To detect an oil leak in an underground oil-filled cable joint without excavation, the most effective method is to monitor the pressure drop in the oil system. Let's examine why this method is the most suitable:

Understanding Oil-Filled Cable Systems: Oil-filled cable systems use pressurized oil to insulate and cool the conductor. A stable pressure within the system is essential for maintaining its integrity. Any leak will manifest as a drop in pressure, providing a reliable indicator of an issue.

Monitoring Pressure Drops: By installing pressure sensors at strategic points along the oil system, engineers can continuously monitor the pressure. A notable drop in pressure indicates a potential leak, allowing for timely intervention before a significant failure occurs.

Comparison with Other Methods:

• \(\text{Use a thermal imaging camera：}\) While effective for detecting hot spots, this method cannot reveal underground oil leaks as it primarily detects temperature variations.
• \(\text{Listen for unusual cable sounds：}\) Sound detection is less precise and reliable for pinpointing oil leaks in buried cables compared to pressure monitoring.
• \(\text{Visually inspect the joint enclosure：}\) Visual inspection requires excavation, which is what the question specifically aims to avoid.

Benefits of Pressure Monitoring: This method is non-intrusive and provides real-time data, allowing for proactive maintenance and reduced risk of major failures or environmental hazards.

Thus, monitoring the pressure drop in the oil system is a practical and effective way to detect oil leaks in underground cables without the need for excavation.