170,896 research outputs found
On the source-frequency dependence of fracture-orientation estimates from shear-wave transmission experiments
No full textCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOShear-wave propagation through anisotropic fractured or cracked media can provide valuable information about these fracture swarms and their orientations. The main goal of this work is to recover information about fracture orientation based on the shear waveforms (S-waveforms). For this study, we carried out ultrasonic S-wave measurements in a synthetic physical model made of epoxy resin (isotropic matrix proxy), with small cylindrical rubber strips as inclusions (artificial cracks) inserted in it to simulate a homogeneous anisotropic medium. In these experiments, we used low, intermediate, and high frequency shear-wave sources, with frequencies 90, 431, and 840. kHz. We integrated and interpreted the resulting S-wave seismograms, cross-correlation panels and anisotropic parameter-analysis curves. We were able to estimate the crack orientation in single-orientation fracture zones. The high frequency peaks associated with scattered S-waves provided interpretable information about the fracture orientations when the propagation direction was parallel to the fracture plane. The analysis was possible utilizing results from frequency-versus-polarization-angle curves. Moreover, we applied a bandpass filtering process to the intermediate and high frequency seismograms in order to obtain low frequency seismograms. A spectral analysis using frequency-wavenumber (F-K) spectra supports this filtering process. The results obtained using an analysis of cross-correlograms and the Thomsen parameter γ extracted from filtered high-frequency data were quite similar to those obtained using a low-frequency source. This highlighted the possibility of using less expensive high-frequency sources to recover information about the fracture set.Shear-wave propagation through anisotropic fractured or cracked media can provide valuable information about these fracture swarms and their orientations. The main goal of this work is to recover information about fracture orientation based on the shear waveforms (S-waveforms). For this study, we carried out ultrasonic S-wave measurements in a synthetic physical model made of epoxy resin (isotropic matrix proxy), with small cylindrical rubber strips as inclusions (artificial cracks) inserted in it to simulate a homogeneous anisotropic medium. In these experiments, we used low, intermediate, and high frequency shear-wave sources, with frequencies 90, 431, and 840. kHz. We integrated and interpreted the resulting S-wave seismograms, cross-correlation panels and anisotropic parameter-analysis curves. We were able to estimate the crack orientation in single-orientation fracture zones. The high frequency peaks associated with scattered S-waves provided interpretable information about the fracture orientations when the propagation direction was parallel to the fracture plane. The analysis was possible utilizing results from frequency-versus-polarization-angle curves. Moreover, we applied a bandpass filtering process to the intermediate and high frequency seismograms in order to obtain low frequency seismograms. A spectral analysis using frequency-wavenumber (F-K) spectra supports this filtering process. The results obtained using an analysis of cross-correlograms and the Thomsen parameter γ extracted from filtered high-frequency data were quite similar to those obtained using a low-frequency source. This highlighted the possibility of using less expensive high-frequency sources to recover information about the fracture set.11481100CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOsem informaçãosem informaçãoAssad, J.M., The effect of orthorhombic anisotropy and its implication for oil recovery and reservoir exploitation (2005) Geophys. Prospect., pp. 121-129Assad, J.M., Tatham, R.H., McDonald, J.A., A physical model study of microcrack-induced anisotropy (1992) Geophysics, 57 (12), pp. 