Deep-borehole seismic observation has become an increasingly important component of modern earthquake monitoring systems due to its outstanding capability for suppressing surface environmental noise, its proximity to earthquake source regions, and its suitability for long-term in situ observations of crustal processes. Conventional surface seismic stations are strongly affected by anthropogenic activities, meteorological disturbances, and near-surface heterogeneities, particularly in densely populated and highly urbanized regions. These factors significantly limit the detection of weak seismic signals, including microearthquakes and foreshocks, and constrain the reliability of earthquake early warning and source characterization. By deploying high-sensitivity instruments at depths ranging from several hundred meters to several kilometers, deep-borehole observation effectively isolates sensors from near-surface noise sources and provides high signal-to-noise ratio （SNR） recordings across a broad frequency band, offering a unique observational window into earthquake generation processes and deep crustal dynamics.

This paper presents a comprehensive review of the development history, international and domestic research progress, key technologies, and future directions of deep-borehole seismic observation. Representative international projects are systematically summarized, including the San Andreas Fault Observatory at Depth （SAFOD） in the United States, the International Ocean Discovery Program （IODP） long-term borehole monitoring systems, deep-borehole observatories in Japanese subduction zones （e.g., NanTroSEIZE）, and the German Continental Deep Drilling Program （KTB）. These projects demonstrate that deep-borehole observatories play a critical role in resolving fault-zone physical properties, monitoring microseismicity and slow-slip events, and constraining stress evolution and fluid–fault interactions under extreme temperature and pressure conditions. International experience highlights the importance of multi-parameter integration, long-term stability under harsh environments, and real-time data transmission for both scientific research and hazard mitigation.

In contrast, deep-borehole seismic observation in China has developed rapidly over the past two decades, supported by major national initiatives such as the Chinese Continental Scientific Drilling （CCSD） project and the Wenchuan Fault Scientific Drilling （WFSD） project. Although a nationwide borehole seismic network with nearly 200 stations has been established, most existing stations are relatively shallow （typically less than 400 m）, which limits their noise suppression capability. Recently, significant progress has been achieved through the construction of 2 km–class deep-borehole observation platforms, including the Changde deep-borehole ultra-broadband seismic observation platform and the Leizhou （Zhanjiang） 2000 m deep-borehole optoelectronic composite observation station. These platforms mark a major milestone in China’s deep-borehole seismic monitoring capacity and provide critical testbeds for advanced instrumentation and engineering solutions.

A key contribution of this review is the establishment of a four-dimensional technical and methodological framework for deep-borehole seismic observation, encompassing sensors, packaging and coupling, acquisition and timing, and data processing and analysis. Within this unified framework, the performance parameters of typical domestic and international projects—such as usable bandwidth, dynamic range, depth adaptability, and long-term stability—are summarized using consistent criteria, forming a comparable reference baseline. In particular, the successful engineering application of the domestically developed very-broadband seismometer TDE-120VB, with a response band from 120 s to 50 Hz, a dynamic range exceeding 145 dB, and proven long-term operation at depths greater than 2000 m, represents a critical breakthrough in the localization of deep-borehole seismic instrumentation in China.

From an engineering perspective, this paper further proposes a recommendatory paradigm for repurposing abandoned energy wells into deep-borehole seismic observatories. Based on practical experience from the Changde platform, key procedures and acceptance criteria for full-chain sealing from wellhead to borehole bottom, grouting consolidation, sensor packaging and coupling, retrievable deployment, and operation and maintenance are systematically summarized. This paradigm provides a feasible pathway toward low-cost, reproducible construction of deep-borehole observation stations, addressing one of the major bottlenecks that currently limits large-scale deployment.

In terms of data processing and analysis, deep-borehole observations offer unprecedented opportunities for detecting weak seismic events and resolving fine-scale source processes. However, traditional detection and phase-picking methods often struggle with the complex and time-varying noise characteristics of borehole environments. Recent advances in artificial intelligence and deep learning, such as PhaseNet, EQTransformer, and physics-informed neural networks, show great potential for improving weak-event detection, phase identification, and source parameter inversion when combined with high-quality deep-borehole data. The integration of seismic observations with other physical fields, including strain, temperature, fluid pressure, and distributed fiber-optic sensing, further enhances the ability to characterize deep crustal processes in a holistic manner.

Despite these advances, several challenges remain. Long-term stability of sensors under extreme temperature and pressure conditions, high drilling and operation costs, and the lack of standardized metadata and quality control frameworks continue to hinder the expansion of deep-borehole seismic networks. Addressing these challenges requires coordinated efforts in materials science, drilling engineering, system integration, and intelligent data processing. Looking forward, the future development of deep-borehole seismic observation is expected to emphasize retrievable and modular platform design, edge-computing data acquisition and processing, physics-constrained artificial intelligence methods, and unified standards for data sharing and interoperability.

In summary, deep-borehole seismic observation represents a powerful and indispensable approach for advancing earthquake science and improving seismic hazard assessment. By synthesizing international experience and recent domestic progress, and by proposing unified technical frameworks and engineering paradigms, this review aims to provide a reference baseline for future deep-borehole observatory construction and operation. The continued integration of advanced instrumentation, intelligent data analysis, and network-scale deployment will play a key role in promoting the large-scale application of deep-borehole seismic observation and fostering interdisciplinary collaboration in Earth system science.