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Geophysics ◽  
2021 ◽  
pp. 1-67
Author(s):  
Yuzhu Liu ◽  
Weigang Liu ◽  
Zheng Wu ◽  
Jizhong Yang

Reverse time migration (RTM) has been widely used for imaging complex subsurface structures in oil and gas exploration. However, because only the adjoint of the forward Born modeling operator is applied to the seismic data in RTM, the output migration profile is biased in terms of the amplitude. To help partially balance the amplitude performance, the RTM image can be preconditioned with the inverse of the diagonal of the Hessian operator. Yet, existing preconditioning methods do not correctly consider the receiver-side effects, assuming that the receiver coverage is infinite or the velocity model is constant. We therefore provide a comparative study aiming to give a clearer understanding on the importance of incorporating the receiver-side effects by developing a frequency-domain scattering-integral reverse time migration (SI-RTM). In the proposed SI-RTM, the diagonal of the Hessian operator is explicitly computed in its exact formulation, and the source-side wavefield and receiver-side Green’s functions are obtained by solving the two-way wave equation. The computational cost is relatively affordable when compared with the more expensive least-squares RTM. In the comparative counterpart, the diagonal of the Hessian operator is approximated by the source-side illumination. We perform two synthetic numerical examples using an overthrust model and a complex reservoir model; the final migration images were significantly improved when the receiver-side effects were accurately considered. A third application of SI-RTM on one field data set acquired from the East China Sea further demonstrates the importance of incorporating the receiver-side effects in normalizing the RTM image. Findings of this study are expected to provide a theoretical basis for improving the ability of RTM imaging of subsurface structures, thereby critically advancing the application of geophysical techniques for imaging complex environments.


Author(s):  
Balkar Singh ◽  
◽  
M. K. Sharma ◽  

In this paper, a novel watermarking technique for the tamper detection of text images is pro- posed. Entropy of every sentence is computed and Markov matrix using the occurrences of the characters is used to generate a character pattern. Entropy and character patterns are converted to Unicode Zero Width Characters (ZWCs) by using a lookup table. The ZWCs of entropy of each sentence is embedded at the end of every sentence after terminator. ZWCs of the character patterns are embedded in the end of the text of the image. On receiver side, ZWCs are extracted and converted to numerical form using the same lookup table. Entropy of every sentence and character patterns are recalculated and compared with extracted values for tamper detection. Comparison of technique with existing state-of-art techniques shows the effectiveness of the proposed technique.


2021 ◽  
Vol 11 (20) ◽  
pp. 9776
Author(s):  
Longsheng Wei ◽  
Lei Zhao ◽  
Jian Peng

A reduced reference quality assessment algorithm for image retargeting by earth mover’s distance is proposed in this paper. In the reference image, all the feature points are extracted using scale invariant feature transform. Let the histograms of image patch around each feature point be local information, and the histograms of saliency feature as global information. Those feature information is extracted at the sender side and transmitted to the receiver side. After that, the same feature information extraction process is performed for the retargeted image at the receiver side. Finally, all feature information of the reference and retargeted images is used collectively to compute the quality of the retargeted image. An overall quality score is calculated from the local and global similarity measure using earth mover’s distance between reference and retargeted images. The key step in our algorithm is to provide an earth mover’s distance metric in a manner that indicates how the local and global information in the reference image is preserved in corresponding retargeted image. Experimental results show that the proposed algorithm can improve the image quality scores on four common criteria in the retargeted image quality assessment community.


Geophysics ◽  
2021 ◽  
Vol 86 (6) ◽  
pp. R913-R926
Author(s):  
Jianhua Wang ◽  
Jizhong Yang ◽  
Liangguo Dong ◽  
Yuzhu Liu

Wave-equation traveltime inversion (WTI) is a useful tool for background velocity model building. It is generally formulated and implemented in the time domain, in which the gradient is calculated by temporally crosscorrelating the source- and receiver-side wavefields. The time-domain source-side snapshots are either stored in memory or are reconstructed through back propagation. The memory requirements and computational cost of WTI are thus prohibitively expensive, especially for 3D applications. To partially alleviate this problem, we provide an implementation of WTI in the frequency domain with a monofrequency component. Because only one frequency is used, it is affordable to directly store the source- and receiver-side wavefields in memory. There is no need for wavefield reconstruction during gradient calculation. In such a way, we have dramatically reduced the memory requirements and computational cost compared with the traditional time-domain WTI realization. For practical implementation, the frequency-domain wavefield is calculated by time-domain finite-difference forward modeling and is transformed to the frequency domain by an on-the-fly discrete Fourier transform. Numerical examples on a simple lateral periodic velocity model and the Marmousi model demonstrate that our method can obtain accurate background velocity models comparable with those from time-domain WTI and frequency-domain WTI with multiple frequencies. A field data set test indicates that our method obtains a background velocity model that well predicts the seismic wave traveltime.