1562-1570Chichinina, T., Obolentseva, I., Gorshkalev, S., Karsten, W., Korsunov, I., Cross-correlation-function method for separation of shear waves S1 and S2 in azimuthally anisotropic media and an example of its application with real data (2012) Society of Exploration Geophysicists, pp. 1-6. , SEG Technical Program Expanded Abstracts 2012Crampin, S., Evaluation of anisotropy by shearwave splitting (1985) Geophysics, 50 (1), pp. 142-152de Figueiredo, J.J.S., Schleicher, J., Stewart, R.R., Dyaur, N., Estimating fracture orientation from elastic-wave propagation: an ultrasonic experimental approach (2012) J. Geophys. Res., 117 (B8)de Figueiredo, J., Schleicher, J., Stewart, R.R., Dayur, N., Omoboya, B., Wiley, R., William, A., Shear wave anisotropy from aligned inclusions: ultrasonic frequency dependence of velocity and attenuation (2013) Geophys. J. Int., 193 (1), pp. 475-488Ding, P., Di, B., Wang, D., Wei, J., Li, X., P and s wave anisotropy in fractured media: experimental research using synthetic samples (2014) J. Appl. Geophys., 109, pp. 1-6Far, M.E., de Figueiredo, J.J.S., Stewart, R.R., Castagna, J.P., Han, D.-H., Dyaur, N., Measurements of seismic anisotropy and fracture compliances in synthetic fractured media (2014) Geophys. J. Int., 197 (3), pp. 1845-1857Holditch, S., Jennings, J., Neuse, S., Wyman, R., The optimization of well spacing and fracture length in low permeability gas reservoirs (1978) Society of Petroleum Engineers, pp. 1-12. , SPE Annual Fall Technical Conference and ExhibitionHudson, J.A., Wave speeds and attenuation of elastic waves in material containing cracks (1981) Geophys. J. R. Astron. Soc., 64 (1), pp. 133-150Kennett, B.L.N., (2002) The Seismic Wavefield: Volume 2, Interpretation of Seismograms on Regional and Global Scales, , Cambridge University PressLi, R., Okoye, P., Uren, N., A study of the effects of transducer size on physical modeling experiments for recovering anisotropic elastic parameters (2000) Geophys. Res. Lett., 27 (22), pp. 3643-3646Lonergan, L., (2007) Fractured Reservoirs, , Geological Society of LondonLou, M., Zhang, Y., Pham, L.D., Shear-wave splitting and fracture orientation analysis from PS waves - examples from synthetic and field OBC data (2001) SEG Annual Meeting, pp. 94-97Nelson, R., (2001) Geologic Analysis of Naturally Fractured Reservoirs, , Gulf Professional PublishingOmoboya, B., de Figueiredo, J.J.S., Dyaur, N., Stewart, R.R., Uniaxial stress and ultrasonic anisotropy in a layered orthorhombic medium (2011) Society of Exploration Geophysicists, pp. 2145-2149. , SEG Technical Program Expanded AbstractsQuan, Y., Harris, J.M., Seismic attenuation tomography using the frequency shift method (1997) Geophysics, 62 (3), pp. 895-905Rathore, J., Fjaer, E., Holt, R., Renlie, L., P- and S-wave anisotropy of a synthetic sandstone with controlled crack geometry (1995) Geophys. Prospect., 43 (6), pp. 711-728Sacchi, M.D., Verschuur, D.J., Zwartjes, P.M., Data reconstruction by generalized deconvolution (2004) Soc. Explor. Geophys., pp. 1989-1992Stewart, R.R., Dyaur, N., Omoboya, B., de Figueiredo, J.J.S., Willis, M., Sil, S., Physical modeling of anisotropic domains: ultrasonic imaging of laser-etched fractures in glass (2013) Geophysics, 78 (1), pp. D11-D19Thomsen, L., Weak elastic anisotropy (1986) Geophysics, 51 (10), pp. 1954-1966Tillotson, P., Sothcott, J., Best, A.I., Chapman, M., Li, X.-Y., Experimental verification of the fracture density and shear-wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry (2012) Geophys. Prospect., 60 (3), pp. 516-525Wei, J., A physical model study of different crack densities (2004) J. Geophys. Eng., 1 (1), pp. 70-76Willis, M.E., Burns, D.R., Rao, R.V.N., Minsley, B.J., Toksoz, M.N., Vetri, L., Spatial orientation and distribution of reservoir fractures from scattered seismic energy (2005) Earth Resources Laboratory, Shortened Title: Fracture Characterization From Coda Waves, , Technical Report, Massachusetts Institute of TechnologyYilmaz, O., (2001) Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data, , SEG BooksZhang, Y., Campman, X., Grandi, S.K., Chi, S., Toksoz, M.N., Willis, M.E., Burns, D.R., Vetri, L., F-K characteristics of the seismic response to a set of discrete parallel fractures (2006) Technical Report, Massachusetts Institute of Technology, , Dept. of Energy, (award number DE-FC26- 02NT15346)Zhou, R., Miandro, R., Chinigo, C., Lou, M., Patval, Z., Dray, N., Fracture orientation estimation using circular VSP (2006) SEG Technical Program Expanded Abstracts, pp. 3427-343
