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The characteristic frequency from the low-frequency scattered fields was discussed when a pulse source is adopted to irradiate an underwater target. The total-field scattered-field source finite-difference time-domain (TSS-FDTD) method is used to analyze the scattered fields from the underwater target. Numerical results illustrate the characteristic frequency from an underwater metallic spheroid.

Micromotion characteristics of conical-shaped high-speed moving targets are presented using the Lorentz finite-difference time-domain (Lorentz-FDTD) method. This research discusses the transformation formulas of the space and time increments, the incident wave, the field components between the moving system, and the laboratory system in case of a micromotion target. The validity and accuracy of the proposed transformation formulas are verified by presenting the results for a moving cone and a hemisphere–cylinder combination target.

Electrical impedance tomography is a non-invasive, safe, portable imaging technique that enables images of electrical impedance to be reconstructed from voltages measured with an array of electrodes placed on the boundary. Typically, an image data set is collected as a series of many impedance measurements designed to sample as much of the volume as possible. The method, as a typical method for geological studies, is applied for submarine target detection. A method based on complete electrode model and singular-value-decomposition method is applied to reconstruct the conductivity distribution of the DOI. Some numerical results are presented with detailed discussions.

To implement the Lorentz finite-difference time-domain (FDTD) method in a 3D case, this communication presents the transformation formulas of the spatial and time increments, the field components, and the incident plane wave between the moving system and the laboratory system. The radiated fields from a moving dipole source are shown to verify the accuracy and validity of the proposed transformation formulas. The relative errors are within an acceptable range. The scattered fields of a moving metallic sphere are also illustrated to verify the validity of the proposed method. And, the scattered fields from a complex pyramid-shaped target are discussed.

An impulse plane wave is introduced in Lorentz finite-difference time domain for calculating the fields of high-speed moving dielectric. Applying the impulse plane wave and the fast Fourier transformation, we can directly read out the frequency shifts of the reflected and transmitted waves by one-time calculation. The proposed approach can greatly save run time and enhance the computation efficiency, especially in terms of considering a large number of incident frequencies. Two cases of moving canonical and noncanonical dielectric targets are considered to verify the accuracy and validity of the proposed method.

In this paper, a method for detecting the shape and position of object embeded in a conductive closed-domain with the acousto-electrical tomography is presented. Due to the difference of conductivity between the inclusion and the background material, we create a forward model to obtain the spatial potnetial distribution in the entire domain. Then, with the Levenberg-Marquardt algorithm the conductivity distribution is reconstructed by applying in the spatial potential distribution. A two-dimensional example is illustrated. Numerical results show that the shape and location of object in the domain can be clearly reconstructed.

In order to improve the computation efficiency for analyzing the underwater electromagnetic propagation at low frequency, a novel parallel total-field scattered-field source finite-difference time-domain (FDTD) method is presented in this letter. The accuracy and validity were verified by comparison to the analytical solution. Numerical results show that the parallel speedup of 13.68 and efficiency of 68.4% are obtained with 20 processors.

In order to precisely calculating the fields in air from an underwater magnetic dipole source, a novel method to introduce a low-frequency source is proposed. This method is built up by combining the analytical solution of the magnetic dipole source in the conductive medium with the total-field scattered-field boundary condition. Numerical results verified the validity and accuracy of the proposed method.

A cascaded total-field scattered-field source finite-difference time-domain (TSS-FDTD) method is used to analyze the underwater propagation properties of a low-frequency electromagnetic wave. In order to obtain the long-distance electromagnetic fields, two or more steps FDTD calculations are included. The relationship between two times FDTD calculations is the field data exchange on the TSS boundary. The grid ratio of two times calculation spaces can be set to an arbitrary positive odd integer, for instance N=5. The numerical results verify the accuracy and validity of the proposed method.

The scattered properties of a three-dimensional almond-shaped target, moving at high speed, is analyzed by the Lorentz-FDTD method. Numerical results show that the amplitude and frequency of the scattered wave are modulated due to the moving speed of the target.

A novel three-dimensional total-field scattered-field source finite-difference time-domain (TSS-FDTD) method is presented to analyze the propagation properties of low-frequency electromagnetic wave in underwater. The proposed method includes two-step FDTD calculations. The first calculation space is meshed with a fine cell size. The second is meshed with a coarse cell size. In addition, the ratio of a coarse cell size to a fine cell size is set to an arbitrary positive integer, such as $N=11$ in this letter. Numerical results prove the accuracy and validity of the proposed method. An analysis of the results shows that the run time spent in TSS-FDTD accounts for 0.35% of that in the conventional FDTD, and the consumed memory is 3.9% of that spent in the conventional FDTD.

In order to reduce the consumed run time for analyzing the propagation properties of a low-frequency electromagnetic wave in air or seawater, a novel approach of modelling the sea and the air with a lossy Drude dispersive medium is proposed. The numerical results verify the accuracy and validity of the proposed approach. The run time consumed can be reduced by increasing the time interval in the proposed approach.

In order to reduce the calculation time step in low frequency, a novel approach of modelling mediums with a lossy Drude dispersive medium is proposed. Numerical example illustrates the scattering properties of underwater targets with different shapes in f=110Hz and f=150Hz.

Based on the conventional finite-difference time-domain (FDTD) method, a novel total- field scattered-field source FDTD method with a dual-meshed technique is presented to deal with the underwater detection problems in low frequency electromagnetic wave. A transformation surface connecting the coarse cell with the fine cell is implemented by applying a total-field scattered-field source (TSS) technique, which is carried out by two-step FDTD simulation. The ratio of a coarse cell size to a fine cell size can be set as an arbitrary integer, such as N = 10. Moreover, an analysis of numerical error illustrates that non-physical reflection fields from the TSS surface are avoided by introducing the TSS surface. We have derived, in detail, the update equations of fields on grids of the TSS surface. Three cases of dealing with different underwater electromagnetic problems are discussed. Numerical results show that by analyzing the magnitude and phase of scattered fields from obstacles underwater we can distinguish the category of the obstacles which belong to either a high resistivity body or a low resistivity body. Therefore, the proposed method provides us an effective tool for analyzing the electromagnetic response of materials underwater.

In order to further detect underwater targets, such as oil, gas, and shipboard equipment and so on, we discuss the underwater radiation fields of a magnetic dipole array with 3×3 elements in the TSS-FDTD method. Numerical results show that the H z and E z amplitude and phase snapshots are symmetrical related to the z axis in the yoz plane in the working frequency f=180Hz. By analyzing the radiated fields, it is more readily to distinguish if there is an abnormal underwater target or not.