Velocity Filter Research Articles

Direct and β-delayed particle decays near closed shells

We have been involved in the investigation of nuclear structure properties of nuclei close to the proton drip line. In these studies we have used on-line isotope separators at Oak Ridge (UNISOR) and Berkeley (OASIS), the ORNL velocity filter, and a fast helium-gas-jet transport system at the LBL 88-in. cyclotron. We describe briefly three areas of our research that have dealt with particle decays: (1) α-decay in medium-weight nuclei, (2) α-decay rates for even-even isotopes, and (3) ß-delayed-proton decay of N = 81 precursors.

The new nuclide225U

In the bombardment of a 270μg/cm 2 180Hf target with48 Ca projectiles at a primary beam energy of E/A=4.24 MeV/u the new nuclide225U was produced.The experiment was performed at the velocity filter SHIP. 225U was found to decay by α emission with Eα=(7880 ±20) keV (≈90%), (7830±20) keV (≈10%) and has a half-life ofT 1/2=(80 +40−20 ms).

DECOMPOSED VIBRATOR PATTERNS TO IMPROVE SEISMIC SURVEY RESULTS1

AbstractEffective noise reduction of single‐sweep recorded data is achieved by application of a velocity filter process on a decomposed vibrator pattern. This technique promises high resolution results with a minimum effect on signal characteristic. A comparison of the stacked section of records vertically stacked in the field with the stacked section of velocity‐filtered receiver gathers shows a significant increase in resolution and signal‐to‐noise ratio.

Slant-stack processing

The slant-stack or T-P domain offers an alternative representation for seismic data. lts theoretical advantages over the conventional time-offset domain have been studied extensively and are well known. In practice, however, the numerical artefacts of the T-P transform, produced by spatial aliasing and aperture limitations, quite often overpower the expected theoretical advantages. Moreover, the presence of random and coherent noise in the data further deteriorates the performance of the transform. Thus, subsequent processing in the T-P domain becomes less effective. In this paper, we present some applications of slant-stack processing for seismic reflection data. The cornerstone is a new improved T-P transform which incorporates a time- and offset-variant velocity filter. The resulting T-P gathers are free of most transform artefacts and exhibit improved signal-to-noise ratios. First, the performance of the transform is shown with applications both to noise-free synthetic data and to field data contaminated with coherent noise. The forward/inverse T-P transform pair is then applied to the common-shot gathers and the results are compared with those obtained from conventional frequency-wavenumber (f-k) filtering. FinaIly, we combine the forward transform with elliptical moveout (EMO) and stacking and obtain stacked sections in the slant-stack domain. This processing sequence avoids the need for an inverse transform and has several advantages over conventional norm al moveout (NMO) and stacking in the time-offset domain.

Velocity filtering of acoustic well logging waveforms

A velocity filter that is based on a particularly convenient and effective combination of an up/down filter and slanting (time-shift) operations is presented. The up/down filter is based on a cascade of spatial and temporal Hilbert transform filters. The Hilbert transform filters and the time-shift operators are finite-duration impulse response (FIR) filters designed according to the minimax criterion. The velocity filter is applied to acoustic well logging waveforms to isolate the initial P, S, and Stoneley waves from their reflected and converted waves at bed boundaries. To achieve this objective, the waveforms are time-shifted to align the primary wave of interest before the velocity filter is applied. Examples demonstrate the effectiveness of this procedure in separating the primary waves from their reflected and converted waves. >

Seismic Data Acquisition over San Juan Volcanic Field, San Juan Sag, South-Central Colorado: ABSTRACT

