Magnetotelluric interpretation of crustal and mantle structure in the Grenville Province

Summary Accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust. A magnetotelluric (MT) station (BAT) in the Grenville Province of the Precambrian Shield in eastern Canada has been in operation since 1975 for time-dependency studies of electrical resistivity changes related to earthquakes. The MT response of the station displays low skew with small to moderate anisotropy. One-dimensional inversion of the apparent resistivity and phase reveals two well-defined conductors in the crust, one at 10 km and the second at the base of the crust. The latter has a resistivity less than 50 Ω m. These results are substantiated by three additional MT stations located up to 40 km distant.
Data from other new MT stations and from stations previously published in the literature are compared with two-dimensional computer model results and with the three-dimensional analogue scale model results of Dosso et al. While additional data for periods less than 100 s would be desirable the results from a number of the MT stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the Grenville. The inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 Ω m. This may coincide with a seismic low-velocity zone observed in the mantle under the Canadian Shield.     

Results of magnetotelluric and gravimetric measurements in western Nicaragua, Central America

Magnetotelluric and gravity data have been collected within a ca. 170 km long traverse running from the Pacific coast of Nicaragua in the west to the Nicaraguan Highland in the east. This part of Nicaragua is characterized by sedimentary rocks of the Pacific Coastal Plain, separated from the Tertiary volcanic rocks of the Highland by the NW-SE-trending Nicaraguan Depression. 2-D interpretation of the magnetotelluric (MT) data, collected at 13 stations, indicates four regions of high electrical conductivity in addition to the conductive coastal region. Two of these are associated with conducting sediments and pyroclastics in the upper part of the crust. Two other conductive structures have been defined at depth around 20 km and the one best defined is located below the depression. From the distribution of seismic events, volcanic activity in the depression and the similarity in geophysical characteristics with areas such as the Rio Grande Rift, this conductor is interpreted as a melt layer or a complex of magma chambers. Models of the upper lithosphere, constrained by the MT model, vertical electrical sounding (VES) data, seismic data and densities, have been tested using gravity data. A model that passes this test shows a gradual thickening of the crust eastwards from the Pacific coast. An anomaly centred over the depression is interpreted to have its origin in a thinning of the crust. In this model the melt layer is situated on top of the bulge of the lower lithosphere. A change in the composition of the crust, from the Pacific Coastal Plain to the Highland, is indicated from the change in character of the MT response and from the density distribution in the gravity model. This may support the hypothesis that the Pacific region is an accreted terrane. MT and gravity data indicate a depth to a resistive and high-density basement in the depression of ca. 2 km. On the basis of this, the vertical setting in the depression is estimated to be of the order of 2.5 km.     

Interpretation of transient electromagnetic soundings over three-dimensional structures for the central-loop configuration

Summary. An assessment is made of the bias of fitting constrained layered-earth models to transient electromagnetic data obtained over 3-D structures. In this assessment we use the central-loop configuration and show that accurate estimates of the depth of burial of 3-D structures can be obtained with layered-earth model fitting. However, layered-earth interpretations are not reliable for estimating depth extents and resistivities of 3-D structures. When layered earths are used for interpretation, it is advantageous in some cases to use data based on the magnetic field instead of the voltage. A magnetic-field definition of apparent resistivity, in contrast to a definition based on the voltage, eliminates apparent-resistivity overshoots and undershoots in the data. A resistivity undershoot in the data can produce an extraneous and misleading layer in an interpretation of a 3-D resistive structure. Due to 3-D effects, apparent-resistivity soundings (magnetic field and voltage) may rise so steeply at late times that it may not be possible to fit a sounding to a reasonable layered-earth model. Truncating such a sounding, over a buried conductor, allows for a reasonable layered-earth fit and an accurate estimate of the depth to the conductor. However, the resistivity of the conductor is overestimated.
Measurements of the horizontal field in the central-loop configuration can map 3-D structures, provided the sensor is located accurately at the centre of the transmitting loop. Horizontal-field calculations show that the transients peak on the flanks of a 3-D structure, but are depressed over the structure's centre. Weak transient responses flanked by two large transient responses, which are opposite in sign, locate the structure. The sign reversal is caused by a corresponding reversal in the currents that are channelled through or deflected away from conductive or resistive structures, respectively.     

