# A High-Precision Energy-Efficient GPS Time-Sync Method for High-Density Seismic Surveys
## 1 Introduction  
In order to meet the needs of more detailed geological structure surveys, the study of seismological methods relies on a denser seismological network, which is used to improve the resolution of seismological research and identify weak seismic phases [1]. Seismic exploration technology is increasingly developing in the direction of multi-channel, high-density, wide-azimuth, and high-efficiency acquisition [2]. The amount of seismic data has increased dramatically. Therefore, the demand for real-time processing of huge data streams at various acquisition sites has continued to increase. However, the cable seismograph has bulky cables and low fieldwork efficiency, which makesthem inappropriate to use in difficult exploration environments. In contrast, the distributed nodal cable-less remote sensing seismograph is non-cable and portable, which makes it capable of adapting to complex exploration environments and the diversity of layout methods [3,4]. The distributed nodal seismic systems are divided into the wireless real-time seismic data acquisition systems and non-real-time storage type seismic data acquisition systems according to the data collection methods. The WTU-508 seismograph of Sercel Inc. and the RT3 seismograph of Wireless Seismic, Inc. are representatives of wireless real-time seismic acquisition systems [5, 6, 7]. These seismographs perform data acquisition and recovery in real-time, so the continuous wireless network support is needed. Wireless transmission of a large amount of seismic data requires a lot of energy, which also limits the application in high-density seismic array exploration. On the other hand, the non-real-time storage type systems perform long-term exploration work in the field and store the collected data in a local large-capacity storage device. After the survey is completed, a large amount of data would be downloaded to the server and aggregated into a database for data interpretation and processing [8]. So, the nodal cable-less storage type seismograph has the characteristics of no cable, large local storage capacity, and long working time capacity, which are highly efficient and suitable for the high-density seismic array exploration. The representatives of cable-less storage type seismograph are the GEIWSR of Jilin University, Quantum of INOVA Company, and Z-land of Fairfield Company [9, 10, 11]. Cable-less storage type seismograph performs distributed data acquisition and data recovery centrally, so it is crucial to distinguish what kind of seismic data is collected by each sensor node at a specific time in the process of seismic exploration. Therefore, the time base must be designed to ensure that the seismic data of the nodes can be time-stamped. The purpose of time synchronization in the whole seismic sensor network is to achieve a globally consistent logical clock so that all events and information in the entire system have a uniform time interpretation [12]. So it is essential to have an accurate and highly reliable time-synchronization performance between each sensor node. Currently, most distributed nodal cable-less storage type seismographs use GPS as a time-base to time-tag seismic data [13]. Figure 1 shows the schematic diagram for time synchronization in the high-density seismic array survey. The sensor nodes are distributed in dozens of kilometers of mountainous terrain and other complex terrain exploration areas densely. The seismic source excites the seismic waves at multiple locations, and all the sensor nodes record the seismic data in real-time, which forms a dense seismic node observation system to realize the fine detection of underground geological structures. At the end of the acquisition task, all nodes carry out centralized seismic data downloading and then unified processing and interpretation are carried out on the large amount of seismic data. All sensor nodes  
Figure 1: Schematic diagram for time synchronization in the high-density seismic array survey.