The San Juan sag, a foreland basin along the Uncompahgre-Brazos uplift, contains Mesozoic sediments overlain by up to 10,000 ft of Tertiary volcanics. Petroleum exploration beneath thick volcanic cover requires successful acquisition of seismic data; however, experience elsewhere has generally produced only marginal results. Commencing in 1983, Meridian Oil recorded 18 seismic lines comprising 260 mi of conventional and portable data. Noise test were performed at two sites to evaluate seismic response and to determine source and receiver parameters. Vibroseis, shothole, and surface dynamite were examined as sources and gave comparable results. Coherent noise trains with velocities from 4,000 to 8,000 ft/sec and dominant frequencies of 10 to 15 Hz were generated in the Tertiary section. The predominant factor affecting data quality was outcrop lithology. Data recorded over alluvial and other Quaternary deposits tended to be good to fair. Data quality over Tertiary volcanics was variable and highly unit dependent, ranging from fair over andesites and volcaniclastics, poor over ash-flow tuffs, and uniformly very poor over basalts. Seismic source appeared to have only a minor effect on quality. Data processing involved no unusual operations. Velocity filtering was used to suppress near-surface generated coherent noise, Refraction statics were applied to linesmore » with significant elevation or lithologic variations. A successful acquisition program helped to establish the presence of a Mesozoic sedimentary section beneath the volcanics. these data revealed large structural features within the sedimentary section as well as a dramatic angular unconformity at the top of the Cretaceous section.« less

Laser Ablation Deposition (LAD) of Metallic Thin Films

ABSTRACTSeveral experiments have been carried out to characterize the laser ablation deposition of metallic thin films as a function of the various process parameters. Ablation thresholds have been investigated for Al, Cu and Au targets as a function of wavelength as well as the fluence dependence of ablation rate. The effect of electric fields on the particle component of the ablated material has been investigated. Measurements of the velocity distribution of these particles have also been carried out for Al, Cu and Au targets as a function of the laser parameters with a view to using a velocity filter to remove the particles from the ablated beam.

Local phase velocity from complex seismic data

We propose a new method that uses the concepts of complex trace analysis for the automatic estimation of local phase velocity. A complex seismic record is obtained from a real seismic record by extending complex trace analysis into higher dimensions. Phase velocity is estimated from the complex data by finding trajectories of constant phase. In two dimensions, phase velocity calculation reduces to a ratio of instantaneous frequency and wavenumber, and thus provides a measure of the dominant plane‐wave component at each point in the seismic record. The algorithm is simple to implement, and computational requirements are small, partly due to a new method for computing instantaneous frequency and wavenumber which greatly simplifies these calculations for 2-D and 3-D complex records. In addition, our approach has the advantage that no a priori velocity input is needed; however, optimum stability is achieved when a limited range of dipping events is considered. Preconditioning the record with an appropriate velocity filter helps reduce the detrimental effects of crossing events, spatial aliasing, and random noise contamination.

A hydrophone vertical seismic profiling experiment

To reduce the cost of VSP data acquisition, it is necessary to record the VSP signal from a vertical array of geophones for a single operation of the source. Until a vertical array of clamped three‐component geophones is available, it seems logical to evaluate the capabilities of a vertical array of hydrophones, which is much easier to fabricate. It is well known that elastic waves in the solid couple to pressure waves in the borehole fluid. It is also well known that this coupling excites in the borehole fluid energy known as tube‐wave noise that dominates the borehole pressure signal after the first arrival. (The borehole acts as a waveguide.) In this paper we test the effectiveness of velocity filtering of the borehole pressure signal to attenuate the slowly propagating tube‐wave noise and enhance the faster propagating body‐wave signals. Our initial test satisfactorily extracted from the hydrophone array data a strong reflected event that was also observed in the conventional clamped geophone VSP taken in the same borehole. We were not as successful in recovering subsequent weaker reflected signals from the hydrophone data, because of the strong incoherent ambient tube‐wave noise. This incoherency resulted from instrumental limitations that allowed us to record, for each shot, only three of the twelve hydrophone channels available in the vertical array.

Determination of the partial electron capture- and spontaneous-fission half-lives of254No

The isotope254No was produced in the fusion reaction48Ca +208Pb. Using the velocity filter SHIP and radiochemical techniques it was found that the nuclide254No with a half-life of 55 s decays byα, EC, and spontaneous-fission. Deduced partial half-lives are (61±2) s forα-decay, (550−160+370) s for EC and [2.2−1.0+2.0]×104 s for spontaneous fission.

