In deep water the source-receiver offsets that are required for accurate velocity determinations cannot be achieved with single-ship multichannel seismic methods. Two ships, one equipped with a multichannel receiving array and the other with a seismic source, have previously been employed to acquire common midpoint, expanding spread profiles, principally to determine deep crustal velocity structures. We extend this method to higher resolution in space and time to determine the velocities of sedimentary layers in deep water offshore Japan in the Nankai Trough. This high-resolution two-ship data acquisition method used a 13.1-L water gun source array; a 1.6-km, 96-channel receiving array with 0.0166-km active group; and shore-based navigation. Analysis of the data was performed in the r-p domain by successive downward continuation of the plane wave data. Interactive velocity analysis methods for both one-dimensional and two-dimensional Earth models are described for both common source/receiver and common midpoint profiles. Results in one of two areas surveyed show a low-velocity zone below the subduction decollement which is consistent with models of low wedge taper, high pore fluid pressure, and reflection polarity reversal described previously by other researchers. The velocity profiles show the expected landward increase in velocity assumed to be due to lateral strain and porosity decrease, but the effect is small, only slightly greater than would be expected in an area of no lateral strain. PROFILES IN NANKAI TROUGH 1689 variations [Diebold and Stoffa, 1981]. This is in contrast to fixed source or receiver, such as ocean bottom seismometer (OBS) profiles. The source ship (R/V Tansei Maru, University of Tokyo) was equipped with two 6.55-L water guns, and the receiving ship (R/V Fred H. Moore, University of Texas) was equipped with a 1.6-km, 96-channel receiving array with a group spacing of 0.0166 km. The small group spacing ensured fine spatial resolution at all offsets. The water gun source yielded a short pulse for good time resolution, and the data were recorded with a 0.002-s sample interval corresponding to a Nyquist frequency of 250 Hz. This was done to ensure recording of the peak power of the water gun wavelet which extends to greater than 80 Hz. Because lateral changes in geology add to the complexity of the velocity analysis, each of the profiles was shot along strike (determined from Sea Beam bathymetric data [Kaiko I Research Group, 1986]) to minimize lateral inhomogeneities.Unfortunately, ocean currents in the more eastern of the two study areas (Figure 1) were too swift to allow the ships to maintain a common midpoint over the seafloor. Since the ship speed through the water affects not only the ground position but the noise level of the towed array, there is a range of speeds outside which the array either is not in the proper position or is too noisy to record high-quality data. In the eastern area, steaming against the current would have required a 12-14 km/hr speed through the water and steami...
We obtain the transverse electric (TE) and transverse magnetic (TM) Fresnel reflection coefficients for different interfaces in the subsoil: air/fresh‐water, air/seawater, fresh‐water/seawater, air/NAPL (non‐aqueous phase liquid), NAPL/water and water/NAPL. We consider a range of NAPL saturations, where the complementary fluid is water with 0.65 ppt (parts per thousand) of NaCl. The common feature is that the TM mode (parallel polarization) has a negative anomaly and the TE mode (perpendicular polarization) has a positive anomaly. For the cases studied in this work, pseudo‐Brewster angles appear beyond 40° for the air/NAPL and NAPL/water interfaces and at near offsets (below 40°) for the water/NAPL interface. Pseudo‐critical angles are present for the water/NAPL interface. Besides the reflection strength, the phase angle can be used to discriminate between low‐ and high‐conductivity NAPL, when the properties of the upper medium are known. A wavenumber–frequency domain method is used to compute the reflection coefficient and phase angle from synthetic radargrams. This method and the curves can be used to interpret the amplitude variations with angle (AVA) of reflection events in radargrams obtained with ground‐penetrating radar (GPR).
Extreme catastrophic events such as earthquakes, terrorism and economic collapses are difficult to predict. We propose a tentative mathematical model for the precursors of these events based on a memory formalism and apply it to earthquakes suggesting a physical interpretation. In this case, a precursor can be the anomalous increasing rate of events (aftershocks) following a moderate earthquake, contrary to Omori's law. This trend constitute foreshocks of the main event and can be modelled with fractional time derivatives. A fractional derivative of order 0 < ν < 2 replaces the first-order time derivative in the classical diffusion equation.We obtain the frequency-domain Green's function and the corresponding time-domain solution by performing an inverse Fourier transform. Alternatively, we propose a numerical algorithm, where the time derivative is computed with the Grünwald-Letnikov expansion, which is a finitedifference generalization of the standard finite-difference operator to derivatives of fractional order. The results match the analytical solution obtained from the Green function. The calculation requires to store the whole field in the computer memory since anomalous diffusion "remembers the past".MSC 2010 : Primary 35R11; Secondary 86A15, 86A17, 86-08
RESUMOForam estudados nove perfis ao longo de uma toposseqüência sobre os sedimentos do Grupo Barreiras, na Fazenda Rio Negro, município de Entre Rios (BA), utilizando a prospecção eletromagnética por meio do Radar Penetrante no Solo -"Ground-penetrating radar -GPR", objetivando analisar a utilização dessa ferramenta na aquisição de informações sobre as feições que ocorrem no solo, mediante a comparação entre os radargramas obtidos e a descrição pedológica. O equipamento utilizado foi um Geophysical Survey System modelo GPR SR system-2, com antena de 80 MHz. A análise radargramétrica confirmou o aparecimento dos fragipãs e duripãs em profundidade, que ocorrem sempre acompanhados de um processo de transformação dos solos do tipo Latossolo Amarelo e Argissolo Amarelo em Espodossolo. Os padrões de reflexão mostram claramente os domínios dos solos argilosos e dos solos arenosos, com e sem a presença dos horizontes endurecidos.Termos de indexação: prospecção do solo, gênese do solo, Grupo Barreiras.(1) Parte da Tese de Doutorado do primeiro autor. Recebido para publicação em setembro de 1999 e aprovado em setembro de 2001.
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