Electrical conductivity anomalies and their relationship with the tectonics of South Australia

Summary. Geomagnetic variation studies have been conducted in the Gawler Craton and Adelaide Geosyncline of South Australia. The magnetometer stations extend from the coast up to the southern edge of the 1970 array of Gough, McElhinny & Lilley. The coast effect is the dominant feature of the data but use is made of the hypothetical event technique to identify two zones of telluric current concentration. Both of them appear to be associated with linear zones of enhanced electrical conductivity within the crust. The Southern Eyre Peninsula anomaly lies within the Gawler Craton and may identify a major fracture or shear in the upper crust. The conductivity anomaly within the Adelaide Geosyncline appears to be the continuation of the Flinders anomaly discovered by the 1970 array study. It correlates well with the arcuate fold pattern of the Southern Flinders Zone of the Geosyncline and with the local pattern of seismicity. In both anomalies the enhanced conductivity is probably caused by saline waters within fractured crustal rocks.     

Modelling of solar quiet magnetic field variations near a conductivity anomaly

Summary. A simplified model of the solar quiet-time ionospheric current system is used to calculate the induced currents in a model earth. The conductivity is assumed to be constant below a depth of about 400 km and zero above that depth. The current induced in the north—south conductivity anomaly under the Rocky Mountains is then estimated from the time-varying potential difference between points at 30 and 45° latitude at the surface of the conducting sphere. The purpose of these calculations is to investigate whether variations in the latitude of the northern hemisphere current system vortex will substantially alter the relationship between the observed magnetic field components at the Earth's surface and the local magnetic field gradient caused by the conductivity anomaly. We find that a 10° shift in the latitude of the ionospheric current focus causes a change of 6 per cent or less in the transfer function from the field components to the gradient in the total field. Thus such latitude shifts cannot explain much of the magnetic field gradient variation at periods near 24 hr that has been observed near Boulder, Colorado.     

The Rocky Mountains And The Southwest: Using Feature Names To Study Two Iconic Subregions In The American West

Geographic regions can be defined in many ways, including via physiography, historical development patterns, language, and culture. After broadly surveying different methods of regionalization and their influences on studies of the American West, this article uses a vernacular‐mapping approach to: first, propose distinctive toponyms that are relatively unique to cultures situated in the American Rocky Mountains and Southwest areas; second, map the spatial distributions of these toponyms across the western American landscape; and, third, compare the resulting distributions to the geographies of western businesses that incorporate regional terms into their corporate names. Notably, while the Rocky Mountains and the Southwest are iconic American regions that have captured the imagination for centuries, their cultural geographies are relatively underexplored in the literature. This article makes a modest contribution to this research gap by using geographic information systems (GIS) to map high concentrations of culturally distinctive feature names. The results reveal that the boundaries of vernacular Southwest and Rocky Mountain regions correspond relatively well with boundaries delineated with physiographic characteristics     

An analysis of intra-event precipitation variability for the United States (1980–2010)

Hourly precipitation data were collected from 143 first-order US weather stations during the period from 1980 to 2009 to assess the internal distribution of precipitation events lasting at least three hours. A total of 46,595 individual precipitation events were identified and evaluated using the mean, standard deviation, skewness, kurtosis, and the number of peaks occurring within an event. Mean event duration is longest along the West and Northwest coasts, the Mid-South, the Mid-Atlantic, and the Northeast; while shorter-duration events are more frequent in the Rocky Mountains, the Southwest, and the Great Plains. Mean event precipitation and standard deviation are greatest along the Gulf Coast and decrease inland. Precipitation events are positively skewed, indicating that more precipitation tends to occur earlier in the event. The most positively-skewed events are also located in regions flanking the Gulf of Mexico, while less-skewed events are common in the Northwest and Rocky Mountain regions. Event kurtosis is negative throughout the entire USA, with the highest negative values generally west of the Front Range, where cyclonic development and transition produce more evenly distributed precipitation within storms. Intra-event precipitation maxima were also evaluated, with western Florida and the desert Southwest having the greatest number per event.     