Synthesis of a holographic image velocity filter using the nonlinear photorefractive effect

We demonstrate a technique by which components of an image moving with a given speed and direction may be selectively enhanced. By using the strong grating velocity sensitivity of the nonlinear photorefractive response, we show that a BSO crystal used in a schlieren imaging system may be used as a real-time velocity filter. The velocity selectivity, tunability and resolution constraints of this system are discussed in detail.

Bistable neutron spin flippers

We have built a bistable spin-flipping device for slow neutrons: it very efficiently flips the spin polarization of neutrons over a wide range of neutron velocities, while leaving unaffected the spins of neutrons in other velocity intervals. Such bistable broad-band flippers can be designed both for π and π 2 spin flips, and also for refocussing flips of the kind used in neutron spin-echo instruments. We further built a neutron velocity filter for polarized neutrons based on this principle, together with a simple reflecting foil neutron velocity high-pass.

Nonlinear identification of stretch reflex dynamics.

The objective of this study was to use nonlinear identification techniques to study the dynamics of stretch reflexes in the human calf muscles (gastrocnemius-soleus). Stochastic perturbations of ankle position were applied while subjects maintained a constant, tonic contraction of gastrocnemius-soleus. Linear models of the relation between ankle velocity and the electromyographic (EMG) activity under these conditions typically accounted for less than 40% of the observed EMG variance. Nonlinear system identification techniques were then applied. The first- and second-order Wiener kernels were computed as the initial stage of this analysis. These did not provide an adequate description of system behavior; subsequent simulation studies showed that the major problem with the Wiener analysis was that the input spectrum was not adequately white. Nevertheless, the shape of the second-order Wiener kernel suggested that a Hammerstein structure consisting of a static nonlinearity followed by a dynamic linear system would be appropriate. Consequently, we used an iterative procedure for Hammerstein system identification to determine the form of the static nonlinearity and the associated linear dynamics. The resulting nonlinear model provided a much better description of the system's behavior than did the linear models (variance accounted for greater than 60%). Furthermore, they confirmed our previous empirical findings; the static nonlinearity closely resembled a half-wave rectifier while the dynamics were typified by a pure delay and a velocity filter. The application of nonlinear identification techniques thus produced a much improved, physically meaningful model of stretch reflex behavior.

The relationship of pre-stack apparent velocity filtering to the symmetry of the CMP stack response

Wearing my processing geophysicist's hat, I, like others, often ask questions to which there appear to be no answers readily available. One such question that had re-occurred to me was 'what is the relationship between pre-stack apparent velocity filtering and the common midpoint (CMP) stack response?' The elements of any answer were roughly -potential reduction in spatial resolution; -reduction in coherent noise; -apparent velocity in common source or geophone space is twice that observed on the CMP stack. During the summer of 1983 Walt Lynn presented some material at Western Geophysical's London office (as part of a week long 'Wave Equation Update School' lecture course given by Jon F. C1aerbout) entitled 'Dip filtering for noise suppression'. Part of this graphical material demonstrated how apparent velocity filtering could be viewed in source-geophone space as removing ciearly definable parts of a pre-stack diffraction. Upon this basis the following text is an attempt to answer my question by doing three things: • Setting down the necessary equations. • Showing how certain symmetry conditions affect the CMP stack. • Computing synthetic examples. The text will consider a point scatterer in the subsurface, how it responds to the CMP method and what happens when pre-stack apparent velocity filtering is included. The most important conclusion will be that certain apparent velocity filter symmetry conditions are necessary if the CMP stack is to respond similarly to positive and negative dips. The analysis will commence by describing the prestack traveltime surface that is associated with the point scatterer, which is sometimes called Cheop's Pyramid. Equations for apparent velocity on the commonly used axes of Cheop's Pyramid will be derived which show how certain areas of the surface are rejected by apparent velocity discrimination. The idea of symmetry will be described and a set of rules will be presented in order to test whether the pre-stack apparent velocity filter will prejudice dip direction on the CMP stack. Some computed examples will be presented along with a brief description of the computational method used. Finally a potential problem with 'marine' three-dimensional (3D) surveys that can be caused by pre-stack apparent velocity filtering will be briefly discussed.

