Effect of the flow conditions on the metrological characteristics of a contact acoustic current meter

We discuss the results of metrological testing of a four-base acoustic current meter performed in a hydrodynamic basin, consider methodological problems encountered in the process of measurements and data processing, and present the experimental dependences of the systematic and random components of the measurement errors on the velocity and direction of the flow. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Robustness of statistical algorithms for location of microseismic sources based on surface array data

Here, we present a statistical simulation study of several array data processing techniques used for the location of sources of elastic waves. The source location problem arises in such engineering applications as radio-communications, acoustics, sonar, meteorology, astrophysics, seismology, etcetera. Recently, this problem has become important for applied geophysics in connection with the modern technology of hydrocarbon production based on the hydraulic fracturing of the surface layers of the Earth’s environment. These techniques are required for the monitoring of hydraulic fracturing through the location of sources of numerous microearthquakes that occur during cracking of crustal rocks. We have established theoretical connections among several location algorithms. In particular, two different theoretical interpretations were proposed for the phase alignment location algorithm, recently proposed for the seismic array data processing. The robustness properties of the phase alignment location algorithm and the adaptive maximum likelihood location algorithm were demonstrated through Monte Carlo simulation in regard to variations of those noise statistical features, which are typical under monitoring of microseismicity at the hydrocarbon production sites. In the framework of the Monte Carlo modeling, we also compared the two mentioned location algorithms with the so-called emission tomography algorithm that is developed for the location of microseismic sources during the monitoring of hydraulic fracturing.     

Recognizing of Microseismic Source Types Based on Small-Aperture Seismic Array Data

Izvestiya, Physics of the Solid Earth - Abstract—In this paper, we consider the problem of recording and processing the microseismic data with their subsequent interpretation and making the...     

Seismicity in the platform regions of Ukraine in the zones of anomalous electrical conductivity

It is established for the first time that there are several regions in Ukraine, in which the earthquakes occurring within platform territory are correlated to the anomalous conductive structures in the Earth’s crust and upper mantle. These regions are identified as (1) Donbass and the eastern part of the Dnieper-Donetsk Depression (DDD); (2) eastern margin of the Ingulets-Krivoi Rog suture zone in the area of the Krivoi Rog-Kremenchug fault zone; (3) the western part of the Cis-Azov megablock; (4) the western boundary of the Ukrainian Shield and its slope; (5) North Dobruja and Pre-Dobrujan Depression. The reconstructed tree-dimensional (3D) geoelectrical models of the Earth’s crust and upper mantle feature anomalously low values of electric resistivity. The earthquake sources in the platform areas of Ukraine are localized above the top and in the upper parts of the crustal anomalies of electrical conductivity.     

The relative contributions of radiative forcing and internal climate variability to the late 20th Century winter drying of the Mediterranean region

The roles of anthropogenic climate change and internal climate variability in causing the Mediterranean region’s late 20th Century extended winter drying trend are examined using 19 coupled models from the Intergovernmental Panel on Climate Change Fourth Assessment Report. The observed drying was influenced by the robust positive trend in the North Atlantic Oscillation (NAO) from the 1960s to the 1990s. Model simulations and observations are used to assess the probable relative roles of radiative forcing, and internal variability in explaining the circulation trend that drove much of the precipitation change. Using the multi-model ensemble we assess how well the models can produce multidecadal trends of realistic magnitude, and apply signal-to-noise maximizing EOF analysis to obtain a best estimate of the models’ (mean) sea-level pressure (SLP) and precipitation responses to changes in radiative forcing. The observed SLP and Mediterranean precipitation fields are regressed onto the timeseries associated with the models’ externally forced pattern and the implied linear trends in both fields between 1960 and 1999 are calculated. It is concluded that the radiatively forced trends are a small fraction of the total observed trends. Instead it is argued that the robust trends in the observed NAO and Mediterranean rainfall during this period were largely due to multidecadal internal variability with a small contribution from the external forcing. Differences between the observed and NAO-associated precipitation trends are consistent with those expected as a response to radiative forcing. The radiatively forced trends in circulation and precipitation are expected to strengthen in the current century and this study highlights the importance of their contribution to future precipitation changes in the region.     