FLORISTIC PATTERNS ON AVALANCHE PATHS IN THE NORTHERN ROCKY MOUNTAINS,USA

Avalanche paths in the northern Rocky Mountains have transverse zones of plant physiognomic types representing major exogenous environmental gradients related to topography. Species composition was analyzed among these transverse zones. Forty plots on two paths in Montana, USA, were ordinated by detrended correspondence analysis, giving ordination axes for both plots and species. The results of the ordination of the plots did not correspond to their differentation in physiognomic zones. Dominant species of similar physiognomy were widely separated in the ordination of species. Endogenous spatial processes are suggested to explain the more complex floristic pattern. (Keywords: avalanche, floristic patterns, Rocky Mountains, vegetation)     

Evidence for 18.6-yr MN term in Japanese air pressure and geophysical implications

Summary Evidence is found in Japanese station pressure records for the 11-yr solar cycle term and a long-period signal near 20 yr which we identify as the 18.6-yr M _N lunar nodal term in atmospheric tides. Amplitudes of the identified M _N term are highly variable with a nominal amplitude of 0.1 mb and, for the seven available stations, the term is in phase (0.0±2.3yr) with the M _N term in 30 temperature records in north-eastern North America. These results are important because of my proposal that enhanced drought conditions in the western United States on a time-scale of ˜ 20 yr are neither 'recurrent' nor 'rhythmic', but rather periodic with a period 18.6yr. And the influence of the Rocky Mountains (as well as the Tibetan Plateau) on standing atmospheric waves vis-à-vis precipitation is the common forcing function for the signal in temperature and drought conditions.     

A comparison of the stability of stationary and non-stationary magnetotelluric analysis methods

A Magnetotelluric (MT) sounding was carried out at a site in south-east Queensland, in the Clarence-Moreton Basin. The synoptic recordings were taken over a period of four months at sampling frequencies from 500 Hz to 5 × 10-5 Hz. The resulting data was analysed by the stationary cross-frequency and the Cone kernel time-frequency distribution (TFD) methods of MT analysis. The results were compared as apparent resistivities on a daily basis for frequencies above 1 Hz, as well as over all the available data. The TFD MT apparent-resistivity results were more stable and less noisy on an daily basis than the cross-frequency results. Similarly the TFD analysis gave less noisy results than the cross-frequency analysis when all available data was processed. Application of these new non-stationary analysis techniques to MT processing should decrease the bias error problem of the MT methods and so increase reliability and repeatability of MT soundings.     

Regional S-wave structure for southern California from the analysis of teleseismic Rayleigh waves

Summary. Teleseismic Rayleigh waves, M_S > 7.0, in the period range 14 to 28 s, are well recorded by the short-period Benioff array within southern California. Multiple arrivals that hinder the determination of local phase velocity curves are detected by narrow band-pass filtering. The records are then windowed on distinct, coherent peaks that move uniformly across the array. Four to seven stations are included in the determination of both the phase velocity across the array and the incidence azimuth. For earthquakes in the western Pacific, the derived incidence azimuths are systematically rotated counterclockwise by 2–16°. Most of this rotation results from refraction at the continental shelf. Phase velocity data for both the southern Mojave—central Transverse Ranges and the Peninsular Ranges are inverted to obtain regional 5-wave velocity models. The starting models are constructed from travel-time studies of local sources, both natural and artificial. Poisson's ratio as a function of depth is calculated for these two regions. The comparison of Poisson's ratio with laboratory ultrasonic studies requires a quartz-rich crust within the southern Mojave—central Transverse Ranges and a mafic crust within the Peninsular Ranges.     

