Journal of Computational Acoustics

A partial friction heterogeneity model of a random-compacted granular pack can be formulated using two parameters ($f_p$,$f(\mu ))$ to account for (1) macroscopic stress heterogeneity distribution of grain packs and (2) microscopic partial friction on the area of grain contact. This proposed partial friction heterogeneity model indicates that surface friction between grain contact is nonignorable, which results in tangential loading path being irreversible. Meanwhile, the tangential stress causes partial sliding on grain contact surface. Macroscopically, the effective shear modulus increases when either fraction of partial slip grain contacts or micro-friction on the area of grain contacts increases.

A new deconvolution algorithm for retrieving a sparse reflectivity series from noisy seismic traces is proposed. The problem is formulated as a constrained minimization, taking the approximation zero norm of reflectivity as the objective function. The resulting minimization is solved efficiently by the trust-region based sequential quadratic programming (SQP) method, which provides global convergence and local quadratic convergence rates under suitable assumptions. The null space decomposition method and the de-biasing method are employed to reduce computational complexity and further improve the calculation accuracy. Synthetic simulations indicate that the spikes on the reflectivity, both their positions and amplitudes, are recovered effectively by the proposed approach.

The present study involves numerical assessment of two types of boundary elements, namely constant and discontinuous quadratic elements based on a hypersingular Burton and Miller boundary integral formulation to tackle spurious frequencies manifesting in exterior problems. Convergence trends of the two types of boundary element with/without the inclusion of hypersingular formulation were studied for various combinations of boundary conditions and over a wide range of frequencies. The results indicate that discontinuous quadratic elements and constant elements give comparable results, with the quadratic elements being computationally more efficient as they take lesser computational time. Nevertheless, the constant element formulation is easier to implement, and it may be used for solving exterior wave propagation problems.

Since the performance of orthogonal frequency division multiplexing (OFDM) system is severely limited by frequency-dependent Doppler shifts over underwater acoustic (UWA) channels, a combined Doppler scale estimation scheme is proposed, containing two steps: (1) the initial Doppler scale estimation via transmitting two identical short OFDM symbols preceded by a cyclic prefix (CP) as a preamble prior to the data transmission; and (2) fine synchronization and Doppler scale estimation for each CP-OFDM block are achieved by the CP-based method, and eventually the resampling methodology is adopted for nonuniform Doppler shifts compensation. The proposed scheme has been tested with real data from the Songhua Lake experiment where the transmitter is moving at a speed of about 2.5 m/s, and the Yellow sea trial in which the transmitter and receiver are floating freely with waves and currents. Excellent performance results are obtained by block-by-block processing, without buffering the whole packet, which is suitable for real-time receiver processing over time-varying UWA channels.

On the Southern Marion carbonate platform, dolomitization is triggered by the circulation of normal or slightly modified seawater and is related to changes in sedimentation rate and sea level change. Dolomitization further modifies formation permeability and fluid flow patterns. Dolostone/calcareous dolostone with large vuggy or moldic porosity is formed by fabric-preserving dissolution and recrystallization, which increases the pore space and facilitates the fluid flow effectively, with permeability ranging from 1mD to 10,000mD. The frame flexibility factor ($\gamma$) is a rock physics parameter which is a proxy of pore structure. We find that at given porosity dolostone with larger pores, higher permeability and higher sonic velocity usually has lower values of frame flexibility factor than limestone. After strong compaction and cementation, the limestone frame occludes fluid flow, prevents dolomitization and has permeability as low as 0.02mD.

Acoustic impedance inversion confirms that the asymmetric geometry of the Southern Marion platform is shaped by the oceanographic currents, which are caused by the southward-flowing East Australian Current. Three layers of dolostone with large pores in the upper platform reveal strong fluid flow within the carbonate platform, leading to dolomitization and dissolution. These three strongly dolomitized zones follow the platform topography, indicating that the diagenetic fluid flow is driven by oceanographic currents. Three large-pore-formation dolomitization events match well with three highstands of sea level events, illustrating that the highstand of sea level induces the formation of dolomitization zones with large pores. This study demonstrates the positive feedback loop of dolomitization and ocean current circulation, as well as the relationship between dolomitization and sea level change, which could be applicable for better understanding subsurface fluid-rock interactions and dolomitization pore systems in other carbonate environments.

