Airborne Gradiometry Error Analysis   总被引：3，自引：0，他引：3  

Christopher Jekeli 《Surveys in Geophysics》2006,27(2):257-275

Gravity gradiometry is one of the older methods of determining the Earth’s local gravitational field, but lies in the shadow of more conventional static and moving-base gravimeter-based systems. While the static torsion balance appears to have been relegated to the museum, support for the airborne and space-borne differential accelerometer (gradiometer) continues so as to overcome limitations in spatial resolution and accuracy inherent in ordinary moving-base gravimetry. One airborne system exists, building on 30 year old technology concepts, and new technologies (e.g., cold-atom interferometry) promise significant improvements. Concomitant advances are required to measure accurately the angular velocity and angular acceleration of the platform, which inseparably combine (in an absolute sense) with the Earth’s gravitational gradients. A numerical analysis of instrument errors, with simulated aircraft dynamics, shows that navigation-grade gyros are just sufficient to account for these effects in gradiometers with 1E/ sensitivity. More accurate instruments, with 0.1 E/ sensitivity, require commensurate sensitivity in the gyros, of the order of 0.01°/h/ = 1.5\times10^{−4 °} \ for typical survey aircraft dynamics. On the other hand, typical orientation errors in the platform, which are problematic for vector gravimetry, are much less of a concern in gradiometry. They couple to the gradient signals and affect only the very low frequencies of the total gradient error.  相似文献   

Characteristics of rain electricity in Nigeria II — Experimental and theoretical relations     

A. I. I. Ette  E. O. Oladiran 《Pure and Applied Geophysics》1980,118(2):765-774

Analysis of data, covering four rainy seasons, of rain current, point-discharge current and potential gradient reveal novel relations in the form (i) $$Q_{r + } /Q_{r - } = k_1 (T_{r + } /T_{r - } )^{1.1} $$ for rain charge and duration ratios; and (ii) $$Q_{p - } /Q_{p + } = k_2 (T_{p - } /T_{p + } )^{1.1} $$ for point charge and duration ratios, where thek's are constants; and (iii) $$i_r = - \alpha (i_p - c)$$ for rain and point-discharge current densities, where α has the same value for all types of rain andc is a constant controlled by the rainfall intensityR. For rain not associated with point discharge the relation takes the familiar form $$i_r = - AR(E - \bar E)$$ Theoretical values are obtained for \ga andA on the basis of the Wilson ion-capture theory as worked out in detail by Whipple and Chalmers.  相似文献   

Variations with time in the detection thresholds of P for seismological stations reporting to the ISC     

J.?B.?YoungEmail author  A.?Douglas  P.?D.?Marshall 《Journal of Seismology》2005,9(4):405-412

To determine maximum-likelihood magnitudes for a given seismological network requires estimates of the detection thresholds of short-period P for the individual stations. Ideally the detection threshold of a station with fixed instrumentation and operating procedures would be constant. However, some stations reporting to the International Seismological Centre (ISC) show systematic variations with time. Other stations are unreliable or report amplitudes (A) and periods (T) only intermittently. Ways are therefore needed for choosing stations that have a consistent performance over many years. Two rough measures are described here that can be used to look at variation in the apparent detection thresholds with time: one is the number of A/T observations (at distances greater than 20^∘) and the other , for the P detections associated each month by the ISC; being the average . Some of the factors that control detection thresholds and clearly revealed by these rough measures are: array stations have lower thresholds than conventional observatories; installation of automatic detectors can lower the threshold; and annual variations in noise level are reflected in annual variations in detection thresholds. For some stations an apparent lowering of the threshold appears not to be due to changes at the station, but is a consequence of the lowering of the ISC reporting thresholds in the region around the station. For stations of the Swiss seismological network there is a slow rise in the threshold from the mid-1970s to 1992 which is difficult to explain but seems to be due to a gradual changes in the detail with which the seismograms are analysed.  相似文献   

On some problems of seismic crustal phase     

葛焕称 《地震学报(英文版)》1995,8(1):57-63

ＯｎｓｏｍｅｐｒｏｂｌｅｍｓｏｆｓｅｉｓｍｉｃｃｒｕｓｔａｌｐｈａｓｅＨｕａｎ－ＣｈｅｎｇＧＥ（葛焕称）（ＳｅｉｓｍｏｌｏｇｉｃａｌＢｕｒｅａｕｏｆＪｉａｎｇｓｕＰｒｏｖｉｎｃｅ,Ｎａｎｊｉｎｇ２１００１４,Ｃｈｉｎａ）Ａｂｓｔｒａｃｔ：Ｉｎｔｈｉｓ...  相似文献   

Quantitative microbial risk assessment: uncertainty and measures of central tendency for skewed distributions   总被引：1，自引：1，他引：0  