VERTICAL SEISMIC PROFILING—SEPARATION OF P‐ AND S‐WAVES*

AbstractIn vertical seismic profile's (VSP's) shot with a large source offset, rays from shot to receiver can have large angles of incidence. Shear waves generated by the source and by conversions at interfaces are likely to be recorded by both the vertical and the horizontal geophones. Varying angles of incidence may give strong variations in the recorded amplitudes. Separation of P‐ and SV‐waves and recovery of their full amplitudes are important for proper processing and interpretation of the data. A P‐S separation filter for three‐component offset VSP data is presented which performs this operation. The separation filter is applied in the k‐f domain and needs an estimate of the P‐ and S‐velocities along the borehole as input. Implementation and stability aspects of the filter are considered. The filter was tested on an 1800 m offset VSP and appeared to be robust. Large velocity variations along the borehole could be handled and results were superior to those obtained by velocity filtering.

Design and implementation of velocity filters using multichannel array processing techniques

A general procedure for designing two-dimensional velocity filters, based on array filters derived originally for multichannel (multiple-input single-output) processing of the recordings of an array of sensors, is presented. The mathematical expressions needed for implementation of the filters using the two-dimensional fast Fourier transform are derived. A specific derivation of the velocity filters, based on a family of multichannel array filters obtained recently for attenuating a coherent interference in the array recordings, is carried out in detail. Two examples illustrating the performance of these velocity filters are also presented.

The experimental work at the velocity filter SHIP — Results and plans

Within the last ten years SHIP has been used for the investigation of evaporation residues from heavy-ion fusion reactions. After a brief description of the separation and identification of the evaporation residues, we will briefly review the experimental work covering the investigation of subbarrier fusion, the observation of radiative fusion, the measurements of ground-state proton decay, and the identification of the heaviest known elements. Recently, SHIP has been completed by the addition of a small magnetic spectrometer to improve the quality of separation.

Munich high-frequency recoil spectrometer

The Munich recoil spectrometer is an instrument to measure recoiling nuclei at or near zero degrees. The suppression of the projectile intensity is achieved with a Wien-type velocity filter applying a high-frequency electric field synchronized with the pulsed beam. A fast time-of-flight detector and a ΔE− E ionization chamber allow the identification of recoiling nuclei according to their A-and Z-values. The whole system is very compact yielding a high collection efficiency with a broad acceptance in both charge and velocity. Therefore, this spectrometer is especially suited for coincidence measurements of recoils with light particles or gamma rays. But this instrument has also been used to measure sub-Coulomb fusion cross sections down to the μb region.

A New Velocity Filter For The Attenuation Of Coherent Seismic Noise

A new method of wave modelling and phase velocity filtering in the FX domain is presented. A shot record is considered to be consisting of two models ? one model is that of relatively slow velocity coherent noise whilst the other is of relatively fast velocity (reflected) waves. The process measures phase velocity as a function of frequency for each trace of a shot record. Then it produces the two separate models ? one with only coherent noise present, the other with only reflected waves present. This has a clear application to shot records on which coherent noise has the same frequency content as reflected signal, and the process requires less computation than the equivalent FK methods.

Windowing of Dispersive Noise in Two Domains

During the collection of seismic data on land, various types of near-surface horizontally propagating waves are generated by the seismic source. These waves are more or less dispersed depending on the elastic properties of the surface layers and can be of sufficient amplitude to cause a difficult coherent noise problem. A process for suppressing noise arising from dispersive direct waves in presented. It is most applicable to data for which such noise severely masks the signal and where conventional velocity filtering alone gives insufficient signal-to-noise (S/N) enhancement. The method involves signal compression and windowing and has been applied successfully to flexural ice-waves and to dispersive ground roll.