The UV to IR transmittance of Antarctic snow
No full textWe report the transmittance of Antarctic snow as a function of the thickness of snow layers, in the range between 350 and 900 nm. Samples were studied near Scott Base, Ross Island, and data are presented for measurements made in early December 1996. The transmittance followed the two flux theoretical expressions well, and scattering and absorption parameters were obtained by curve fitting. Scattering dominated the transmittance at shorter wavelengths. The absorption at shorter wavelengths varied from one site to another, being lowest for snow samples measured furthest away from Ross Island, suggesting the effects of blown volcanic dust. The snow albedo derived from the transmittance parameters is compared with those of other workers.PT: J; CR: BOHREN CF, 1974, J GEOPHYS RES, V79, P4527 DAVIS A, 1990, J GEOPHYS RES-ATMOS, V95, P11729 GRENFELL TC, 1977, J GLACIOL, V18, P445 GRENFELL TC, 1984, J GEOPHYS RES-OCEANS, V89, P3573 GRENFELL TC, 1994, J GEOPHYS RES-ATMOS, V99, P18669 ISHIMARU A, 1978, WAVE PROPAGATION SCA, V1, CH10 KORTUM G, 1969, REFLECTANCE SPECTROS, CH4 PEROVICH DK, 1991, GEOPHYS RES LETT, V18, P1233 WARREN SG, 1984, APPL OPTICS, V23, P1206 WEILER CS, 1994, ANTARCT RES SER, V62 WISCOMBE WJ, 1980, J ATMOS SCI, V37, P2712; NR: 11; TC: 11; J9: J GEOPHYS RES-ATMOS; PG: 9; GA: ZK155Source type: Electronic(1
True-amplitude single-stack redatuming
No full textFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOBased on the chaining of diffraction-stack migration and isochron-stack demigration, we derive a general true-amplitude Kirchhoff-type single-stack operator for 3D and 2.5D redatuming. It consists of performing a single weighted stack along adequately chosen stacking surfaces or lines. The corresponding traveltimes and weight functions can be calculated using quantities obtained from dynamic ray tracing. The operator simplifies when specified for zero-offset data. For simple types of media, we derive analytic expressions for the stacking lines and weight functions and demonstrate their functionality with numerical examples. © 2014 Elsevier B.V.Based on the chaining of diffraction-stack migration and isochron-stack demigration, we derive a general true-amplitude Kirchhoff-type single-stack operator for 3D and 2.5D redatuming. It consists of performing a single weighted stack along adequately chosen stacking surfaces or lines. The corresponding traveltimes and weight functions can be calculated using quantities obtained from dynamic ray tracing. The operator simplifies when specified for zero-offset data. For simple types of media, we derive analytic expressions for the stacking lines and weight functions and demonstrate their functionality with numerical examples.10595111FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO05/00305-0sem informaçãoBerryhill, J.R., Wave-equation datuming (1979) Geophysics, 44, pp. 1329-1344Berryhill, J.R., Wave equation datuming before stack (1984) Geophysics, 49 (11), pp. 2064-2067Bevc, D., (1995) Imaging Under Rugged Topography and Complex Velocity Structure, , (Ph.D. dissertation), Stanford UniversityBeylkin, G., Imaging of discontinuities in the inverse scattering problem by inversion of a generalized Radon transform (1985) J. Math. Phys., 26 (1), pp. 99-108Bleistein, N., (1984) Mathematical Methods for Wave Phenomena, , Academic Press, New YorkBleistein, N., Two-and-one-half dimensional in-plane wave propagation (1986) Geophys. Prospect., 34, pp. 686-703Bleistein, N., On the imaging of reflectors in the earth (1987) Geophysics, 52, pp. 931-942Červený, V., de Castro, M.A., Application of dynamic ray tracing in the 3-D inversion of seismic reflection data (1993) Geophys. J. Int., 