Interference effects of aircraft on earth's electromagnetic response at very low frequency and low frequency          下载免费PDF全文

C. Shan  L. B. Pedersen 《Geophysical Prospecting》2015,63(1):211-224

Over the last few decades, very low frequency electromagnetics has been widely and successfully applied in mineral exploration and groundwater exploration. Many radio transmitters with strong signal‐to‐noise ratios are scattered in the very low frequency band and low frequency band. Based on experiences gained from ground measurements with the radio‐magnetotelluric technique operating in the frequency interval 1–250 kHz, broadband magnetometers have been used to cover both very low frequency (3–30 kHz) and low frequency (30–300 kHz) bands to increase the resolution of the near‐surface structure. The metallic aircraft as a conductive body will distort the magnetic signal to some extent, and thus it is important to investigate aircraft interference on the electromagnetic signal. We studied noise caused by rotation of an aircraft and the aircraft itself as a metallic conductive body with three methods: 3D wave polarization, determination of transmitter direction and full tipper estimation. Both very low frequency and low frequency bands were investigated. The results show that the magnetic field is independent of the aircraft at low frequencies in the very low frequency band and part of the low frequency band (below 100 kHz). At high frequencies (above 100 kHz), the signals are more greatly influenced by the aircraft, and the wave polarization directions are more scattered, as observed when the aircraft turned. Some aircraft generated noise mixed with radio transmitter signals, detected as ‘dummy’ signals by the 3D wave polarization method. The estimated scalar magnetic transfer functions are dependent on the aircraft flight directions at high frequencies, because of aircraft interference. The aircraft eigenresponse in the transfer functions (tippers) between vertical and horizontal magnetic field components was compensated for in the real part of the estimated tippers, but some unknown effect was still observed in the imaginary parts.  相似文献   

A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran     

B. Oskooi  I. Mansoori  L. B. Pedersen  H. A. Koyi 《Pure and Applied Geophysics》2015,172(2):491-502

  相似文献   

Depth distribution of chironomids and an evaluation of site-specific and regional lake-depth inference models: a good model gone bad?     

Les C. Cwynar  Andrew B. H. Rees  Cy R. Pedersen  Stefan Engels 《Journal of Paleolimnology》2012,48(3):517-533

We used 39 surface samples from Marcella Lake, Yukon Territory, to examine the distribution of chironomid head capsules in relation to depth and to develop a site-specific (intralake) inference model for reconstructing past lake levels. Ten of the 34 most-frequently occurring taxa encountered in the surface-sediment samples are significantly related to depth. We then applied the site-specific inference model and a previously developed regional model to samples from deep- and shallow-water cores from Marcella Lake. The inferences were compared to an independent Digerfeldt-type reconstruction of lake level history and to moisture inferences drawn from pollen data. Although the site-specific model was good in having better performance statistics than the regional model, it was bad at producing depth reconstructions because most samples from the long cores lacked suitable analogues in the site-specific training set. None of the chironomid-based reconstructions was a good match to the Digerfeldt-type reconstruction. Inconsistencies remain between the paleohydrological inferences derived from the chironomid depth models, the Digerfeldt-type reconstruction and pollen-inferred reconstructions of past moisture regimes.  相似文献   

Heat flow in rift basins above a hot asthenosphere     

Tom Pedersen 《地学学报》1993,5(2):144-149

When continental rifting takes place above a hot asthenosphere, pressure-release melting of adiabatically upwelling mantle may generate large volumes of basaltic melts which subsequently are emplaced at crustal levels and cool. To correctly estimate the heat flow from tectonic subsidence and crustal thinning, it is necessary to account for the melt volumes. A simple physical model of heat flow that incorporates a crustal growth correction on lithospheric extension estimates, as well as the heat in the emplaced magma, has been developed. The principal result is that heat flow may be substantially increased for several million years after rifting, even for a moderately heated asthenosphere.  相似文献   

Non-linear surface wave phase velocity inversion based on ray theory     

Marianne Bruneton  Véronique Farra  Helle Anette Pedersen  the SVEKALAPKO Seismic Tomography Working Group 《Geophysical Journal International》2002,151(2):583-596

  相似文献   

Magma development of the Leka Ophiolite Complex, central Norwegian Caledonides     

H. Furnes  R.B. Pedersen  J. Hertogen  B.A. Albrektsen 《Lithos》1991,27(4):259-277

The magmatic products of the Leka Ophiolite Complex of Lower Ordovician age, indicate formation in different tectonic settings and generation from different mantle sources. Harzburgites of the mantle tectonite, clinopyroxenes from wehrlites of the ultramafic cumulates, the metabasalts of the dyke complex and earliest pillow lavas (IAT/MORB, boninites) all show characteristics compatible with formation above a subduction zone in an intra-oceanic setting. Nd-isotopes indicate that some of the IAT and boninites may have been derived from a source contaminated by continental material, and a CAB source differ significantly from that of the IAT and boninites. The later pillow lavas of MORB composition show only minor influence of subduction-related processes (minor or no negative Ta-anomalies), and the supposed latest volcanic sequence of an alkaline OIB-type, none at all. The MORB- and OIB-type magmas are thought developed by spreading in a back-arc setting, in which the latter magma type developed in a remote position from the subduction zone.  相似文献   

