# A High-Precision Energy-Efficient GPS Time-Sync Method for High-Density Seismic Surveys
## 1 Introduction
need to be laid over a large area and have a long working period, so the sensor nodes need to be energy-efficient. Seismic data with poor synchronization capability will cause considerable errors in the interpretation of seismic data. When the acquisition time increases, the frequency drift of the crystal oscillator on the acquisition station will bring significant time-error accumulation. This requires that the time synchronization system must be able to perform error compensation or error correction. In addition, the synchronization error will increase as the number of nodes increases, and this is the most essential difference between distributed systems and other systems [14]. Thus, it places higher requirements on the time synchronization for large scale exploration task. It is urgent to develop a time tagging system for seismic data and a multi-node time-synchronous system for high-density seismic array survey. The GEIWSR seismograph developed by Jilin University uses the high-precision GPS timing method, which has solved the problem of poor synchronization accuracy between distributed sensor nodes. Then they performed multiple field experiments that showed that the GEIWSR has an accurate time-synchronization ability [15]. However, the long-term operation of GPS equipment consumes a lot of energy. In addition, there is no GPS signal in the complex area (such as the jungle and other harsh environments), which brings great difficulties to the seismic exploration task. In 2013, the Fairfield Nodal Company launched the Z-land nodal seismograph, a small and cable-less portable storage type seismic acquisition system. It realizes time marking for seismic data with the built-in GPS receiver and the local clock system. Thus, the multi-node time-synchronization is ensured under the condition of low energy consumption [16]. However, the local clock of the instrument will drift and result in a large error when it works for a long time due to the change of working temperature, operating state, and so on. The clock accuracy of the Z-land system is only 100 us by discontinuous GPS on-off with local clock, which is not very suitable for high-density seismic array surveys. The continuous working time of the Quantum nodal seismic exploration system developed by INOVA Company can reach 50 days in the field with the 24-h operation, but its time synchronization accuracy is about 40 us [17]. Although it can meet the general field exploration requirements, it is still not very suitable for high-density seismic exploration applications. Some other equipment adopts a time synchronization scheme based on the Network Time Protocol (NTP). This scheme is suitable for time synchronization in small wireless sensor networks, and it is mainly based on the network transmission protocol. As the number of nodes and the exploration distance increase, the synchronization accuracy will also deteriorate. Therefore, a high-precision energy-efficient time-synchronization method that is not limited by the number of nodes and exploration distance is urgently needed. In this paper, we present a high-precision energy-efficient time-synchronization method named the time-sharing indexed interpolation intercept (T3I) method, which combines GPS and high-precision TCXO for high-density seismic array surveys. In order to acquire a high-precision time-stamped system, we use the excellent frequency stability over temperature, low supply voltage, and a highly reliable crystal unit to achieve time stamping of the overall seismic data. An accurate time series between GPS pulse per second (PPS) interrupt and GPS serial interrupt is used to provide a reasonable time-service process. Further, the GPS synchronization time information is recorded in a large-capacity SD card. When the seismic data is collected by the control center, the seismic data with time stamp will be intercepted by segment time through the server. For energy-efficient design, it is put into sleep mode during the periods when no time service is provided.