A lake sediment-based proxy of floods in the Rocky Mountain Front Ranges,Canada

We analyzed lake sediment deposits and local hydrometric records to assess the potential for developing a high-resolution record of sediment delivery from the Rock Lake catchment, situated in the non-glacierized Front Ranges of the Rocky Mountains, Canada. Rhythmic couplets of silt–clay characterized the clastic sediments recovered from the deep central basin of the lake. Contemporary sediment yield to Rock Lake (10.7 ± 1.8 Mg km⁻² year⁻¹) is comparable to other studied Canadian Cordillera lakes that have sedimentary lithologies and absence of glacier cover, but distinct rhythmic deposition is relatively unique to this basin. Spatial patterns of deposition within the lake were assessed by correlating rhythmites between multiple sediment cores and by sub-bottom, acoustic profiling. Bracketed dates for a spatially continuous sequence of eight thick rhythmites were established by correlating laminations between core samples collected more than 30 years apart. We identified a consistent pattern between the rhythmite and hydrometric data series between 1975 and 2006 and determined that specific flooding events caused by summer rainstorms are associated with each of the eight thick rhythmites. We observed a good relationship between rhythmite thickness and total flood volume that exceeded a threshold discharge. Acoustic profiling showed that the lake could be a good candidate for longer-term proxy development. We discuss how some of the methods used in this study could benefit ongoing paleoenvironmental assessments based on lacustrine rhythmite series.     

Separation of local and regional information in distorted GDS response functions by hypothetical event analysis

Electromagnetic investigations are usually intended to examine regional structures where induction takes place at a given period range. However, the regional information is often distorted by galvanic effects at local conductivity boundaries. Bahr (1985) and Groom & Bailey (1989) developed a physical distortion model for decomposing the MT impedance tensor, based upon local galvanic distortion of a regional 2-D electromagnetic field. We have extended their method to predict the magnetic variation fields created at an array of sites. The magnetic response functions at periods around 1000 s may be distorted by large-scale inhomogeneities in the upper or middle crust. In this period range, the data measured by a magnetometer array contain common information that can be extracted if the data set is treated as a unit, for example by using hypothetical event analysis. With this technique it is always possible to recover the regional strike direction from distorted data, even if a strong, spatially varying regional vertical field component is present in the data set. The determination of the regional impedance phases, on the other hand, is far more sensitive to deviations from the physical distortion model.
The approach has been used to investigate the Iapetus data set. For the array, which covers an area of 200  km × 300  km in northern England/southern Scotland, the technique revealed a common regional strike azimuth of ca . N125° E in the period range 500–2000  s. This direction differs from the strike indicated by the induction arrows, which seem influenced mainly by local current concentrations along the east–west-striking Northumberland Trough and a NE–SW-striking mid-crustal conductor. Both impedance phases are positive and differ by ca . 10°, which supports the assumptions of distortion fields in the data set and that the regional structure is 2-D.     

Conductivity anomalies in the Baltic Shield in Finland

In 1981–84 six arrays of 30 or 31 magnetometers were operated on the Baitic Shield in central and southern Finland to deduce information about the electrical conductivity within the Earth's crust. The magnetometer sites cover the area with 20–60 km spacing allowing the identification of large-scale electrical structures within the crust. Each array recorded the three orthogonal magnetic field components on analogue film for about two months simultaneously at all the stations. Altogether 17 magnetic variation events of 2–6hr length have been digitized and analysed.
This paper includes results of the last array in south-western Finland, a comparison of two methods of induction vector estimation and combination of results from all the arrays. Four zones of anomalously high electrical conductivity have been observed. They are all interpreted as lying at depths greater than 5 km. The 'Oulu Anomaly' is wider than the others and rather short, about 100 km. The 'Southern Finland Anomaly' is probably a 500 km-long narrow zone. This and the 'Ostrobothnian Anomaly' surround three sides of the large Central Finland Granite area. The 'Outokumpu Anomaly' is probably deeper lying than the others. Blocks separated by these anomalies are presented. The blocks have different average resistivities.     

