# A High-Precision Energy-Efficient GPS Time-Sync Method for High-Density Seismic Surveys
## 2 Development of the GPS Time-Synchronization Method
### Description of the Cable-Less Storage Seismograph  
The time synchronization platform designed in this paper is mainly for the new generation of nodal cable-less storage seismograph GEIWSR-III developed by Jilin University. The characteristics  
Figure 2: Double-receiver model of seismic reflection survey.  
of the seismograph should be fully considered when designing time synchronization. The structural block diagram of GEIWSR-III is shown in Figure 3.  
GEIWSR-III is mainly composed of a 5-Hz moving coil type geophone, a data recording unit, a power management unit, and a wireless monitoring unit. The geophone used in the GEIWSR-III is a moving coil velocity-type geophone with a natural frequency of 5 Hz. It has the sensitivity with 30 V/m/s and a wide, stable frequency band from 5 Hz to 200 Hz. In addition, it weighs only 80 g, which greatly improves the portability of the instrument. The data recording unit is the main time- synchronization platform, which includes the MCU, a data acquisition unit, a GPS positioning unit, an SD card storage unit, a wired Ethernet data download unit, and the status indication unit. In addition, the node also includes a power management module to provide a high-efficiency power supply for the entire system. The wireless monitoring unit equips a low-power ZigBee in each sensor node. It serves as a data quality monitoring system to provide status information of the node for users in a timely manner, which greatly improves exploration efficiency.  
The time-synchronization system should complete the synchronization task among the nodes on the premise of not interfering with the normal acquisition work. The control center and each sensor node adopt the wireless multi-hop communication system to realize seismic data acquisition tasks. The time synchronization system is applied in the actual workflow of each node in the high-density seismic exploration environment. However, the actual working process of this system is more complicated. This article only discusses the links that are closely related to clock synchronization. The simplified system working flowchart is shown in Figure 4. After the system is powered on, the control center calls the station first and the wireless monitoring system of each sensor node responds to its own address number immediately. When a faulty node emerges, manually troubleshooting or repair is required. According to the survey line arrangement, the control center re-addresses each sensor node. After the sensor node receives its new address, it performs self-tests, and then sends the self-test parameter to the control center. When receiving the parameters of each node, the control center judges whether the node is normal. If normal, it configures the acquisition parameters for each node and starts the acquisition task. Otherwise, it will perform troubleshooting and repair manually. When the collection task is over, the control center sends the stop collection instruction and shutdown instruction. After receiving the instruction, each node executes the stop collection task and the shutdown process, and then the collection task ends. The acquisition process of GEIWSR-III can meet the basic exploration requirements, but the single acquisition time cannot be too long. Otherwise, the time deviation between the sensor nodes will continue to accumulate, which will cause the asynchronous data to affect the  
Figure 3: Block diagram of the time-synchronization platform GEIWSR-III.  
data processing and interpretation. Thus, the time-synchronization problem of the GEIWSR-III has greatly limited the efficiency of seismic exploration. If the time-synchronization can be performed, it can realize long-term continuous acquisition, even for dozens of days.