113, pp. 776-779Hanitzsch, C., Schleicher, J., Hubral, P., True-amplitude migration of 2D synthetic data (1994) Geophys. Prospect., 42 (5), pp. 445-462Hubral, P., Krey, T., Interval velocities from seismic reflection time measurements (1980) Soc. Expl. Geophys., Tulsa, OK, , K. Larner (Ed.)Hubral, P., Schleicher, J., Tygel, M., Three-dimensional paraxial ray properties - part II. Applications (1992) J. Seism. Explor., 1 (4), pp. 347-362Hubral, P., Schleicher, J., Tygel, M., A unified approach to 3-D seismic reflection imaging - part I: Basic concepts (1996) Geophysics, 61 (3), pp. 742-758Martins, J., Schleicher, J., Tygel, M., Santos, L.T., 2.5-D true-amplitude Kirchhoff migration and demigration (1997) J. Seism. Explor., 6 (2-3), pp. 159-180Schleicher, J., Tygel, M., Hubral, P., 3-D true-amplitude finite-offset migration (1993) Geophysics, 58 (8), pp. 1112-1126Schleicher, J., Tygel, M., Hubral, P., (2007) Seismic True-amplitude Imaging. No. 12 in Geophysical Developments, , SEGSchneider, W.A., Phillip, L.D., Paal, E.F., Wave-equation velocity replacement of the low-velocity layer for overthrust-belt data (1995) Geophysics, 60, pp. 573-580Schuster, G.T., Zhou, M., A theoretical overview of model-based and correlation-based redatuming methods (2006) Geophysics, 71 (4), pp. SI103-SI110Sheriff, R.E., Factors affecting seismic amplitudes (1975) Geophys. Prospect., 23, pp. 125-138Tegtmeier, S., Gisolf, A., Verschuur, E., 3D sparse data Kirchhoff redatuming (2004) Geophys. Prospect., 52 (6), pp. 509-521Tygel, M., Schleicher, J., Hubral, P., Pulse distortion in depth migration (1994) Geophysics, 59, pp. 1561-1569Tygel, M., Schleicher, J., Hubral, P., Dualities involving reflectors and reflection-time surfaces (1995) J. Seism. Explor., 4, pp. 123-150Tygel, M., Schleicher, J., Hubral, P., A unified approach to 3-D seismic reflection imaging - part II: theory (1996) Geophysics, 61 (3), pp. 759-775Tygel, M., Schleicher, J., Hubral, P., Santos, L.T., 2.5D true-amplitude Kirchhoff migration to zero offset in laterally inhomogeneous media (1998) Geophysics, 63 (2), pp. 557-573Wapenaar, C.P.A., Kirchhoff-Helmholtz downward extrapolation in a layered medium with curved interfaces (1993) Geophys. J. Int., 115, pp. 445-455Wapenaar, C.P.A., Cox, H.L.H., Berkhout, A.J., Elastic redatuming of multicomponent seismic data (1992) Geophys. Prospect., 40 (4), pp. 465-482Wiggins, J.W., Kirchhoff integral extrapolation and migration of nonplanar data (1984) Geophysics, 49, pp. 1239-1248Yilmaz, O., Lucas, D., Prestack layer replacement (1986) Geophysics, 51, pp. 1355-136
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
Trimpi Occurrence And Geomagnetic Activity: Analysis Of Events Detected At Comandante Ferraz Brazilian Antarctic Station (l = 2.25)
No full textWe present an analysis of the occurrence of Trimpi events observed at Comandante Ferraz Brazilian Antarctic Station (EACF), at L = 2.25, as observed by the amplitude of very low frequency (VLF) signals transmitted from Hawaii (NPM 21.4 kHz) from April 1996 to August 1999. The event parameters (total duration, amplitude variation, time incidence, and type (negative or positive)) were analyzed for 4394 events detected in the first year (solar minimum and relatively low Trimpi activity). The Trimpi occurrence was compared with geomagnetic activity for the whole period. A higher incidence of Trimpi events was observed near the equinoxes, with minima at the solstices following the same behavior as the Ap indices annual variation. During the day, their incidence peaked at ~0700 and ~1045 UT in accordance with the occurrence of lightning flashes in the geomagnetic conjugate regions but apparently not in agreement to the daily mean geomagnetic variation (Kp index) found to be peaked (in entire years sampling average) at 0300 UT and weaker at 0900 UT for the whole 4-year period. The relationship between Trimpi occurrence and rise in solar activity for cycle 23 is discussed. A sudden strong incidence of Trimpi events around 5 May 1998 and 27 August 1998 was attributed to pronounced magnetic storms (Dst index). Copyright 