The geochemistry of manganese carbonate in Panama Basin sediments     

T.F. Pedersen  N.B. Price 《Geochimica et cosmochimica acta》1982,46(1):59-68

The Mn distribution in Panama Basin area sediments and interstitial waters is discussed. Striking surficial Mn enrichments produced by a well-known diagenetic recycling process characterize the sediments of the region. Thermodynamic solubility calculations indicate that in at least one core interstitial waters approach saturation with respect to MnCO₃. A mixed carbonate phase of composition (Mn₄₈ Ca₄₇ Mg₅)CO₃ was recovered from an ash band in the same core. The association of this material with the coarse volcaniclastic debris is thought to result from facile manganous carbonate precipitation in sediment horizons of coarser mean grain size. Since sulphate reduction in the upper two metres of Panama Basin sediments is fairly minor, little increase in alkalinity is observed, and it is postulated that significant production of carbonate alkalinity is not a prerequisite for manganous carbonate generation in hemipelagic sediments. A more important factor appears to be the availability of Mn oxides for solution during early diagenesis. Stable C isotopic analyses indicate that little C of organic origin is used in the precipitation reaction in either Panama Basin or Loch Fyne (Scotland) sediments.  相似文献   

A cummingtonite-porphyritic dacite with amphibole-rich xenoliths from the tertiary central volcano at Króksfjördur,NW Iceland     

Asger Ken Pedersen  Niels Hald 《Lithos》1982,15(2):137-159

An instrusive dacite and a salic pumice, emplaced late in the evolution of the Miocene (c. 10 m.y.) Króksfjördur volcano, NW Iceland, contain a varied assemblage of xenolithic metaigneous rocks. Mineral and rock chemistry shows that the dacite is very similar to calc-alkaline salic rocks from the SW Pacific. It contains phenocrystic plagioclase, quartz, pyroxene, cummingtonite, hornblende, biotite, two oxides, apatite and zircon in a rhyolitic glass. The rock equilibrated at 700 to 750°C. P ～ 1.6 Kbar and P_H2O ～ 1 Kbar. The xenoliths are layered gabbros, granophyres and various fine-grained hornfelses and show that the dacite magma was residing within a gabbro intrusion capped by granophyre prior to the eruption. The hornfelses are amphibole-plagioclase, amphibole-pyroxene-plagioclase and pyroxene-plagioclase rocks formed during high-temperature metamorphism of basic dykes cutting the gabbro intrusion. The gabbros and hornfelses mostly record higher metamorphic temperatures (850–940°C) than the dacite, and indicate that they were equilibrated during the ascent of a magma body into a hydrous high-level region within the volcano. During a following thermal decline, the hydrated magma cooled to form the first cummingtonite-bearing low-T magma to be recorded from the ocean ridge systems.  相似文献   

Polyphase deformation of the massive sulphide ore of the Black Angel Mine,central West Greenland     

F. D. Pedersen 《Mineralium Deposita》1981,16(1):157-176

The massive Fe-Zn-Pb sulphide sheets constituting the Angel Zone ore body of the Black Angel Mine, show evidence of three phases of deformation at greenschist facies metamorphic grade. During an early phase an originally layered sulphide ore type was isoclinally folded. Subsequent thrusting parallel to the ore body transformed the layered ore into massive and porphyroclastic ore tectonites. Late, open folds refolded the earlier structures and caused localized differential mobilization of the sulphides. The microstructures of the layered ore tectonite indicate a period with static grain growth, interpreted as the result of prograde metamorphism, followed by a dynamic recrystallization under low stress and at low strain rates, which is correlated with the early isoclinal folding. The microstructures of the massive and the porphyroclastic ore tectonites indicate syntectonic recrystallization under high stress and at high strain rates, corresponding to the thrusting of the ore bodies. The microstructures of the mobilized sulphides show evidence of repeated plastic/cataclastic deformation and recrystallization, corresponding to highly variable strain and strain rate conditions during the mobilization. Post-deformational annealing took place at elevated temperature and was largely controlled by inhibition-dependent grain growth and to a minor extent by orientation-dependent grain growth.  相似文献   

Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)     

Hanne D. Pedersen  Dieke Postma  Ole Larsen 《Geochimica et cosmochimica acta》2005,69(16):3967-3977

Iron oxides may undergo structural transformations when entering an anoxic environment. These transformations were investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with ⁵⁵Fe-labelled iron oxides. ⁵⁵Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesised at 10°C and 25°C, respectively, a goethite and a hematite. The iron oxides were then submerged in Fe²⁺ solutions (0-1.0 mM) with a pH of 6.5. In the presence of aqueous Fe²⁺, an immediate and very rapid release of ⁵⁵Fe was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe²⁺ no release was observed. ⁵⁵Fe was not released from hematite, even at the higher Fe²⁺ concentration. The release rate is mainly controlled by characteristics of the iron oxides, whereas the concentration of Fe²⁺ only has minor influence. Ferrihydrite and 5-nm-sized lepidocrocite crystals attained complete isotopic equilibration with aqueous Fe(II) within days. Within this timeframe ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the timeframe of the experiment; however, the continuous slow release of ⁵⁵Fe suggests that isotopic equilibrium will ultimately be attained.Our results imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly, iron oxides should properly be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a reevaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides, and the significance of stable iron isotope signatures.  相似文献