Electronics ◽  
2021 ◽  
Vol 10 (19) ◽  
pp. 2343
Author(s):  
Ya-Qiong Jia ◽  
Guo-Ping Jiang ◽  
Hua Yang ◽  
Bin Yu ◽  
Ming-Di Du

A new fractional-order multi-carrier M-ary differential chaos shift keying system with multilevel code-shifted modulation (MC-MDCSK-MCS) is presented in this paper. The proposed system adopts multiple subcarriers, on which multiple MCS-MDCSK-modulated signals are transmitted simultaneously. Moreover, M-ary modulation has been combined with the proposed system to achieve a higher bit rate. On the receiver side, the recovered reference signal is first averaged and then used for MCS-MDCSK demodulation, which helps improve performance. We analyze the bit error rate (BER) of the proposed system and verify our theoretical derivations with the simulation results over additive white Gaussian noise (AWGN) and Rayleigh fading channels. Finally, related comparisons are completed, which show that the MC-MDCSK-MCS system is excellent and promising.


2021 ◽  
Author(s):  
Dushyant Singh Chauhan ◽  
Gurjit kaur ◽  
Dinesh Kumar

Abstract The performance of FSO communication link is subject to numerous atmospheric factors in wireless communication like fog, rainfall, and haze which leads to deteriorate the performance of a system in term of BER and power at the receiver side. Due to numerous advantages of OCDMA over other access techniques, it allows various users to access a channel simultaneously without intervention with the other user. It has the ability to provide security, large number of users, privacy, reduce interference from multiple users and operate asynchronously. So, in this paper, Multi-Diagonal codes along with the fiber brags grating filters for Spectral amplitude coded OCDMA technique is developed for FSO system and performance analysis is performed in terms of BER and received power.


Electronics ◽  
2021 ◽  
Vol 10 (14) ◽  
pp. 1639
Author(s):  
Kaiwen Chen ◽  
Ka Wai Eric Cheng ◽  
Yun Yang ◽  
Jianfei F. Pan

Receiver-side control has been a reliable practice for regulating the transferred energy to the batteries in the electric vehicle (EV) wireless power transfer (WPT) systems. Nonetheless, the unpredictable fluctuation of the mutual inductance in dynamic wireless charging brings extreme instability to the charging process. This overshoot that appears in instant vibrations may largely increase the voltage/current stress of the system, and even cause catastrophic failure in the battery load. In addition, the speed of the vehicles may lead to untraceable steady-state operation. However, existing solutions to the above two issues suffer from either long communication time delay or significantly compromised output regulation. In this paper, the slow dynamics and the overshoot issues of the WPT system are elaborated in theory, and the small-signal model mainly considering mutual inductance disturbance is established. A simple feedforward control is proposed for overshoot damping and fast system dynamics. Experimental results validate that the overshoot can be reduced by 13% and the settling time is improved by 50% in vehicle braking or acceleration. In constant speed driving, the battery charging ripple is decreased by 12% and ensures better system stability.


2021 ◽  
Vol 25 (4) ◽  
pp. 84-94
Author(s):  
Sajjad R. Abdulridha ◽  
◽  
Fadhil S. Hasan ◽  

The main drawback of the Orthogonal Frequency Division Multiplexing (OFDM) with Differential Chaos Shift Keying (DCSK) that is named (OFDM-DCSK) is the high Peak to Average Power Ratio (PAPR). In this paper, clipping and companding techniques are suggested to overcome the PAPR problem in the OFDM-DCSK system. For the clipping technique, the clipping function is applied before transmitting the signal without the need for an inverse function at the receiver side. While for companding techniques, the commanding function is applied at the end of the transmitter side and the corresponding decompanding function is applied at the receiver to recover the original signal. Different companding techniques are investigated including Hyperbolic, A-Law, and Mu-Law companding function that are compared with the Palm clipping technique. The MATLAB simulation result shows that the Mu-Law technique has the best PAPR reduction (7.22 dB) with a good bit error rate (BER) performance when the number of subcarriers is equal to 512.


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