to produce a film of geomagnetic change since 1600 a d the non dipole geomagnetic field is found to be constantly changing no fixed or standing non dipole features are observed non dipole foci are seen to have lifetimes of a few hundred years the westward drift which was an important feature of the 18th and early 19th century geomagnetic field was less pronounced in the 17th century the growth evolution decay and replacement of non dipole foci but not their movement are found to have been the major features producing century long secular directional magnetic variation most of the low degree and order spherical harmonic coefficients have changed significantly over the last few hundred years in particular the change in sign of the axisymmetric quadrupole around 1837 a d is noted sustained century long intensity changes however appear to have been dominated by variations in the intensity of the centred dipole rather than by non dipole field fluctuations

multi stage mountain building vs relative plate motions in the betic cordillera deduced from integrated microstructural and petrological analysis of porphyroblast inclusion trails abstract the tectonic evolution of the betic cordillera has been investigated through integrated microstructural and petrological analysis of 93 samples of garnetiferous grt cld plg ky st phengite schists from its lowermost allochthon the nevado filabride domain porphyroblast inclusion trails in these samples exhibit well developed preferred vertical and horizontal orientations and truncational relationships these features indicate a complex history of multiple crenulation cleavage development during alternating contractional and gravity induced deformations associated foliation intersection axes preserved in porphyroblasts fia were determined separately for 2 groups of samples using the asymmetry technique bell et al 1995 and the fitpitch method aerden 2003 respectively broadly consistent results establish a succession of 4 fia sets with specific trends which from the oldest to the youngest fia set are ne sw fia set 1 nw se fia set 2 w e fia set 3 and nnw sse fia set 4 this trend sequence is also reflected in the orientations of successive fold generations and associated lineations more remarkably the fia trend sequence mimics known changes in the plate motions of africa and adria relative to iberia in the 50 10 ma interval this is also the time span indicated by published radiometric ages garnet isopleth thermobarometry applied to selected samples and compositional zoning of garnets indicate that the earliest 3 fia sets formed along a prograde metamorphic path from ca 5 kb 500 c to 10 kb 550 c whereas the latest stages of fia set 3 and fia set 4 developed during exhumation exhumation involved a period of heterogeneous reheating that was spatially and temporally linked to emplacement of the overlying alpujarride complex

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

understanding the hydrogeology of the venice lagoon subsurface with airborne electromagnetics abstract the occurrence of alternating dry wet conditions in transitional environments such as wetlands deltas and lagoons usually challenges the use of traditional direct and geophysical surveys for comprehensive hydrogeologic investigations moreover significant mixing between continental fresh groundwater and marine salty surface waters generally takes place in these flat coastal areas airborne electromagnetics aem is a promising tool in this respect as it provides in a fast and cost effective manner large scale distribution of bulk electrical conductivities that can be used profitably to develop hydrogeologic models the results of a skytem aem survey in the venice lagoon italy show the capability of this technique to significantly improve the knowledge of the hydrogeologic setting of the lagoon and nearby coastland subsurface irrespective of the different features characterizing the area the environment consists of salt marshes mud flats shallows tidal channels islands together with reclaimed farmlands crossed by natural watercourses and drainage channel networks in particular the aem shows i the presence of fresh water with resistivity larger than 20 m underneath the central part of the lagoon at depths from 10 to 25 m below m s l ii the interface between different relevant stratigraphic units e g the clayey layer bounding the holocene pleistocene sedimentation and iii the occurrence of areas with possible submarine fresh groundwater discharge moreover the source and inland extent of the saltwater contamination in the shallow coastal aquifers along the southern margin of the lagoon are clearly revealed aem data were complemented with very high resolution seismic vhrs acquisitions the integrated analysis of the two data sets allows distinguishing between lithostratigraphic heterogeneity and variability of the subsurface fluids

