USING GEOPHYSICS TO REDUCE THE POTENTIAL OF INADVERTENT RETURNS IN HDD BORES: A CASE STUDY IN THRUST-FAULTED DEVONIAN STRATA, NORTH CENTRAL PENNSYLVANIA

Horizontal directional drilling (HDD) technologies have become a cost-effective option for installing new utility infrastructure in environmentally sensitive areas with minimal surface disturbance. However, completion of HDD bores in structurally and lithologically complex environments often introduces an elevated risk of inadvertent returns (IRs) of drilling mud or other types of surficial impacts when the bore path intercepts unidentified or mischaracterized geological hazards. Streams, wetlands, and other water bodies are particularly at-risk due to the increasing number of HDD projects being conducted near these features.

Traditional methods of geological hazard assessment, which typically include a desktop review of published literature and maps, onsite reconnaissance, and a geotechnical boring program, can fail to resolve some subsurface hazards capable of contributing to IRs. Adding geophysical surveys to the preliminary hazards assessment phase of an HDD project can reduce the risk of IRs or other surficial impacts by more accurately locating and characterizing potential unseen hazards including faults, joints, fractures, and lithological contacts.

A recent case study is presented in which three-dimensional electrical resistivity (3D ER) surveying techniques were utilized to preliminarily assess the potential for IRs associated with a proposed 490 meter-long (1,600 feet-long) HDD bore for a new drinking water supply line beneath the Susquehanna River through thrust-faulted, Devonian-age strata in North Central Pennsylvania. The 3D ER results were used to guide a realignment of the HDD bore path and design a targeted confirmatory drilling program. The HDD project was ultimately completed without any occurrences of IRs, despite the bore path’s interception of multiple shallow thrust faults and lithological contacts, including transitions from siliceous shale of the Hamilton Group to the calcareous shale and limestone of the Onondaga Formation.