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Public Reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comment regarding this burden estimates or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and Finite-difference, time-domain ͑FDTD͒ calculations are typically performed with partial differential equations that are first order in time. Equation sets appropriate for FDTD calculations in a moving inhomogeneous medium ͑with an emphasis on the atmosphere͒ are derived and discussed in this paper. Two candidate equation sets, both derived from linearized equations of fluid dynamics, are proposed. The first, which contains three coupled equations for the sound pressure, vector acoustic velocity, and acoustic density, is obtained without any approximations. The second, which contains two coupled equations for the sound pressure and vector acoustic velocity, is derived by ignoring terms proportional to the divergence of the medium velocity and the gradient of the ambient pressure. It is shown that the second set has the same or a wider range of applicability than equations for the sound pressure that have been previously used for analytical and numerical studies of sound propagation in a moving atmosphere. Practical FDTD implementation of the second set of equations is discussed. Results show good agreement with theoretical predictions of the sound pressure due to a point monochromatic source in a uniform, high Mach number flow and with Fast Field Program calculations of sound propagation in a stratified moving atmosphere.

SummaryThis paper reviews the physical fundamentals and engineering implementations for efficient information exchange via wireless communication using physical waves as the carrier among nodes in an underwater sensor network (UWSN). The physical waves under discussion include sound, radio, and light. We first present the fundamental physics of different waves; then we discuss and compare the pros and cons for adopting different communication carriers (acoustic, radio, and optical) based on the fundamental first principles of physics and engineering practice. The discussions are mainly targeted at underwater sensor networks (UWSNs) with densely deployed nodes. Based on the comparison study, we make recommendations for the selection of communication carriers for UWSNs with engineering countermeasures that can possibly enhance the communication efficiency in specified underwater environments.

Water level records from a high‐speed recorder in the Wali well have been compared with long‐period seismograms to study the response of the well to seismic waves. Water level oscillations caused by two teleseismic events have been studied. The data show that the peak gain of well water level relative to aquifer pressure occurred at periods of 19–23 s for both events, in contrast with previous theory that predicted peak gain at periods of 35–39 s for a well with 565 m open to the aquifer. We present a more exact analysis of the vertical flow field in the open part of the well bore, which yields a theoretical response with peak gain much closer to the observed periods. The more exact analysis shows that during the seismically induced oscillations little flow takes place more than 200 m below the top of the aquifer.

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This paper describes a generalized approximate method for assessing the effect of topography on the state of stress at depth in the crust and applies it to the site of the Cajon Pass Scientific Drilling Project. While previous studies have addressed idealized topography with regular shapes, we develop a three‐dimensional (3‐D) method for arbitrary surface topography. The analysis leads to four sets of elastic boundary value problems that are solved by convolutional methods based on the Green's functions of Boussinesq's problem and Cerruti's problem. The effect of topography on the state of stress can be divided into two parts by the linear superposition theorem. The first part is the effect on gravitational stress in the crust. In this case the effect of the topography can be expressed by the summation of the zero‐order vertical load and the first‐order horizontal shear at the base of the relief. The second part is the effect of topography on tectonic stress. In this case the effect of topography involves only the first‐order horizontal shear. The total effect of the topography on the state of stress in the crust can be obtained by adding the effect on gravitational stress with the effect on tectonic stress. The stresses caused by a simple case of 3‐D topography are calculated with this approach to serve as illustration. The identity of this study with previous studies (McTigue and Mei, 1981; Savage et al., 1985) for the same two‐dimensional case demonstrates the correctness of this method. In applying this technique to the Cajon Pass Scientific Drilling Project, we considered stresses induced by the topography associated with the San Gabriel and the San Bernadino mountains as a function of depth at the site. The regional topography induces stresses of several megapascals at the drilling site which decrease rapidly with depth. The induced maximum horizontal compressive stress is approximately in the N‐S direction. If we assume that in the absence of topography there is little right‐lateral shear stress acting on the San Andreas fault at depth (a weak fault assumption), the regional topography tends to cause a slight increase of right‐lateral shear on the San Andreas fault at a shallow depth in the crust. On the other hand, if we assume the strong fault model, the topography has no effect on the sense of shear on the San Andreas fault at all. Therefore it is clear that the small amount of left‐lateral shear stress on the San Andreas fault observed in the upper 3.5 km at the Cajon Pass site is not associated with the topographic effect.

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Field observations are tested against modal propagation theory to find the practical limitations upon derivation of layer permittivities and signal attenuation rates from a radar moveout profile over two-layer ground. A 65-MHz GPR pulse was transmitted into a 30-60-cmthick surface waveguide of wet, organic silty to gravelly soil overlying a drier refracting layer of sand and gravel. Reflection profiles, trench stratigraphy, resistivity measurements, and sediment analysis were used to quantify the propagation medium and possible attenuation mechanisms.Highly dispersive modal propagation occurred within the waveguide through 35 m of observation. The fastest phase velocity occurred at the waveguide cutoff frequency of 30 MHz, which was well received by 100-MHz antennas. This speed provides the refractive index of the lower layer, so the near-cutoff frequencies must match a lower layer refraction. A slower, lower frequency phase of the dispersed pulse occurred at about 60-70 MHz, with an average attenuation rate of about 0.4 dB/m. Similar events appear to have reflected back and forth along the waveguide. Modal theory for the average layer thickness shows all primary events to be different aspects of a TE 1 mode, predicts the correct 30-70-MHz phase speeds and low-frequency cutoff phenomenon, but also predicts that the 60-70-MHz group speed should be slightly lower than observed. An Airy phase was apparently out of the bandwidth. Two-dimensional finite-difference timedomain modeling qualitatively simulates the main field results.After accounting for an inverse dependency of amplitude on the square of the range, the high resistivity of the surface layer accounts for the 0.4-dB/m attenuation rate for the 60-70-MHz phase of the pulse. However, erratic amplitudes, interface roughness, and the reflected packets indicate scattering. We conclude that permittivities can be well estimated from dispersive moveout profiles given an average surface layer thickness, and the wide bandwidth of GPR antennas allows the full dispersion to be seen. Attenuation rates appear to be derivable from the higher frequency part of our dispersive event, for which attenuation might be least affected by the waveguide dispersion.