review of electromagnetic induction for mapping barrier island framework geology abstract the geologic framework controls on modern barrier island transgression and the relationship of these controls to subsurface structure hydrology and island geomorphology are not well understood recent evidence suggests that alongshore variations in pre holocene geology of barrier islands modify nearshore hydrodynamic processes and sediment transport ultimately affecting how barrier islands will respond to relative sea level rise explorations of holocene barrier island geology are usually based on cores to supplement bathymetric onshore offshore seismic and or ground penetrating radar gpr surveys the advantages and limitations of these methods with respect to barrier island investigations are briefly described in this review alternative near surface geophysical methods including electromagnetic induction emi sensors are increasingly being used for coastal research because they are non invasive provide continuous subsurface information across a variety of sub environments and are capable of characterizing large areas in a short time although these emi sensors have shown promise in coastal applications a number of issues primarily related to subsurface hydrology need to be addressed to fully assess the limitations of this technique this paper reviews the theory methodology and applications of emi in support of geologic framework studies with particular reference to barrier islands resolution of these issues will allow emi sensors to complement and offer significant advantages over traditional methods in support of an improved understanding of large scale barrier island evolution

mapping the magnetic susceptibility and electric conductivity of marine surficial sediments by benthic em profiling distribution accumulation and diagenesis of surficial sediments in coastal and continental shelf systems follow complex chains of localized processes and form deposits of great spatial variability given the environmental and economic relevance of ocean margins there is growing need for innovative geophysical exploration methods to characterize seafloor sediments by more than acoustic properties a newly conceptualized benthic profiling and data processing approach based on controlled source electromagnetic csem imaging permits to coevally quantify the magnetic susceptibility and the electric conductivity of shallow marine deposits the two physical properties differ fundamentally insofar as magnetic susceptibility mostly assesses solid particle characteristics such as terrigenous or iron mineral content redox state and contamination level while electric conductivity primarily relates to the fluid filled pore space and detects salinity porosity and grain size variations we develop and validate a layered half space inversion algorithm for submarine multifrequency csem with concentric sensor configuration guided by results of modeling we modified a commercial land csem sensor for submarine application which was mounted into a nonconductive and nonmagnetic bottom towed sled this benthic em profiler neridis ii achieves 25 soundings second at 3 4 knots over continuous profiles of up to a hundred kilometers magnetic susceptibility is determined from the 75 hz in phase response 90 signal originates from the top 50 cm while electric conductivity is derived from the 5 khz out of phase quadrature component 90 signal from the top 92 cm exemplary survey data from the north west iberian margin underline the excellent sensitivity functionality and robustness of the system in littoral 0 50 m and neritic 50 300 m environments susceptibility versus porosity crossplots successfully identify known lithofacies units and their transitions all presently available data indicate an eminent potential of csem profiling for assessing the complex distribution of shallow marine surficial sediments and for revealing climatic hydrodynamic diagenetic and anthropogenic factors governing their formation

surface water groundwater exchange in transitional coastal environments by airborne electromagnetics the venice lagoon example 1 a comprehensive investigation of the mixing between salt fresh surficial water and groundwater in transitional environments is an issue of paramount importance considering the ecological cultural and socio economic relevance of coastal zones acquiring information which can improve the process understanding is often logistically challenging and generally expensive and slow in these areas here we investigate the capability of airborne electromagnetics aem at the margin of the venice lagoon italy the quasi 3d interpretation of the aem outcome by the spatially constrained inversion sci methodology allows us to accurately distinguish several hydrogeological features down to a depth of about 200 m for example the extent of the saltwater intrusion in coastal aquifers and the transition between the upper salt saturated and the underlying fresher sediments below the lagoon bottom are detected the research highlights the aem capability to improve the hydrogeological characterization of subsurface processes in worldwide lagoons wetlands deltas

continuous geomagnetic field models for the past 7 millennia 1 a new global data compilation 1 a global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled it consists of 16 085 results of inclination 13 080 of declination and 3188 of intensity for the time span 5000 bc to 1950 ad declination and inclination data come partly from existing databases and partly from original literature a new global compilation of intensity data for the millennial scale is included data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set the global distribution of the data is very inhomogeneous in both time and space all the data are compared to predictions from the previous 3000 year global model cals3k 1 this collection of data will be useful for global secular variation studies and geomagnetic field modeling although southern hemisphere data are still underrepresented in particular we will use it in a further study to update and extend the existing global model cals3k 1 the huge increase in data compared to the previous compilation will result in significant changes from current models as we might have missed some suitable data we encourage the reader to notify us about any data that have not been included yet and might fit in as improving our global millennial scale models remains our aim for the future the data files described in this paper are available from the earthref digital archive erda at http earthref org cgi bin erda cgi n 331