2003 by the American Geophysical Union.108A1Bilitza, D., (1995) Adv. Space Res, 20 (9), pp. 1751-1754. , International reference ionosphere-Status, 96Bolton, S., One year variation in the near Earth solar wind ion density and bulk flow velocity (1990) Geophys. Res. Lett, 17, pp. 37-40Budden, K.G., (1961) The Waveguide Mode Theory of Wave Propagation, , Logos Press, London, UKCarpenter, D.L., Inan, U.S., Seasonal, latitudinal and diurnal distributions of whistler-induced electron precipitation events (1987) J. Geophys. Res, 92, pp. 3429-3435Friedel, R.H.W., Hughes, A.R.W., Characteristics and frequency of occurrence of Trimpi events recorded during 1982 at Sanae, Antarctica (1990) J. Atmos. Terr. Phys, 52, pp. 329-339Helliwell, R.A., Katsufrakis, J.P., Trimpi, M.L., Whistler-induced amplitude perturbation in VLF propagation (1973) J. Geophys. Res, 78, pp. 4679-4688Inan, U.S., Bell, T.F., Chang, H.C., Particle precipitation induced by short-duration VLF waves in the magnetosphere (1982) J. Geophys. Res, 87, pp. 6243-6264Inan, U.S., Carpenter, D.L., Lightning-induced electron precipitation events observed at L ∼ 2.4 as phase and amplitude perturbations on subionospheric VLF signals (1987) J. Geophys. Res, 92, pp. 3293-3303Inan, U.S., Lehtinen, N.G., Lev-Tov, S.J., Johnson, M.P., Bell, T.F., Hurley, K., Ionization of the lower ionosphere by gamma-rays from a magnetar: Detection of a low energy (3-10 keV) component (1999) Geophys. Res. Lett, 26, pp. 3357-3360Johnson, M.P., Inan, U.S., Lauben, D.S., Subionospheric VLF signatures of oblique (nonducted) whistler-induced precipitation (1999) Geophys. Res. Lett, 26, pp. 3569-3572Johnson, M.P., Inan, U.S., Lev-Tov, S.J., Bell, T.F., Scattering pattern of lightning-induced ionospheric disturbances associated with early/fast VLF events (1999) Geophys. Res. Lett, 26, pp. 2363-2366Lauben, D.S., (1998) Precipitation of radiation belt electrons by obliquely propagating lightning-generated whistler waves, , Ph.D. thesis, Dept. of Electr. Eng, Stanford Univ, Stanford, Calif, OctoberLauben, D.S., Inan, U.S., Bell, T.F., Poleward-displaced electron precipitation from lightning-generated oblique whistlers (1999) Geophys. Res. Lett, 26, pp. 2633-2636Lev-Tov, S.J., Inan, U.S., Bell, T.F., Altitude profiles of localized D region density disturbances produced in lightning-induced electron precipitation events (1995) J. Geophys. Res, 100, pp. 21,375-21,383Orville, R.E., Huffines, G.R., Cloud-to-ground lightning in the United States: NLDN results in the first decade, 1989-98 (2000), pp. 1179-1193. , 5Poulsen, W.L., Bell, T.F., Inan, U.S., Three-dimensional modeling of subionospheric VLF propagation in the presence of localized D region perturbations associated with lightning (1990) J. Geophys. Res, 95, pp. 2355-2366Rycroft, M.J., Interactions between whistler mode waves and energetic electrons in coupled system formed by the magnetosphere, ionosphere and atmosphere (1991) J. Atmos. Terr. Phys, 53, pp. 849-858Rycroft, M.J., Israelsson, S., Price, C., The global atmospheric electric circuit, solar activity and climate change (2000) J. Atmos. Sol. Terr. Phys, 62, pp. 1563-1576Wait, J.R., (1962) Electromagnetic Waves in Stratified Media, , Pergamon, New YorkZajac, B.A., Rutledge, S.A., Cloud-to-ground lightning activity in the contiguous United States from 1995 to 1999 (2000) Mon. Weather Rev, 129 (5), pp. 999-1019Zieger, B., Mursula, K., Annual variation in near-Earth solar wind speed: Evidence for persistent north-south asymmetry related to solar magnetic polarity (1998) Geophys. Res. Lett, 25, pp. 841-84
Experiments on the structure and stability of mode-2 internal solitary-like waves propagating on an offset pycnocline
