MULTI-VALUED CROSSCORRELATION FUNCTION FOR m-SEQUENCES

Based on the theory of exponential sums an d quadratic forms over finite field, the crosscorrelation function values betwee n two maximal linear recursive sequences are determined under some conditions.

A normalized wavefield separation cross-correlation imaging condition for reverse time migration based on Poynting vector

Prestack reverse time migration（PSTM） is a common imaging method; however low-frequency noises reduce the structural imaging precision. Thus, the suppression of migration noises must be considered. The generation mechanism of low-frequency noises is analyzed and the up-, down-, left-, and right-going waves are separated using the Poynting vector of the acoustic wave equation. The computational complexity and memory capacitance of the proposed method are far smaller than that required when using the conventional separation algorithm of 2D Fourier transform. The normalized wavefield separation crosscorrelation imaging condition is used to suppress low-frequency noises in reverse time migration and improve the imaging precision. Numerical experiments using the Marmousi model are performed and the results show that the up-, down-, left-, and right-going waves are well separated in the continuation of the wavefield using the Poynting vector. We compared the imaging results with the conventional method, Laplacian filtering, and wavefield separation with the 2D Fourier transform. The comparison shows that the migration noises are well suppressed using the normalized wavefield separation cross-correlation imaging condition and higher precision imaging results are obtained.

Research on High Speed Communication Method of ELF-EM While Drilling Based on Adaptive Combined Filtering Algorithm

In terms of the electromagnetic wave measurement while drilling,the extremely low frequency electromagnetic(ELF-EM)signal below 20Hz is usually chosen as the downhole data carrier.To improve the transmission rate of ELF-EM signal and the signal to noise ratio(SNR)at the receiving end,the DQPSK modulation was proposed as the modulation method for the communication of electromagnetic wave system.Different from the traditional IQ orthogonal modulation and coherent demodulation methods,the proposed phase selection modulation and correlation algorithm demodulation are easier to implement and more practical.With regard to the communication synchronization,a fast algorithm,which based on the normalized cross-relation number,was used for waveform matching,and the maximum point of the correlation coefficient was used as the starting point of communication synchronization.The communication simulation results show that the proposed DQPSK modulation signal based on the adaptive combined filtering algorithm has better terminal error rate and transmission rate than the traditional modulation method.Under the same carrier frequency and code width,the transmission rate of DQPSK modulation is 4 to 5 times and 2 times that of PPM modulation and 2DPSK modulation respectively.The communication modulation and demodulation modes as well as the decoding algorithm with combined adaptive filter proposed in this paper can effectively solve practical engineering problems.

Weighting Function Modification Used for Phase Transform-Based Time Delay Estimation

Generalized cross-correlation is considered as the most straightforward time delay estimation algorithm.Depending on various weighting function,different methods were derived and a straightforward method,named phase transform(PHAT)has been widely used.PHAT is well-known for its robustness to reverberation and its sensitivity to noise,which is partly due to the fact that PHAT distributes same weights to the frequencies dominated by signal or noise.To alleviate this problem,two weighting functions are proposed in this paper.By taking a posteriori signal-to-noise ratio(SNR)into account to classify reliable and unreliable frequencies,different weights could be assigned.The first proposed weighting function borrows the idea of binary mask and distributes same weights to frequencies in same set,whereas,the second one assigns weights based on coherence function.Experiments showed the robustness of proposed methods to reverberation and noise for improving the performance of time delay estimation through various criteria.

Acoustic Shock Emission in a Collision of a Drop with Water Surface

The collision of droplets with a water surface is being actively developed in the interests of many applied problems—transfer of matter through the ocean-atmosphere boundary,underwater acoustic noise of the marine environment,measurement of precipitation intensity,various technologies,and much more.One of the research priorities is acoustic radiation,in particular,shock sound arising at the moment a drop contacts the surface.The impact of the drop is preceded by processes that affect the shape of the drop,because of which it noticeably deviates from the spherical one.As a result,the final(contact)velocity changes–one of the most important parameters of collision.The study of the listed factors is carried out in this work by means of broadband acoustic measurements and high-speed video recording.

Cyclogram and cross correlation: A comparative study to quantify gait coordination in mental state

The purpose of this study to evaluate the effect of mental task on gait coordination. The comparison between two techniques Crosscorrelation and Cyclo- gram has been performed. A set of gait experiments was developed and conducted to evaluate the effect of mental task on gait coordination. The perimeter derived from the geometric figure, cyclogram perimeter (CP), of the knee-knee cyclogram is the main descriptor considered in this study. For crosscorrelation it is the peak value of cross correlation coefficient (CCC) that has been taken for comparison. The sensitivity of both the techniques in terms of percentage has been calculated. Crosscorrelation is highly sensitive (mean=20.4 S.D.=2.3), towards the change in gait coordination with mental task, in comparison to cyclogram perimeter (mean=2.2 S.D.=1.2). The results have strength to assess the progress of rehabilitation among Parkinson patients.

Suppression of internal multiples based on virtual events

In practical seismic exploration, internal multiples generated when the wave impedance of medium is strong, and seismic records are recorded. The method of virtual event repress internal multiples is to move scattered points from underground to the surface, similar to the method of the surface-related multiple elimination (SRME). The method of SRME belongs to the prediction-subtraction approaches to eliminate internal multiples, prediction method is based on building a brand new way of seismic wave propagation (virtual reflection and virtual event), so that it has forward and backward wave propagation, and through convolution with significant wave to predict the internal multiples. Due to required data needing field information of full-wave, the authors use Seislet transform interpolating the missing data to ensure the premise of internal multiples prediction. The test data show that the above method has achieved good results.

Fluctuant characteristics of two-phase flow behind a bottom aerator

Experimental observations show that the random process of two-phase flow behind an aerator is an ergodic process and its amplitude distribution is similar to a normal distribution. The maximum pressure fluctuation is at the re-attachment point where the jet-trajectory flow over the aerator re-attaches to the bottom of the channel, and its amplitude is 2—3 times larger than when there is no aerator. There is a dominant frequency of 1.24 Hz in the model, but the coherence in the frequency domain is not obvious for other frequencies beside the dominant frequency. There is a large vortex at the re-attachment point behind the aerator but correlation among the measurement points is not obvious in the time domain.

Auto-Cross-and Triple-Correlations of Sequences

Autocorrelations, crosscorrelations and triple correlations are three important concepts for signal design and analysis, although the triple ones are not so popular as auto-and cross-ones. This poper concentrates on designing new polyphase sequences with good correlations and on recovering signals from their triple correlations. The close relationship between Costas armys and the triple correlations Of m-sequences are initially recognized.