airborne electromagnetic bathymetry and estimation of bedrock topography in broken bay australia abstracta helicopter transient electromagnetic tem survey was flown over shallow coastal waters in broken bay australia overlying several palaeovalleys and exposed reef sections the infilled palaeovalleys contain unconsolidated sediments with variable thicknesses exceeding 100 m previous marine seismic reflection and vibrocore studies provide an estimate of sediment thickness and surficial sediment conductivity respectively which combined with known bathymetry sonar soundings and measured seawater conductivity can be used to generate a crude 1d geoelectric ground truth model consisting of two layers seawater sediment overlying a relatively resistive basement the primary focus was to examine the accuracy of interpreted water depths obtained by 1d inversion of airborne tem data assuming a two layer over resistive basement model by comparing these depths with known water depths the secondary focus was to compare the interpreted sediment thickness i e the thickness of the second geoelectr

lake sediment record of the geomagnetic secular variation in britain during holocene times summary a palaeomagnetic record of the geomagnetic secular variation during the last 10000 years has been obtained from 10 cores of sediment from loch lomond scotland lake windermere north england and llyn geirionydd north wales a time scale is provided by 30 radiocarbon age determinations and pollen analyses on several of the cores the main swings and much fine detail of both declination and inclination records repeat well between cores and between lakes and the overall record is much more detailed than previous european records the new record shows that neither declination nor inclination swings have been periodic over the past 10000 years but that the main swings have become progressively shorter in duration during that time each swing is characterized by fine detail which enables use of the record as a secondary method of dating other european sediments the motion of the geomagnetic vector has been predominantly clockwise throughout the time period spanned but confirms a period of anticlockwise motion from 1100 to 600 bp first discovered by british archaeomagnetic investigations the record agrees with british and czechoslovakian archaeomagnetic records but not with japanese archaeomagnetic or north american lake sediment records this suggests that the secular changes are controlled by local growing and decaying drifting sources rather than by wobbling of the main geomagnetic dipole

geomagnetic field variations in northern sweden during the holocene quantified from varved lake sediments and their implications for cosmogenic nuclide production rates palaeomagnetic analyses were conducted on two varved lake sediment sequences in northern sweden the magnetic properties of the sediment sequences are dominated by stable single domain magnetite with characteristics typical of bacterial magnetosomes alternating field demagnetization measurements indicate that the single domain magnetite is the dominant carrier of a stable natural remanent magnetization temporal variations in inclination and declination were obtained from a total of four cores and the data points were stacked according to their independent calendar year varve ages statistically significant patterns in inclination and declination form a regional palaeomagnetic secular variation psv curve which possesses features that are identical in form to the uk holocene psv master curve however the calibrated radiocarbon ages of uk features identified prior to 1500 bc are approximately 500 years older than their swedish varve dated equivalents which points to dating errors and or drifting of the geomagnetic field the sediments meet the uniformity criteria proposed for palaeointensity reconstruction and estimates of relative geomagnetic field intensity are calibrated against global dipole moment compilations a calculated nuclide production curve is derived from the reconstructed geomagnetic field intensity which empirically demonstrates the dominant modulation of cosmogenic nuclide production by dipole moment between 5000 bc and ad 1500

synchronizing holocene lacustrine and marine sediment records using paleomagnetic secular variation high sediment accumulation rates in lacustrine and shallow marine archives around iceland offer the potential to compare high resolution paleoclimatic reconstructions from terrestrial and marine archives however direct comparisons are hampered by difficulties in stratigraphic correlation and in deriving accurate age models for lacustrine archives icelandic paleomagnetic secular variation psv has the potential to synchronize these records here we compare holocene psv from a well dated marine core on the north iceland shelf with psv from two lacustrine archives with comparable sediment accumulation rates hvt03 1a a glacier dominated lake and hak03 1b in a nonglacial catchment geochemically characterized tephra layers combined with unique high amplitude structures in the psv records provide secure tie points every 200 yr once the records are synchronized the chronology from the marine core can be reliably transferred to the two lacustrine records the resultant lacustrine age models reveal large changes in sediment accumulation rate at submillennial scales that escape detection in conventional age models with independent dates every 1 k y sediment accumulation rate changes occur at similar times in both lakes despite very different catchment properties low and regular accumulation rates during the holocene thermal maximum suggest regionally stable vegetated catchments followed by a stepped landscape destabilization during the transition into neoglaciation culminating with maximum sedimentation rates during the little ice age psv allows synchronization between multiple records from nearby marine and lacustrine archives providing improved age models and a means of assessing leads and lags between marine and terrestrial environments

