Estimation Of Formation Parameters Using Full Waveform Acoustic and Shear Wave Logs

A combination of borehole Stoneley waves from full waveform acoustic logs and direct shear wave logs was used to estimate formation permeability and shear wave velocity. Data sets used here were collected by Area's array full waveform acoustic logging tool and shear wave logging tool. The P- and S-wave velocities of the formation are determined by threshold detection with cross-correlation correction from the full waveform and the shear wave log, respectively. The full waveform acoustic logging data are also processed using the Extended Prony's method to estimate the borehole Stoneley wave phase velocity and attenuation as a function of frequency. Two different borehole models are considered for the inversion of Stoneley wave velocity and attenuation data. They are the isotropic elastic and the porous isotropic borehole models. Inversion parameters include shear wave velocity and formation permeability. Inverted shear wave velocities and permeabilities are compared with the shear wave log and the core permeability measurements, respectively, for an integrated interpretation and possible identification of shear wave anisotropy.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-86ER13636

Inversion For Permeability From Stoneley Wave Velocity And Attenuation

The in situ permeability of a formation is obtained by the inversion of Stoneley wave phase velocity and attenuation, which are evaluated by applying the Extended Prony's method to the array sonic logging data. The Maximum Likelihood inversion is used together with logarithmic parameterization of the permeabilities. Formation shear wave velocity is also inverted for. This process is tested on both synthetic and field data. Logarithmic parameterization contributes to rapid convergence of the algorithm. Permeabilities estimated from field data are in good agreement with core measurements.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu

Fourth-Order Finite Difference Acoustic Logs In A Transversely Isotropic Formation

In this paper we present a finite difference scheme for seismic wave propagation in a fluid-filled borehole in a transversely isotropic formation. The first-order hyperbolic differential equations are approximated explicitly on a staggered grid using an algorithm that is fourth-order accurate in space and second-order accurate in time. The grid dispersion and grid anisotropy are analyzed. Grid dispersion and anisotropy are well suppressed by a grid size of 10 points per wavelength. The stability condition is also obtained from the dispersion analysis. This finite difference scheme is implemented on the nCUBE2 parallel computer with a grid decomposition algorithm. The finite difference synthetic waveforms are compared with those generated using the discrete wavenumber method. They are in good agreement. The damping layers effectively absorbed the boundary reflections. Four vertically heterogeneous borehole models: a horizontal layered formation, a borehole with a radius change, a semi-infinite borehole, and a semi-infinite borehole with a layer, are studied using the finite difference method. Snapshots from the finite difference results provide pictures of the radiating wavefields.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Multiwavelength properties of a new Geminga-like pulsar: PSR J2021+4026

In this paper, we report a detailed investigation of the multiwavelength properties of a newly detected gamma-ray pulsar, PSR J2021+4026, in both observational and theoretical aspects. We firstly identify an X-ray source in the XMM-Newton serendipitous source catalogue, 2XMM J202131.0+402645, located within the 95% confidence circle of PSR J2021+4026. With an archival Chandra observation, this identification provides an X-ray position with arcsecond accuracy which is helpful in facilitating further investigations. Searching for the pulsed radio emission at the position of 2XMM J202131.0+402645 with a 25-m telescope at Urumqi Astronomical Observatory resulted in null detection and places an upper-limit of 0.1~mJy for any pulsed signal at 18~cm. Together with the emission properties in X-ray and gamma-ray, the radio quietness suggests PSR J2021+4026 to be another member of Geminga-like pulsars. In the radio sky survey data, extended emission features have been identified in the gamma-ray error circle of PSR J2021+4026. We have also re-analyzed the gamma-ray data collected by FERMI's Large Area Telescope. We found that the X-ray position of 2XMM J202131.0+402645 is consistent with that of the optimal gamma-ray timing solution. We have further modeled the results in the context of outer gap model which provides us with constraints for the pulsar emission geometry such as magnetic inclination angle and the viewing angle. We have also discussed the possibility of whether PSR J2021+4026 has any physical association with the supernova remnant G78.2+2.1 (gamma-Cygni).Comment: 11 pages, 14 figure

Finite Difference Modelling Of Acoustic Logs In Vertically Heterogeneous Biot Solids

This paper discusses the results of tests carried out on a finite difference formulation of Biot's equations for wave propagation in saturated porous media which vary in range and depth (Stephen, 1987). A technique for modeling acoustic logs in two dimensionally varying Biot solids will give insight into the behavior of tube waves at permeable fractures and fissures which intersect the borehole. The code agrees well with other finite difference codes and the discrete wavenumber code for small porosity in the elastic limit of Biot's equations. For large porosity (greater than one per cent) in the elastic limit or for the acoustic limit, good agreement is not obtained with the discrete wavenumber method for vertically homogeneous media. The agreement is worst for amplitudes of the pseudo-Rayleigh wave. The amplitude of the Stoneley wave and the phase velocities of both waves could be acceptable for some applications. An example is shown of propagation across a horizontal high porosity stringer in a Berea sandstone. Reflections from the stringer are observed but given the inaccuracies of the pseudo-Rayleigh waves for vertically heterogeneous media the amplitudes for the stringer model are questionable. We propose a three stage approach for further work: 1) Use the Virieux scheme instead of the Bhasavanija scheme for the finite difference template. The Virieux scheme has been shown in other studies to be more accurate for liquid-solid interfaces. 2) Run the present code for lower frequency sources to emphasize Stoneley waves and diminish pseudo-Rayleigh waves. Stoneley waves are most sensitive to permeability variations which are the primary objective of Biot wave studies. 3) Develop a finite difference code for Biot media with the fluid-solid boundary conditions specifically coded. This code would be suitable for studying constant radius boreholes in vertically varying Biot media.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu

Acoustic Logging In Randomly Stratified Formations

The propagation of borehole acoustic waves in the presence of various types of heterogeneous formations is investigated by modeling them as stratified media with varying velocity-depth distributions. Two types of formations are modeled, using translational and cyclic random models, respectively. Borehole acoustic wavefields for the heterogeneity formation models are simulated using finite-difference techniques. The wavefield modeling results show that the borehole acoustic waves can be significantly affected by the formation heterogeneities. Specifically, the scattering due to heterogeneity can cause significant amplitude attenuation and travel time delay for the transmitted waves. The borehole guided waves are also sensitive to the formation heterogeneity. The effects of the random formation heterogeneity on the borehole acoustic waves are controlled by two factors: the degree of heterogeneity variation and the heterogeneity scale length relative to the wavelength.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Doping Effect of Nano-Diamond on Superconductivity and Flux Pinning in MgB2

Doping effect of diamond nanoparticles on the superconducting properties of MgB2 bulk material has been studied. It is found that the superconducting transition temperature Tc of MgB2 is suppressed by the diamond-doping, however, the irreversibility field Hirr and the critical current density Jc are systematically enhanced. Microstructural analysis shows that the diamond-doped MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50-100 nm) with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20 nm) inside the grains. High density of dislocations and diamond nanoparticles may take the responsibility for the enhanced flux pinning in the diamond-doped MgB2.Comment: 16 pages, 6 figure