Research On Key Technologies Of Seismic Exploration Instrument Based On Distributed Acoustic Sensing

As one of the most important methods in geophysical exploration,seismic exploration is widely used in the fields of geological resource exploration and geological disaster warning.Most existing seismic exploration instruments use electromechanical geophones for sensing,and electromechanical geophones are point sensors.In oil and gas exploration and natural seismic monitoring,point sensors need to be arrayed to ensure the continuity of the seismic wave distribution.There is a problem that the layout is difficult and it is difficult to be on duty for a long time,and its detection capacity and density are also limited by cost.Unlike point sensors,in Distributed Acoustic Sensing(DAS),optical fiber is both a sensor and a transmission medium.Using optical fiber can realize a massive sensor array.In the exploration application,only one optical cable needs to be deployed or the existing communication optical cable is directly used,and the deployment difficulty and cost are relatively low.In addition,DAS has a large detection capacity and can achieve a detection length of tens of kilometers under the premise of ensuring highlevel and high-density acquisition.It has important potential application value in the field of seismic exploration with increasingly deeper and more refined exploration targets.At present,major foreign DAS manufacturers do not sell instruments to China,but only provide testing services.The domestic DAS equipment is mainly oriented to the fields of perimeter security and structural health monitoring.The combination with seismic exploration is not mature.Its frequency response range and other indicators cannot meet the needs of seismic exploration.There are also problems such as low integration,immature supporting software,and insufficient real-time calculation and display capabilities for seismic exploration data.In response to the above problems,this paper proposes a system design and process manufacturing technology for seismic exploration instruments based on the DAS principle.In the design process,the parameters of pulsed light,modulator,detector,electrical signal filter and amplifier were optimized and verified to improve the key parameters such as the frequency response range,spatial resolution,and minimum detectable strain of the system;proposed the PXIe(PXI Express)standard seismic survey instrument structure and the corresponding manufacturing process technology of highly integrated optical board,control and signal conditioning board to improve the portability and reliability of the instrument.A data acquisition technique of undersampling is proposed.According to Shannon's band-pass sampling theorem,the data acquisition card receiving parameters are reasonably configured,and the sampling rate and subsequent signal processing pressure are reduced without loss of performance.For seismic exploration applications,the software's professional display capabilities have been improved.On the basis of waterfall charts,time domain charts,and spectrum charts commonly used by DAS,a real-time waveform display interface dedicated to seismic exploration has been added,which makes it more advantageous in displaying transient disturbance.The parameter calibration results show that the maximum monitoring distance of the seismic survey instrument reaches 60 km,the highest spatial resolution is better than 4 m,the frequency response range is 0.1 Hz to 500 Hz,the minimum detectable strain reaches 0.5 n?,and the final instrument external dimension is only 330 × 185 × 300 mm,weight 12 kg,power consumption less than 90 W,integration and portability have improved compared with domestic DAS equipment with similar performance.The seismic exploration instrument participated in the active source-based ground fissure condition monitoring test conducted in Guangming Village,Xibei Town,Wuxi City.During the test,the existing buried and underground optical cables were successfully used to monitor seismic waves.The distance between the monitoring channels reaches 4 m,and the output waveform matching the existing geophones is obtained,which shows a higher sensitivity than the existing geophones.The later inversion of the test data reflects the local ground fissure evolution state,which promotes the application and promotion of DAS equipment in the field of seismic exploration.