K. K. Benke  A. J. Hamilton 《Stochastic Environmental Research and Risk Assessment (SERRA)》2008,22(4):533-539

In the past, arithmetic and geometric means have both been used to characterise pathogen densities in samples used for microbial risk assessment models. The calculation of total (annual) risk is based on cumulative independent (daily) exposures and the use of an exponential dose–response model, such as that used for exposure to Giardia or Cryptosporidium. Mathematical analysis suggests that the arithmetic mean is the appropriate measure of central tendency for microbial concentration with respect to repeated samples of daily exposure in risk assessment. This is despite frequent characterisation of microbial density by the geometric mean, since the microbial distributions may be Log normal or skewed in nature. Mathematical derivation supporting the use of the arithmetic mean has been based on deterministic analysis, prior assumptions and definitions, the use of point-estimates of probability, and has not included from the outset the influence of an actual distribution for microbial densities. We address these issues by experiments using two real-world pathogen datasets, together with Monte Carlo simulation, and it is revealed that the arithmetic mean also holds in the case of a daily dose with a finite distribution in microbial density, even when the distribution is very highly-skewed, as often occurs in environmental samples. Further, for simplicity, in many risk assessment models, the daily infection risk is assumed to be the same for each day of the year and is represented by a single value, which is then used in the calculation of p _Σ, which is a numerical estimate of annual risk, P _Σ, and we highlight the fact that is simply a function of the geometric mean of the daily complementary risk probabilities (although it is sometimes approximated by the arithmetic mean of daily risk in the low dose case). Finally, the risk estimate is an imprecise probability with no indication of error and we investigate and clarify the distinction between risk and uncertainty assessment with respect to the predictive model used for total risk assessment.  相似文献   

Dynamics of magma in a volcanic conduit — Special emphasis on viscosity of magma with bubbles     

D. Shimozuru 《Bulletin of Volcanology》1978,41(4):333-340

A possible dynamic process for magma flow in a volcanic conduit is briefly described. In many of the governing equations, viscosity of magma is involved, and hence, the effective viscosity of magma with small concentration of bubbles was calculated under the assumption of small Reynolds number. The result is $$\eta _\ell = \eta _u (1 + \Phi ),$$ where η_o is the viscosity of a liquid and ? is the volume concentration of bubbles. Thus, the effective viscosity increases with nucleation of gas bubbles in magma. This result reduces the effect of a thermal feedback evele which is postulated as a possible thermodynamical process in viscous magma in a volcanic conduit.  相似文献   

Sull'approssimazione numerica di una variabile casuale normale,con applicaztone agli errori d'osservazione     

Prof. Dr. Beniamino Gulotta 《Pure and Applied Geophysics》1950,18(1):64-77

Riassunto Data una variabile casuale X che segue la legge normale di probabilitl con valor medio a ed error medio y 1'A. considera un'altra variabile casuale che prende il valore intero r quando r–1/2

Summary Given a random variable X following the normal probability law, with expectation a and standard error p, the author considers another random variable , that takes the entire value r when r–1/2  相似文献   

Continuous evaluation of seismic hazard induced by the deposit extraction in selected coal mines in Poland     

E. Glowacka  A. Kijko 《Pure and Applied Geophysics》1989,129(3-4):523-533

A probabilistic relation between seismic activity and the volumeV of extracted deposits in mines is derived $$\Sigma E = C \cdot V^B ,$$ whereC andB are parameters characterizing mining works and the state of rock mass. Assuming that the measure of seismic hazard is the amount of seismic energy released in a given time interval, it is shown how the hazard can be evaluated continuously. The derived relations were tested in selected coal mines in Upper Silesia.  相似文献   

Travel times and body wave magnitude     

Seweryn J. Duda 《Pure and Applied Geophysics》1971,87(1):13-37

Summary Presently used -charts for magnitude determinations have been obtained principally on the base of direct observations of ground motion amplitudes, and periods, of several components of seismic waves (such as PZ, PH, SH), as function of epicentral distance. However, observations alone can serve for the definition of a magnitude scale for events with only one particular focal depth. In order to make the magnitude concept applicable to all focal depths, it must be decided when two events with different focal depths should be assigned the same magnitude. Any respective decision, which will assure consistent magnitudes, must be based on the velocity vs. depth, and eventually the anelasticity vs. depth profiles of the Earth.It is concluded that in magnitude studies the anelasticity is of minor importance. Thus, after the amplitudes are compensated for the radiation pattern at the focus, the observed variation of amplitudes along the surface of the Earth, as function of epicentral distance, is practically due only to the velocity heterogeneity inside the Earth. Assuming a dependence of the velocity on the distance from the center of the Earth (no lateral velocity heterogeneities are permitted), a set of new -charts is obtained, independent of direct amplitude observations, for PZ-, PH-, and SH-waves. A refinement in the magnitude definition warrants the magnitude figures obtained with the new -charts to be uniform with regard to focal depths. Examples show the new -charts to decrease the scatter of magnitude determinations between stations.Since the efficiency in generating longitudinal and transverse waves is most probably not the same for all events, separate P-wave and S-wave magnitudes are advocated.  相似文献   

