Retrieving shallow shear-wave velocity profiles from 2D seismic-reflection data with severely aliased surface waves

The inversion of surface-wave phase-velocity dispersion curves provides a reliable method to derive near-surface shear-wave velocity profiles. In this work, we invert phase-velocity dispersion curves estimated from 2D seismic-reflection data. These data cannot be used to image the first 50 m with seismic-reflection processing techniques due to the presence of indistinct first breaks and significant NMO-stretching of the shallow reflections. A surface-wave analysis was proposed to derive information about the near surface in order to complement the seismic-reflection stacked sections, which are satisfactory for depths between 50 and 700 m. In order to perform the analysis, we had to overcome some problems, such as the short acquisition time and the large receiver spacing, which resulted in severe spatial aliasing. The analysis consists of spatial partitioning of each line in segments, picking of the phase-velocity dispersion curves for each segment in the f-k domain, and inversion of the picked curves using the neighborhood algorithm. The spatial aliasing is successfully circumvented by continuously tracking the surface-wave modal curves in the f-k domain. This enables us to sample the curves up to a frequency of 40 Hz, even though most components beyond 10 Hz are spatially aliased. The inverted 2D VS sections feature smooth horizontal layers, and a sensitivity analysis yields a penetration depth of 20–25 m. The results suggest that long profiles may be more efficiently surveyed by using a large receiver separation and dealing with the spatial aliasing in the described way, rather than ensuring that no spatially aliased surface waves are acquired.Fil: Onnis, Luciano Emanuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Osella, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Carcione, Jose M.. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; Itali

Accurate calibration of test mass displacement in the LIGO interferometers

We describe three fundamentally different methods we have applied to calibrate the test mass displacement actuators to search for systematic errors in the calibration of the LIGO gravitational-wave detectors. The actuation frequencies tested range from 90 Hz to 1 kHz and the actuation amplitudes range from 1e-6 m to 1e-18 m. For each of the four test mass actuators measured, the weighted mean coefficient over all frequencies for each technique deviates from the average actuation coefficient for all three techniques by less than 4%. This result indicates that systematic errors in the calibration of the responses of the LIGO detectors to differential length variations are within the stated uncertainties.Comment: 10 pages, 6 figures, submitted on 31 October 2009 to Classical and Quantum Gravity for the proceedings of 8th Edoardo Amaldi Conference on Gravitational Wave

Assessing the ability of the 14C projection-age method to constrain the circulation of the past in a 3-D ocean model

Radiocarbon differences between benthic and planktonic foraminifera (B-P ages) and radiocarbon projection ages are both used to determine changes of the past ocean circulation rate. A global 3-D ocean circulation model with a constant modern ocean circulation is used to study which method is less influenced by atmospheric Δ14C variations. Three factors cause uncertainties: first, the long equilibration time of the ocean after atmospheric Δ14C changes; second, different mixing processes in the ocean, which cause an ocean response of smaller amplitude than the atmospheric forcing; and third, the unknown source region and corresponding initial surface 14C reservoir age of subsurface waters. The model suggests that B-P ages and projection ages have lower uncertainties the closer they are to deepwater formation zones. In the North Atlantic the B-P age method is less influenced by atmospheric Δ14C variations than the projection-age method. Projections ages vary less in the Pacific as long as atmospheric Δ14C decreases linearly. A more irregular atmospheric Δ14C evolution leads to age variations of similar magnitude with both methods. On the basis of the model experiment, we suggest a potential improvement of the projection-age method

An Exploration of the Performance and Acoustic Characteristics of UAV-Scale Stacked Rotor Configurations

As interest grows in rotor- and propeller-driven electric vertical takeoff and landing (eVTOL) aircraft for the Urban Air Mobility market, there is a potential for previously studied concepts to reemerge due to the opportunities afforded by novel technologies and operating modes. One such concept is the stacked rotor, which consists of multiple co-rotating rotors positioned co-axially with a small axial offset. The goal of the work presented in this paper is to determine whether stacked rotors offer a compelling advantage for eVTOL aircraft in terms of both performance and acoustic characteristics. Results are presented for new experimental tests and computational modeling of multiple stacked rotor configurations, and comparisons are made with conventional rotor configurations. Testing of thirteen separate configurations each using the same blade shaperevealed a configuration that resulted in an increase in the rotor power loading efficiency by more than 7% and reduced noise by more than 3 dBA when compared with a conventional rotor with all blades located in the same rotational plane