No full textThe structure and stability of mode-2 internal solitary-like waves is investigated experimentally. A rank-ordered train of mode-2 internal solitary waves is generated using a lock release configuration. The pycnocline is centred either on the mid-depth of the water column (the 0% offset case) or it is offset in the positive vertical direction by a fraction of 5%, 10% or 20% of the total fluid depth. It is found that offsetting the pycnocline has little effect on the basic wave properties (e.g wave speed, wave amplitude and wave length) but it does significantly affect wave stability. Instability takes the form of small K-H-like billows in the rear of the wave and small scale overturning in the core of the wave. In the 0% offset case, instability occurs on both the upper and lower interfaces of the pycnocline and is similar in extent and vigour over the two interfaces. As the offset percentage is increased, however, instability is more pronounced on the lower interface with little or no evidence of instability being observed on the upper interface. In the 20% offset case a mode-1 tail is associated with the wave and the wave characteristics resemble qualitatively the recent field observations of Shroyer et al [E. L. Shroyer, J. N. Moum and J. D. Nash, J. Geophys. Res. 115, C07001 (2010)].Peer reviewe
A deterministic algorithm for experimental design applied to tomographic and microseismic monitoring surveys
No full textMost general experimental design algorithms are either: (i) stochastic and hence give different designs each time they are run with finite computing power, or (ii) deterministic but converge to results that depend on an initial or reference design, taking little or no account of the range of all other possible designs. In this paper we introduce an approximation to standard measures of experimental design quality that enables a new algorithm to be used. The algorithm is simple, deterministic and the resulting experimental design is influenced by the full range of possible designs, thus addressing problems (i) and (ii) above. Although the designs produced are not guaranteed to be globally optimal, they significantly increase the magnitude of small eigenvalues in the model–data relationship (without requiring that these eigenvalues be calculated). This reduces the model uncertainties expected post-experiment. We illustrate the method on simple tomographic and microseismic location examples with varying degrees of seismic attenuation
Stress permutations: 3-D distinct element analysis accounts for a common phenomenon in brittle tectonics.
No full textProperties of mass-loading shocks: 1. Hydrodynamic considerations
No full textThe one-dimensional hydrodynamics of flows subjected to mass loading are considered anew, with particular emphasis placed on determining the properties of mass-loading shocks. This work has been motivated by recent observations of the outbound Halley bow shock (Neubauer et al., 1990), which cannot be understood in terms of simple hydrodynamical or magnetohydrodynamical descriptions. By including mass injection at the shock, we have investigated the properties of the Rankine-Hugoniot conditions on the basis of a geometric formulation of the entropy condition. Such a condition, which is more powerful than the usual thermodynamical formulation, serves to determine those solutions to the Rankine-Hugoniot conditions which correspond to a physically realizable downstream state. On this basis a concise theoretical description of hydrodynamic mass-loading shocks is obtained. We show that mass-loading shocks have more in common with combustion shocks than with ordinary nonreacting gas dynamical shocks. It is shown that for decelerated solutions to the Rankine-Hugoniot conditions to exist, the upstream flow speed u0 must satisfy u0 > ucrit > cs, where cs is the sound speed. Besides the usual supersonic-subsonic transition, mass-loading fronts can also admit a decelerating supersonic-supersonic transition, the structure of which consists of a sharp decrease in the flow velocity preceding a recovery and an increase in the final downstream flow speed. We suggest the possibility that such structures may describe the inbound Halley bow shock (Coates et al., 1987a). Both parallel and oblique shocks are considered, the primary difference being that oblique shocks are subjected to a shearing stress due to mass loading. It is conjectured that such a shearing may destabilize the shock
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