Acoustic impedance in carbonates is influenced by factors such as porosity, pore structure/fracture, fluid content, and lithology. Occurrence of moldic and vuggy pores, fractures and other pore structures due to diagenesis in carbonate rocks can greatly complicate the relationships between impedance and porosity. Using a frame flexibility factor ($\gamma$) derived from a poroelastic model to characterize pore structure in reservoir rocks, we find that its product with porosity can result in a much better correlation with sonic velocity ($V_p$ = $A-B\ast \varphi \ast \gamma$) and acoustic impedance ($AI$ = $C-D\ast \varphi \ast \gamma )$, where A, B, C and D is 6.60, 0.03, 18.3 and 0.09, respectively for the deep low-porosity carbonate reservoir studied in this paper. These new relationships can also be useful in improving seismic inversion of ultra-deep hydrocarbon reservoirs in other similar environments.

Direction of arrival (DOA) estimation is of great interest due to its wide applications in sonar, radar and many other areas. However, the near-field interference is always presented in the received data, which may result in degradation of DOA estimation. An approach which can suppress the near-field interference and preserve the far-field signal desired by using a spatial matrix filter is proposed in this paper and some typical DOA estimation algorithms are adjusted to match the filtered data. Simulation results show that the approach can improve capability of DOA estimation under near-field inference efficiently.

A new algorithm involving flank array geometry calibration under strong multipath conditions is proposed to address the problem of installation errors and the shell deformation. The derived linear mapping relationship between geometric error of sensors and signal eigenvector reduces the calculating difficulty in near calibration mode. By regarding the reflection as coherent visual sources in known position, the compensation strategy of strong multipath matching is put forward. Cramer–Rao bound (CRB) analysis for the calibration mode is employed. Numerical simulations and lake trials verify the efficiency of the proposed algorithm and illustrate the performance improvement under strong multipath conditions.

We have conducted simultaneous ultrasonic velocity and pore volume change measurements on a carbonate rock sample. By including of pressure dependent porosity data, we have improved Cheng’s pore aspect ratio spectrum inversion methodology and made the inverted pore aspect ratio spectrum more realistic. Tang’s unified velocity dispersion and attenuation model is modified and extended to poroelastic media with complex pore structure under undrained condition. Using improved pore aspect ratio spectra inversion methodology and modified Tang’s model, we have explored the potential application of pore aspect ratio spectrum in prediction of seismic wave dispersion and attenuation.

Highly dispersive surface waves resulting from the combination of strong lateral seafloor heterogeneities, shallow water depths and hard sea bottom severely degrade seismic reflection data quality. Considering that seismic signals are nonstationary and surface waves have various spectra over time, we proposed S-transform-based time frequency wavenumber analysis technique which allows the dynamic analysis of spectrum over time. The data is first transformed from the time-space domain to the time-wavenumber domain through one-dimensional Fourier transform over the spatial variable, then the variable-factor S-transform is applied over time. Nonstationary filtering is then designed to identify and separate surface waves in complex wavefields, based on its low frequency and low velocity properties, in the time-frequency-wavenumber (TFK) domain. Application to field data illustrates that with this technique, not only is the surface wave effectively suppressed, but also the reflective signals are enhanced, which confirm the validity of the method.

The extremely long underwater channel delay spread causes severe inter-symbol interference (ISI) for underwater acoustic communications. Passive time reversal processing (PTRP) can effectively reduce the channel time dispersion in a simple way via convolving the received packet with a time reversed probe signal (PS). However, the PS itself may introduce extra noise and interference (self-correlation of the PS). In this paper, we propose a virtual time reversal processing (VTRP) for single input single output (SISO) orthogonal frequency division multiplexing (OFDM) systems. It convolves the received packet with the reversed estimated channel, instead of the PS to reduce the interference. Two sparse channel estimation methods, matching pursuit (MP), and basis pursuit denoising (BPDN), are adopted to estimate the channel impulse response (CIR). We compare the performance of VTRP with the PTRP and without any time reversal processing through MATLAB simulations and the pool experiments. The results reveal that VTRP has outstanding performance over time-invariant channels.

Molecular dynamics (MD) simulations have been performed to study sound wave propagation in a simple monatomic gas (argon) and the thermo-acoustic effects on a single walled carbon nanotube (CNT). The objective of this study was to understand the acoustic behavior of CNTs in the presence of acoustic waves propagating in gaseous media. A plane sound wave was generated within a rectangular domain by oscillating a solid wall comprising Lennard-Jones (LJ) atoms with the same intermolecular potential as the gas molecules. A CNT was aligned parallel to the direction of the flow at the wall at the opposite end of the domain. Interatomic interactions in the CNT were modeled using the REBO potential. The behavior of the sound wave propagation in argon gas without the CNT was validated by comparison with a previous study. The simulation results show that the thermo-acoustic behavior of CNTs can be simulated accurately using MD and that large-scale MD can be performed in the ultrasonic frequency range. This investigation will contribute to an improved understanding of the acoustic absorption mechanism of these nanoscopic fibers.