a time domain electromagnetic survey of the tag hydrothermal mound a conductivity survey of the tag mound was made using innovative transient electric dipole dipole instrumentation the receiver was deployed on the sulfide mound 30 m south of the central black smoker while the submersible alvin carried the transmitter on a survey path around the receiver at a radius of approximately 70 m conductivity measurements of the seafloor between the transmitter and receiver were made at 5 minute intervals the survey lasted 4 hours measurements made at 12 sites are initially interpreted in terms of an apparent conductivity of a uniform seafloor a more complex model in which the seafloor has two layers is then used apparent conductivities range from 1 4 to 15 9 s m generally higher than that of seawater the results suggest possible focusing of hydrothermal convection in the northern quadrant of the mound adjacent zones of anhydrite accumulation and fluid convection in the north eastern quadrant heterogeneity and layering in the kremlin zone and unexpected spatial variability in the western quadrant the data from several sites show evidence of distortion attributed to 3 d anomalies

marine downhole to seafloor dipole dipole electromagnetic methods and the resolution of resistive targets most active marine electromagnetic surveys use an array of transmitting and receiving electric dipoles located on the sea floor there are many reports of successful detections of buried resistors with these arrays nevertheless the concept has a significant drawback large offsets between the transmitter and receiver with respect to the target depth are needed a logical alternative is the use of a vertical electric dipole transmitter in a borehole deeper than the target combined with receivers located at the seafloor because current must pass through the target even data recorded at short offsets with respect to target depth should be sensitive to the target we derived the equations for the electric and magnetic field components produced by the vertical electric dipole in a 1d layered medium the equations are used to model the fields for resistive layers an eigenparameter analysisis applied to the different field components and the resolution is compared with the standard seafloor inline array

quantifying aem system characteristics using a ground loop quantitative interpretation of time domain airborne electromagnetic aem data is hampered by uncertainty in altimetry system geometry transmitter waveform data averaging and timing we present a simple calibration method that serves to define these issues by the use of a closed multiturn loop of known electrical and physical properties that is insulated from the ground beneath it by predicting the secondary response of the aem receiver and comparing it with the measured data we have identified and quantified systematic errors mentioned above in several systems in addition we identify an alternative subprocess that uniquely calculates altimeter and geometry errors by measuring the current induced in a ground loop of known properties and comparing it with predictions the ground loop method is used best over resistive cover to minimize limitations caused by nonuniform conductive ground and is a calibration tool that makes aem data consistent with quantitative models fluctuating geometric errors caused by bird swing limit the accuracy of applying the geometry corrections from one flyover to an entire survey

long term nongeocentric axial dipole directions and a geomagnetic excursion from the middle pleistocene sediments of the humboldt river canyon pershing county nevada a paleomagnetic record has been obtained from the middle pleistocene sediments of the humboldt river canyon pershing county nevada the age of the sediments is constrained by two tephra layers the 610 000 year old lava creek tephra and the approximately 495 000 year old dibekulewe tephra the magnetization of the sediments is strong and stable and the similarity of data from replicate sections strongly suggests that the data set represents an accurate record of the magnetic field except for the presence of postmagnetization rotations of less than 10 the influence of postmagnetization rotations was eliminated in one of the sections hr1a by rotating the paleomagnetic directions in accordance with the direction of the lava creek welded tuff previously determined from five sampling sites near yellowstone national park after correcting the mean directions we observed nongeocentric axial dipole non gad mean directions of moderate amplitude 19 13 in declination and 22 8 in inclination in the record from the hrla section which may represent as much as 115 000 years of magnetic field behavior in the youngest part of the humboldt river canyon sediments a zone of anomalous directions was recorded which we interpret to be a detailed record of an unconfirmed geomagnetic excursion this record exhibits behavior consistent with that of previously reported polarity transitions because of the widespread nature of the tephras especially the lava creek tephra the prospects are very good for improving estimates of the magnitude and duration of the non gad paleomagnetic directions and also for confirming the excursion