Detecting　boulders　before　tunnel　excavation　has　become　essential　routine　work　to　ensure　safe,　economic　and　efficient　tunnelling.　Here,　a　horizontal　cross-hole　seismic　method　to　determine　the　distribution　of　boulders　in　tunnelling　areas　is　developed　by　integrating　the　horizontal　directional　drilling　technique　and　cross-hole　seismic　method.　Compared　to　the　cross-hole　seismic　method,　the　proposed　method　has　obvious　advantages　of　higher　resolution　and　less　limitation　by　ground　conditions.　The　basic　techniques　and　process　of　this　method　are　introduced　in　detail.　To　verify　the　feasibility　and　efficacy　of　the　proposed　method,　a　220　kV　power　cable　tunnel　crossing　the　Guangming　River　is　taken　as　an　example　for　detecting　boulders　beneath　the　river.　A　four-borehole　detection　plan　is　implemented,　and　four　profiles　are　obtained.　The　results　reveal　several　high-velocity　anomalies,　which　are　identified　as　dense　sand　according　to　additional　logging　surveying.　This　example　indicates　that　the　proposed　method　can　provide　sufficient　information　about　boulders　in　underground　engineering.　©　2022　IEEE.