Comparative soil CO2 flux measurements and geostatistical estimation methods on Masaya volcano, Nicaragua     

Jennifer L. Lewicki  Deborah Bergfeld  Carlo Cardellini  Giovanni Chiodini  Domenico Granieri  Nick Varley  Cynthia Werner 《Bulletin of Volcanology》2005,68(1):76-90

We present a comparative study of soil CO₂ flux () measured by five groups (Groups 1–5) at the IAVCEI-CCVG Eighth Workshop on Volcanic Gases on Masaya volcano, Nicaragua. Groups 1–5 measured using the accumulation chamber method at 5-m spacing within a 900 m² grid during a morning (AM) period. These measurements were repeated by Groups 1–3 during an afternoon (PM) period. Measured ranged from 218 to 14,719 g m⁻² day⁻¹. The variability of the five measurements made at each grid point ranged from ±5 to 167%. However, the arithmetic means of fluxes measured over the entire grid and associated total CO₂ emission rate estimates varied between groups by only ±22%. All three groups that made PM measurements reported an 8–19% increase in total emissions over the AM results. Based on a comparison of measurements made during AM and PM times, we argue that this change is due in large part to natural temporal variability of gas flow, rather than to measurement error. In order to estimate the mean and associated CO₂ emission rate of one data set and to map the spatial distribution, we compared six geostatistical methods: arithmetic and minimum variance unbiased estimator means of uninterpolated data, and arithmetic means of data interpolated by the multiquadric radial basis function, ordinary kriging, multi-Gaussian kriging, and sequential Gaussian simulation methods. While the total CO₂ emission rates estimated using the different techniques only varied by ±4.4%, the maps showed important differences. We suggest that the sequential Gaussian simulation method yields the most realistic representation of the spatial distribution of , but a variety of geostatistical methods are appropriate to estimate the total CO₂ emission rate from a study area, which is a primary goal in volcano monitoring research.Editorial responsibility: H Shinohara  相似文献   

Electric conductivities of a cracked solid     

Alan Hoenig 《Pure and Applied Geophysics》1978,117(4):690-710

Calculations on the basis of the self-consistent approximation are used to study the effects of randomly distributed elliptical cracks and of non-randomly distributed circular cracks, either dry or saturated by a highly conductive material phase, on the electric conductivities of a cracked body. Analytic and numeric results are given for two special non-random distributions. In the first, the cracks are assumed randomly distributed in planes parallel to a given plane. In the second, the crack normals are randomly distributed in parallel planes. The results of the theoretical calculations indicate that the magnitudes of the crack induced variations of the dry cracked rock depend upon a crack density parameter ? rather than upon the crack porosity. Here, ? is defined as $$\varepsilon = \frac{{2N}}{\pi }< \frac{{A^2 }}{P} > $$ whereN is the average number of cracks per unit volume, andA andP are the crack area and perimeter respectively. (For circular cracks of radiusa, ?=N〈a³〉.) Although a straightforward relationship does connect ? with the porosity, it may be more meaningful for laboratory experiments to concentrate upon measuring crack-induced variations as functions of crack density rather than of porosity. For saturated cracked rocks, the results of the calculations indicate that, in addition to ?, variations in conductivity depend also upon a saturation parameter Ω, which relates crack aspect ratio α to matrix and fluid conductivities σ and σ_F $$\Omega = \frac{{{\sigma \mathord{\left/ {\vphantom {\sigma {\sigma _F }}} \right. \kern-\nulldelimiterspace} {\sigma _F }}}}{\alpha }.$$   相似文献   

An algorithm for the optimum distribution of a regional seismic network — II. An analysis of the accuracy of location of local earthquakes depending on the number of seismic stations     

Andrzej Kijko 《Pure and Applied Geophysics》1977,115(4):1011-1021

Summary Seven optimal networks consisting of 4 to 10 stations are compared for a given region, where velocity-depth profiles and the distribution of seismic intensity are known. Assuming that the standard error of arrival time is _t =0.05 s and the standard errors of the parameters of velocity-depth profiles are equal to 5% of their values, the average standard errors of the origin time and focus coordinates are estimated. The application of optimum methods to the planning of seismic networks in the Lublin Coal Basin is presented, and maps of standard errors of origin time , depth and epicenter ( _xy ) for the case of an optimum network of 6 seismic stations are given.  相似文献