Research On Nodal Seismic Acquisition System And Its Denoising Algorithm

Since seismic exploration technology gradually moving towards fine,multiefficient,high-density and multi-wave multi-component acquisition,the level of seismic data increased dramatically.Distributed system with wire or not cannot adapt to the change,however,the nodal seismograph,characterized by cable-free,the ability to adapt complex terrain environment and the diversity of layout methods,can easily adapt the change.Therefore,the paper studied the cable-free nodal seismic data acquisition system,especially on the problems such as low resolution,low acquisition efficiency,low clock accuracy and low signal-to-noise ratio of seismic data.Research was carried out a semi-blind,nodal seismic acquisition system.The nodal system was developed on the i.mx6 platform and Linux,which mainly integrated low pass filter,three-component high-precision ADS1282 acquisition unit,STM32-based GPS/BD independent time-scale unit,data storage and command transmission unit and the PC data processing unit.The main work of this topic focuses on the following three aspects:Firstly,aiming at problems among the nodes at home and abroad,like low resolution and work efficiency,it developed the high-resolution,high-precision,lownoise,high-efficiency three-component acquisition unit.On the hardware,to improve the resolution of weak seismic signals and achieve low noise sampling,the threechannel 32-bit ?-? ADS1282 ADC was used to sample in an oversampling rate.The i.mx6 efficiently collected data by time division multiplexing technology through one SPI interface.On the software,the Linux kernel module was used to dynamically load the ADS1282 device and driver,then ran related processes to control device.Then,compared with application driver,the kernel driver avoided lots of system call which can cause the occupation and waste of system resources.And it separated the real-time operation from the non-real-time operation,which can improve the real-time performance and working efficiency of the system.Secondly,aiming at the problem of tens of milliseconds of error in the Linux system timing scheme,it developed the STM32-based GPS/BD independent time-scale unit with microsecond-level clock synchronization accuracy.STM32 taken as the secondary MCU to parse the time information,i.mx6 as the main CPU was used to collect seismic data and send the hardware time-request interrupt to STM32.And the GPS/BD receiver received the high-precision global satellite clock signal,after that,STM32 can get the UTC time through the serial port.At the same time,the local crystal oscillator was used to count the PPS pulse in seconds to obtain the microsecond time,and then it would be added to the UTC time string,which finally would return to i.mx6.This way can better avoid the system timing of the Linux on i.mx6,avoid the tens of millisecond delay caused by system call,process scheduling and control,thus improving the time precision.Thirdly,the wavelet threshold denoising algorithm of the low-frequency seismic data preprocessing unit was improved,improving the SNR and amplifing the lowfrequency effective signal.On the one hand,Traditional way in one-dimensional seismic data preprocessing process to suppress the noise was inefficient.On the other hand,there was the gain control need n the conventional seismic software to observe the trend of seismic wave propagation.Therefore,an improved wavelet with gain control function was proposed.In the threshold denoising algorithm,wavelet multiscale decomposition algorithm and threshold processing method were used to realize noise suppression.And the denoising effect of several improved threshold functions in the wavelet threshold denoising algorithm was compared and analyzed.The improved wavelet threshold denoising algorithm proposed by this paper has high reliability,effectively realizes the gain control and noise suppression,and improves the SNR of seismic data.The results show that the nodal seismic acquisition system meets the design requirements and achieves expected goals,and proposes some innovative methods.Firstly,the acquisition system has low internal noise,low signal distortion rate,good inter-channel isolation,low crosstalk,and can collect weak seismic signals with high precision,low noise and high work efficiency.Secondly,the time precision of the time-scale unit can reach 0.1us and the synchronization accuracy between the level and time-scale units is better than 20 us.Finally,the improved wavelet threshold denoising algorithm works well in the denoising process of the standard noisy Doppler signal and the actual noisy seismic signal,and can gain the effective signal.But the equipment is still with high power consumption for its embed Linux system,and the denoising algorithm is with some shortcomings in the application of weak SNR seismic data,both of them need a further study.It is hoped that the research through this subject can further promote the progress and development of the nodal system.