The Research Of Ternary Pseudo-random Encoding Sweep Technique And The Signal Generator For Vibrators

Vibrator is the most successful non-explosive source developed as yet. The excitation waveform can be artificially controlled which is a known function of the time, and it is transmitted into the ground with a long time, in other words, that use a long-time excitation waveform with small energy which can be controlled to replace a short-time excitation waveform with big energy from impact seismic source. The idea of Vibrators seismic exploration is that we send the continuous wave which has frequency, amplitude and phase that can be artificially controlled to the under-ground, and the wave will be reflected back when there are underground rock interface as spreading which then will be received and recorded by the receiving device set up on the earth surface—detector, and thus we realized the detection of the underground target after becoming the received signal to the delayed pulse signal. The major problems of Vibrators seismic exploration is that the resolution will be decline if the sidelobe of the correlation wavelet is too larger than the main lobe and the SNR will also be down if the end vibration (the relevant noise) generating after correction is too strong.Although the pulse function—the autocorrelation function of the white noise is the ideal autocorrelation function, pure random white noise can not be realized in a limited time. Therefore, the research of the pseudo-random sweeps signal whose autocorrelation function is closer to the pulse function is an effective way to improve the results of Vibrators seismic exploration.This paper uses multiple M-sequence to study the theory and method to construction ternary pseudo-random sequence with the ideal characteristics. Firstly, we use the coefficients of primitive polynomial in the finite fields to construct the recursive formula which can generate multiple M-sequence. Secondly, according to a specific mapping, we map the M-sequence to the ternary pseudo-random sequence with the ideal correction characteristics whose value is {-1, 0, 1}. This paper discusses the specific construction method of this ternary pseudo-random sequence in detail and completes the construction of ternary pseudo-random sequence which used to generate the ternary pseudo-random encoding sweeps signal.Ternary pseudo-random encoding sweeps signal is get by ternary pseudo-random sequence after modulation using the signal modulation technique of the sin carrier. In this paper, the elements of 1 and -1 are separately defined as a cycle of sine wave whose initial phase are 180O and 0O, and the element of 0 is defined the zero level. After the software simulation for the autocorrelation function of the ternary pseudo-random encoding sweeps signal, the conclusion is validated that the ternary pseudo-random encoding sweeps signal has better autocorrelation characteristics than the binary encoding M-sequence.In this paper, the system structure and the operation mode of the DDS chip of AD9852 are discussed, and the generator of the ternary pseudo-random encoding sweeps signal based on the AD9852 is designed and realized. Above all, using the Single-Tone operation mode of AD9852, MCU command AD9852 to produce sine wave corresponding to the elements of ternary pseudo-random sequence which only has a cycle each. And then, the ternary pseudo-random encoding sweeps signal required will be obtained after linking this single-cycle waveform. In addition, in the paper, the design of the hardware and software for signal generator are discussed in detail, also the problems encountered in the process of the system debugging are analyzed and the solutions are presented. At last, the quality of the output waveform is improved which provide a guarantee for the correct operation of the electric control part of Vibrators.At last, the developed ternary pseudo-random encoding sweeps signal generator is embedded into the electromagnetic driving Vibrators with the independent intellectual property rights in the laboratory, and the experiments of excitation acquisition are carried through. Through these experiments we found that, although the ternary pseudo-random encoding signals have the ideal characteristics, the distortion problem in the ternary pseudo-random encoding sweeps signal is more serious than in the binary situation and the autocorrelation function characteristics of the ternary pseudo-random encoding signals is worse than the binary, which is caused by the distortion of the substrate vibration signal because of the springs of the mechanical inertia. In order to improve the quality of signal substrate, a new ternary encoding method which has ideal correlation characteristics is presented and deduced theoretically in this paper. The experimental results show that, the correlation characteristics of the substrate signal using this method to get encoding sweeps signal is superior to the use of the binary pseudo-random sweeps as well as ordinary ternary pseudo-random encoding sweeps.