Enhancement of Apparent Resistivity Sensitivity in Offshore DC Surveys Using Local Seafloor Insulation

The global transition toward renewable energy has accelerated the deployment of offshore wind farms, where reliable mapping of the shallow subsurface is crucial for wind-turbine foundation design and installation. Detecting shallow structures and their lateral variability in marine sediments helps reduce geotechnical uncertainties related to subsoil heterogeneity, which strongly influence construction risks, performance, and costs. Among geophysical methods, Direct Current (DC) resistivity surveys are effective for lithological and structural characterization; however, their offshore application remains strongly limited by two main factors: current leakage into the highly conductive seawater column and the resulting reduction in apparent resistivity (ρₐ) sensitivity to seabed targets.

We present a marine acquisition enhancement technique that deploys an electrically insulating sheet directly above the electrode cable placed on the seafloor—a previously patented concept—to restrict vertical current leakage and promote lateral current diffusion along the seabed, thereby increasing electrical interaction with subsurface formations.

A numerical parametric study was conducted using the finite element method (COMSOL Multiphysics), followed by a comparison between insulated marine models and equivalent terrestrial reference models without seawater. The analysis investigated the effects of: (1) seawater depth (Wd) = 1–60 m, (2) seawater-to-subformation resistivity contrast (Cr) = 1.5–100, (3) Wenner electrode spacing (a) = 2–30 m, and (4) insulation width (L) = 1–100 m, symmetrically covering 30 electrodes with 2 m spacing (Fig. 1a).

For Cres = 1.5, conventional marine acquisition without insulation produces large ρₐ errors relative to the terrestrial reference (Fig. 1b), exceeding 20% at small spacings (a = 2–6 m) in shallow water (Wd = 1 m), and remaining between 50% and 60% for most spacings when Wd increases to 28–60 m. The insulating sheet significantly enhances sensitivity: in shallow water (Wd = 1 m), long sheets (L = 60–100 m) as well as intermediate coverage (L = 30 m) reduce ρₐ errors to less than 2% over the entire spacing range (a = 2–30 m), closely reproducing the terrestrial response. Shorter sheets (L ≤ 10 m) still reduce errors to below 10% at intermediate to large spacings.

As water depth increases (Wd = 28–60 m), resistivity recovery becomes partial: even long sheets reduce ρₐ errors to approximately 10% at a = 30 m, compared to more than 50% without insulation. The effectiveness of leakage control also decreases geometrically at larger spacings, where errors tend to stabilize or slightly increase. Furthermore, when Cres increases to 10, relative errors rise for all sheet lengths, reaching approximately 15% at a = 30 m even for L = 60–100 m. This behavior is attributed to stronger lateral current diffusion within the subsurface, which diminishes the influence of insulation on current pathways.

This comparative analysis confirms that seafloor insulation systematically improves ρₐ sensitivity relative to conventional marine acquisition, particularly for larger insulation coverage. The proposed technique provides an effective solution for enhancing shallow structural detection in offshore DC resistivity surveys and offers quantitative guidelines for optimized survey design in offshore wind-turbine foundation site investigations.