Table 1 Existing geophysical methods for deep onshore-offshore coastal probing along with representative studies.

Magnetotellurics (MT)

Gustafson et al.⁴⁶ recorded 100-Hz offshore MT responses, probing deep beneath the coastal white ribbon. Blatter et al.⁸¹ jointly inverted offshore MT/CSEM data.

60 km

Marine CSEM

Attias et al.²⁹ describe marine CSEM data offshore Hawai’i aimed at connecting terrestrial to offshore freshwater reservoirs. The survey vessel was navigated close to the shoreline.

500 m

Semi-airborne CSEM

Abd Allah et al.⁸² describe the time-domain GREATEM⁴⁸ configuration wherein a terrestrial grounded electric dipole source is deployed and airborne receivers measure magnetic field variations along flight lines crossing the white ribbon. A similar semi-airborne system SATEM⁴⁹ exists but has not been used for deep-probing white ribbon studies. The frequency-domain semi-airborne system DESMEX⁸³ for terrestrial applications achieves depth penetration of several km.

0.4–2.0 km

Seismic reflection

Seismic reflection has been used to map deep aquifers beneath the coastal white ribbon⁵² in the North Atlantic and Mediterranean, offshore Florida, and elsewhere. Offshore seismic data are sometimes tied to onshore boreholes⁸⁴. Seismic imaging provides structural boundaries and lithology which can be used to convert marine-CSEM resistivity maps into subsurface fluid distributions⁸⁵.

3 km

Borehole

IODP Expedition 313⁸⁶ drilled three shallow-water boreholes in ~35 m water depth specifically for OFG exploration with core recovery to ~500 m depth.

500 m

Satellite-based

Gravity data have mapped offshore geology on the continental shelf including the ocean-continent boundary⁵³ important to the petroleum industry. The sensitivity of gravity data to deep-white ribbon hydrogeology is insufficient to map offshore aquifers and/or establish onshore connectivity. Similar remarks hold for magnetic data⁵⁴ for which the sensed property is magnetization.

basement