Robust phase algorithms for estimating apparent slowness vectors of seismic waves from regional events

In this paper, we consider the problem of estimating the apparent slowness vector p of a plane P wave caused by a regional seismic event and recorded by a small-aperture seismic array. The case is considered when strong non-stationary and non-Gaussian random interferences act on the array sensors. In this case, the well-known estimate of wideband frequency-wave-number analysis (WFK) becomes ineffective due to large estimation errors. We have proposed three new algorithms for estimating the vector p that are robust i.e. resistant to changes in the statistical properties of the random interferences. They mainly use information about the slowness vector contained in the phases of the spectra of seismograms recorded by the array sensors. An intensive Monte Carlo simulation was carried out to compare the accuracy of the proposed phase-based estimates with the accuracy of WFK-estimate in the case when non-stationary and non-Gaussian anthropogenic interferences act on the array sensors. In the simulation we used synthetic mixtures of signals caused by a seismic event and anthropogenic seismic interferences. These signals and interferences were recorded by real small-aperture seismic arrays. It was shown that the proposed phase-based estimates of the vector p provide significantly better accuracy than traditional WFK-estimate in the case of strong anthropogenic interferences, and have approximately the same accuracy in the case of white Gaussian noise.

     

The north equatorial countercurrent in the West Atlantic during the summer of 1988

Experimental data provided by a hydrophysical profiling probe are invoked to examine the vertical and horizontal structures of the north equatorial countercurrent in the Atlantic Ocean. The kinematics of waters and the hydrological element distribution are compared. The countercurrent's major parameters have been identified and volume transport has been calculated.Translated by V.Puchkin.     

Influence of local and remote SST on North Atlantic tropical cyclone potential intensity

We examine the role of local and remote sea surface temperature (SST) on the tropical cyclone potential intensity in the North Atlantic using a suite of model simulations, while separating the impact of anthropogenic (external) forcing and the internal influence of Atlantic Multidecadal Variability. To enable the separation by SST region of influence we use an ensemble of global atmospheric climate model simulations forced with historical, 1856–2006 full global SSTs, and compare the results to two other simulations with historical SSTs confined to the tropical Atlantic and to the tropical Indian Ocean and Pacific. The effects of anthropogenic plus other external forcing and that of internal variability are separated by using a linear, “signal-to-noise” maximizing EOF analysis and by projecting the three model ensemble outputs onto the respective external forcing and internal variability time series. Consistent with previous results indicating a tampering influence of global tropical warming on the Atlantic hurricane potential intensity, our results show that non-local SST tends to reduce potential intensity associated with locally forced warming through changing the upper level atmospheric temperatures. Our results further indicate that the late twentieth Century increase in North Atlantic potential intensity, may not have been dominated by anthropogenic influence but rather by internal variability.     

The 1976/77 transition in precipitation over the Americas and the influence of tropical sea surface temperature

Most major features of the interdecadal shift in boreal winter-spring precipitation over the American continents associated with the 1976–1977 transition are reproduced in atmospheric general circulation model (GCM) simulations forced with observed sea surface temperature (SST). The GCM runs forced with global and tropical Pacific SSTs produce similar multidecadal changes in precipitation, indicating the dominant influence of tropical Pacific SST. Companion experiments indicate that the shift in mean conditions in the tropical Pacific is responsible for these changes. The observed and simulated “post- minus pre-1976” difference in Jan–May precipitation is wet over Mexico and the southwest U.S., dry over the Amazon, wet over sub-Amazonian South America, and dry over the southern tip of South America. This pattern is not dramatically different from a typical El Niño-induced response in precipitation. Although the interdecadal (post- minus pre-1976) and interannual (El Niño?La Niña) SST anomalies differ in detail, they produce a common tropics-wide tropospheric warmth that may explain the similarity in the precipitation anomaly patterns for these two time scales. An analysis of local moisture budget shows that, except for Mexico and the southwest U.S. where the interdecadal shift in precipitation is balanced by evaporation, elsewhere over the Americas it is balanced by a shift in low-level moisture convergence. Moreover, the moisture convergence is due mainly to the change in low-level wind divergence that is linked to low-level ascent and descent.