The mapping of induced currents around the Kenya Rift: a comparison of techniques

Summary. Single-station and inter-station transfer functions are derived from data recorded by a magnetometer array in and around the Kenya Rift Valley. Different methods of presentation of the transfer function estimates are compared in terms of their ability to define lateral variations in electrical conductivity. When, as in East Africa, the conductivity structure is complex and three-dimensional, by far the most useful method of presentation is to use the transfer functions to simulate the anomalous internal fields associated with regional current flow at a particular azimuth. The use of inter-station rather than single-station transfer functions is almost essential where the amplitudes of anomalous horizontal fields are of the same size or greater than the normal fields. Horizontal field transfer functions prove to be just as useful as those usually calculated for the vertical component, and provide strong additional constraints on the internal current system.
Maps of simulated vertical and horizontal fields of internal origin indicate the importance of current channelling around the Rift Valley. A conductor just to the east of Nairobi apparently funnels regionally induced currents into two conductors associated with the Rift and dome; one at shallow depths beneath the Rift floor, and a deeper body to the east.     

大尺度山地上空臭氧亏损及其变化趋势

通过对卫星资料的分析,揭示了全球大尺度山地上空有大气臭氧亏损,其极大值出现在春季或初夏,线性回归分析表明:在1979～1991年中,全球大尺度山地上空的臭氧总量呈下降趋势。     

Interpretation of long-offset transient electromagnetic data from the Odenwald area, Germany, using two-dimensional modelling

The standard 1-D inversion approach for the interpretation of transient electromagnetic (TEM) data usually fails in the presence of near-surface conductivity anomalies. Since multidimensional inversion codes are not routinely available, the only alternative to discarding the data may be trial-and-error forward modelling. We interpret data from a long-offset transient electromagnetic (LOTEM) survey which was carried out in 1995 in the Odenwald area, using 2-D finite-difference modelling. We focus on a subsegment of the LOTEM profile, which was shot with two different electric dipole transmitters. A model is found which consistently explains the electric and magnetic field data at eight locations for both transmitters. First, we introduce a conductive dyke under the receiver spread to explain sign reversals in the magnetic field transients. A conductive slab under one of the transmitters is required to obtain a reasonable quantitative fit for that transmitter. Consideration of the electric field data then requires a modification of the layered earth background. Finally, we study the response of a crustal conductor, which was the original target of the survey. The data are sensitive to the conductor, and for the investigated subset of the data the fits are slightly better without the conductive layer.     

Localized velocity anomalies in the lower mantle

Summary. Two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000–2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of S, ScS, P and PcP travel times and amplitudes from deep earthquakes in Peru, Bolivia, Argentina and the Sea of Okhotsk. Comparison of the relative patterns of ScS-S differential travel times and S travel-time residuals across North American WWSSN and CSN stations for the different source regions provides baselines for interpreting which phases have anomalous times. A region of low S and P velocities is located beneath Northern Brazil and Venezuela at depths of 1700–2700 km. This region produces S -wave delays of up to 4 s for signals from deep Argentine events recorded at eastern North American stations. The localized nature of the anomaly is indicated by the narrow bounds in azimuth (15°) and take-off angle (13°) of the arrivals affected by it. The long period S -waves encountering this anomaly generally show 30–100 per cent amplitude enhancement, while the short-period amplitudes show no obvious effect. The second anomaly is a high-velocity region beneath the Caribbean originally detected by Jordan and Lynn, who used travel times from deep Peruvian events. The data from Argentine and Bolivian events presented here constrain the location of the anomaly quite well, and indicate a possible short- and long-period S -wave amplitude diminution associated with it. When the travel-time data are corrected for the estimated effects of these two anomalies, a systematic regional variation in ScS-S station residuals is apparent between stations east of and west of the Rocky Mountains. One possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath North America.     

3-D magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins

20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.