Study Of Design For Reliability Of Rf And Analog Circuits

Due to continued device dimensions scaling, CMOS transistors in the nanometer regime have resulted in major reliability and variability challenges. Reliability issues such as channel hot electron injection, gate dielectric breakdown, and negative bias temperature instability (NBTI) need to be accounted for in the design of robust RF circuits. In addition, process variations in the nanoscale CMOS transistors are another major concern in today‟s circuits design. An adaptive gate-source biasing scheme to improve the RF circuit reliability is presented in this work. The adaptive method automatically adjusts the gate-source voltage to compensate the reduction in drain current subjected to various device reliability mechanisms. A class-AB RF power amplifier shows that the use of a source resistance makes the power-added efficiency robust against threshold voltage and mobility variations, while the use of a source inductance is more reliable for the input third-order intercept point. A RF power amplifier with adaptive gate biasing is proposed to improve the circuit device reliability degradation and process variation. The performances of the power amplifier with adaptive gate biasing are compared with those of the power amplifier without adaptive gate biasing technique. The adaptive gate biasing makes the power amplifier more resilient to process variations as well as the device aging such as mobility and threshold voltage degradation. Injection locked voltage-controlled oscillators (VCOs) have been examined. The VCOs are implemented using TSMC 0.18 µm mixed-signal CMOS technology. The injection locked oscillators have improved phase noise performance than free running oscillators. iv A differential Clapp-VCO has been designed and fabricated for the evaluation of hot electron reliability. The differential Clapp-VCO is formed using cross-coupled nMOS transistors, on-chip transformers/inductors, and voltage-controlled capacitors. The experimental data demonstrate that the hot carrier damage increases the oscillation frequency and degrades the phase noise of Clapp-VCO. A p-channel transistor only VCO has been designed for low phase noise. The simulation results show that the phase noise degrades after NBTI stress at elevated temperature. This is due to increased interface states after NBTI stress. The process variability has also been evaluated

ANALYSIS OF TIME-LAPSE 3-D VSP DATA FOR SEISMIC MONITORING OF CO2 FLOOD IN WEYBURN FIELD, SASKATCHEWAN

This study explores the potential of VSP data in reservoir studies and particularly in separating the effects of CO2 pore pressure and saturation after fluid injection into the reservoir. It helps establishing robust and reliable links between physical properties of hydrocarbon reservoirs and seismic data. It also reduces the uncertainties of the AVA analysis through constructing a detailed model of first-arrival amplitude decay combining geometric spreading, scattering, and inelastic dissipation. The inversion of seismic data reveals anisotropic variations of geometric attenuation (wavefront curvatures and scattering, denoted ) and the effective attenuation parameter () with depth. Statistical analysis of model uncertainties quantitatively measures the significance of these results. This model correctly predicts the observed frequency-dependent first-arrival amplitudes at all frequencies and can be used for reflected waves. Scattering and geometric spreading (focusing and defocusing of wave fronts) significantly affect seismic amplitudes at lower frequencies and shallower depths. Using of complete well logs, a model of P- and S-wave scattering is derived from direct-wave attenuation observations by numerical and analytical methods. Both approaches reveal fluctuations in the transmitted-energy flux within different depth intervals, and particularly at frequencies above 60 Hz. A randomization of well logs suggests that the upper envelope of the transmitted energy flux (corresponding to strongest transmission) is a reasonable estimate for random scattering. The lower envelope corresponding to the strongest reflectivity appears to be a useful characteristic of the fluctuations in the scattered wavefields. Once these ‘random’ and ‘fluctuation’ attenuations are modeled, they can be isolated from the intrinsic and geometric effects. Finally, three amplitude-based methods are used to separate the effects of CO2 pressure and saturation in the Weyburn reservoir. Based on these results, an area close to an injection well within the southern part of the study area is interpreted as having the highest pressure, and the area between the northern and eastern wells show the highest CO2 saturation. Near the center of the study area, the effects of CO2 saturation and pressure appear to be the weakest