A study of saline incursion across an inter‐tidal zone on Anglesey, Wales using airborne conductivity data

Abstract

This study considers fixed-wing airborne electromagnetic data obtained by a recent highresolution airborne geophysical survey across the island of Anglesey, Wales. The survey detected an extensive (~10 km in length) highly conductive, near-surface zone across lowlying marsh and salt-marsh areas. The four frequency electromagnetic survey data enable a continuous assessment of subsurface conductivity variations across the inter-tidal zone and onshore into the Holocene sediments and bedrock. Inshore sea-water conductivities exceed 2000 mS/m but reduce across sandbank areas to a range of values, typically exceeding 500 mS/m. Saline incursion onshore is observed largely at lower values and at a range of scales extending to over 10 km. Within defined salt-marsh areas, a main front of near-surface incursion can be mapped and extends several hundred metres inshore. The main body of subsurface saltwater incursion is largely confined to a trough containing Holocene deposits above Upper Carboniferous (Coal) Measures which were historically mined. The main conductive zone becomes more compact, both vertically and laterally, with increasing distance inland. Modelling of the data indicates that, onshore, the major concentrations of dissolved solids are confined to the upper 30 m within undulating subsurface zones. Historical tidal salt loading of the Holocene sediments may account for the main features detected within this low-lying and fault-bounded area now protected by tidal gates.

A lithological assessment of the resistivity data acquired during the airborne geophysical survey of Anglesey, North Wales

“…3, the lowest onshore resistivity values are found over the Malltraeth Marsh (Fig. 2b) and display resistivities of less than 1 ohm m. Beamish (2012) investigated this region and concluded salt loading of Holocene tidal flat deposits was responsible for the reduced resistivity. It is also noted that the effects of saline intrusion are seen on the east coast of Anglesey and along the northern coast of the mainland; they appear to coincide with regions of superficial coastal zone deposits.…”

“…Thin blown sand and coastal zone deposits are also sparsely mapped in some coastal locations. Beamish (2012) considers the effect on the electromagnetic data from the most significant post glacial (Holocene) deposits, a region of salt loaded silty tidal flat deposits now protected by tidal gates. This area, known as the Malltraeth Marsh (see Fig.…”