The seismometer-detached ocean bottom seismograph and its experiments in the Gakkel Ridge,Arctic Ocean

Yu Jiahui; Niu Xiongwei; Jin Zidi; Niu Linghui; Ruan Aiguo; Ding Weiwei; Zhu Xiaoyi; Liu Zhangju; Liu Bowen; Zhuang Cantao; Li Jiabiao

doi:10.11939/jass.20230152

Abstract：The ocean bottom seismograph(OBS)is an indispensable instrument for investigat-ing the deep crust-mantle structure.China's 12th Arctic scientific expedition marked a signific-ant milestone in marine seismic exploration by achieving the first large-scale active exploration along the challenging Gakkel Ridge in the Arctic Ocean,where ice coverage poses formidable obstacles.Impressively,the expedition successfully recovered 42 out of 43 deployed OBSs,with all five seismometer-detached OBSs retrieved intact.This paper introduces a seismometer-detached OBS specifically developed to overcome the unique challenges encountered during the expedition,particularly in terms of OBS recovery and positioning.Its main features include: 1)The newly developed seismometer-detached OBS integrates cutting-edge technology to enhance its functionality and performance under harsh Arctic conditions.One of its key innova-tions is the combination of ultra-short baseline beacons and advanced acoustic response short baseline array positioning systems,which improves positioning accuracy through dual position-ing technology.The positioning accuracy error of the short baseline array is within±100 m,and the mature Sonardyne ultra-short baseline array loaded on the Xuelong-2 polar scientific research icebreaker can achieve a positioning accuracy up to one thousandth of the water depth.The use of such a dual positioning system can ensure accurate positioning of the instrument even in areas with dense ice layers.This ground-breaking design significantly improves the accuracy and efficiency of OBS recovery operations,enabling researchers to locate and recover instru-ments with unprecedented reliability and precision. 2)Referencing the current status of seismometer-detached OBSs both domestically and in-ternationally,a titanium alloy seismic instrument chamber with a buoyancy of 17 kg in water has been designed.This chamber is incorporated into the seismic instrument separation struc-ture,independently installed inside the OBS hull via flexible cables.Upon reaching the sea-floor,it is timed to release,optimizing the coupling between the seismic instrument and the seabed.This structure also serves to reduce the impact of bottom currents on the seismometer.By enhancing this coupling,the OBS effectively improves the signal-to-noise ratio for record-ing seismic data,thus minimizing ambient noise and interference.Consequently,researchers can obtain clearer and more accurate seismic records,thereby facilitating a deeper understand-ing of the Earth's geological processes and tectonic activities. 3)The OBS utilizes advanced domestic seismic instruments and buoyancy materials.On one hand,this provides new options for the materials used in OBS development,to some ex-tent alleviating the problem of insufficient component production,which is advantageous for the large-scale production and industrialization.On the other hand,it enhances the flexibility of the OBS's design,enabling the possibility of loading multi-functional modules onto the OBS.Additionally,using domestically produced seismometers makes it easier to optimize and devel-op hardware and software according to scientific research requirements.This signifies a signi-ficant step towards achieving self-sufficiency in marine instrument technology.This domestic production capacity not only enhances China's scientific research capabilities,but also pro-motes innovation and technological advancement in the field of marine instruments,establish-ing independent intellectual property rights for key technologies of marine seismographs. China's 12th Arctic scientific expedition has yielded promising results,with the seismo-meter-detached OBS demonstrating exceptional performance in recording seismic signals.It was capable of collecting seismic data in the low-level horizontal seismic ambient noise environ-ment along the Gakkel Ridge of the Arctic Ocean.Notably,the OBSs have exhibited low hori-zontal seismic ambient noise levels,underscoring their suitability for seismic exploration in ice-covered marine environments.Additionally,these OBSs successfully collected the waveform records of a small teleseismic,two micro-earthquakes and three-component active seismic ex-ploration.This validates to some extent the effectiveness of the seismometer-detached structure in improving the signal-to-noise ratio,indicating that this type of OBS can meet the require-ments for submarine exploration under the ice. The successful development and application of the seismometer-detached OBS provide valuable experience for future submarine seismic exploration in extreme environments.Look-ing ahead,there are opportunities for further enhancement in various aspects such as real-time data transmission,longer battery life,and integration of more modularized sensors.The use of flexible buoyancy materials also liberates the submarine seismograph from the limitations im-posed by traditional glass chambers.This enables the configuration of different types of modu-larized sensors,thus paving the way for the development of a versatile submarine observation platform with powerful functionality.