Marine survey and samplings by Thomé de Gamond were conducted during 1833–67, basing the seabed depth at a maximum of 55 meters (180 ft) and the connection of geological strata. Surveying was conducted for many years, with 166 marine and 70 land-deep boreholes being bored and more than 4000 kilometers of marine geophysical study completed. Surveys were attempted in 1958–59, 1964–65, 1972–74 and 1986–88.
The surveying which was undertaken in 1958–59 provided for plunged tube and bridge plans as well as a bored tunnel which aided in the investigation of a wide area. Marine geophysics surveying for civil engineering tasks was in its early childhood during this time. The positioning and resolution from seismic profiling was very poor. The surveys undertaken during 1964-65 centered on a northerly route which left the English coast at Dover harbor; applying 70 boreholes, a region of intensely weathered rock with high permeability was found just south of Dover harbor.
A more southerly course of surveying was looked into in 1972–73 and the path was established to be possible. At Sangatte on the French side a deep barb with adits was made. The 1986–97 survey reinforced the previous findings and the type of the gault clay and tunneling medium were also investigated. It was discovered that chalk marl made up 85% of the route. Geophysical techniques from the oil industry were applied.
Once the geological surveying was concluded the route for the three tunnels was depicted through a layer of soft impermeable rock known as chalk marl. No other medium was suitable for tunnelling purposes. The chalk marl permitted the tunnel boring machines (TBMs) beginning from the British coast to be projected for a speedy approach, and these TBMs often made out over 300 meters of tunnel in a week.
