geosciences Review Seismic Reflection Methods in Offshore Groundwater Research Claudia Bertoni 1,*, Johanna Lofi 2, Aaron Micallef 3,4 and Henning Moe 5 1 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK 2 Université de Montpellier, CNRS, Université des Antilles, Place E. Bataillon, 34095 Montpellier, France; johanna.lofi@gm.univ-montp2.fr 3 Helmholtz Centre for Ocean Research, GEOMAR, 24148 Kiel, Germany; [email protected] 4 Marine Geology & Seafloor Surveying, Department of Geosciences, University of Malta, MSD 2080 Msida, Malta 5 CDM Smith Ireland Ltd., D02 WK10 Dublin, Ireland; [email protected] * Correspondence: [email protected] Received: 8 April 2020; Accepted: 26 July 2020; Published: 5 August 2020 Abstract: There is growing evidence that passive margin sediments in offshore settings host large volumes of fresh and brackish water of meteoric origin in submarine sub-surface reservoirs. Marine geophysical methods, in particular seismic reflection data, can help characterize offshore hydrogeological systems and yet the existing global database of industrial basin wide surveys remains untapped in this context. In this paper we highlight the importance of these data in groundwater exploration, by reviewing existing studies that apply physical stratigraphy and morpho-structural interpretation techniques to provide important information on—reservoir (aquifer) properties and architecture, permeability barriers, paleo-continental environments, sea-level changes and shift of coastal facies through time and conduits for water flow. We then evaluate the scientific and applied relevance of such methodology within a holistic workflow for offshore groundwater research. Keywords: submarine groundwater; continental margins; offshore water resources; seismic reflection 1. Introduction Groundwater resources are declining due to over-exploitation and climate change [1]. In particular, present-day sea-level rise and over-pumping are threatening the availability of freshwater in coastal areas [2,3], where aquifers are experiencing a high level of depletion, saline contamination and pollution from agricultural, urban and industrial activities [4–13]. In the last decade, there has been growing evidence that passive margin sediments in offshore settings can host large volumes of low salinity water in sub-surface reservoirs (Figure1)[ 4,14–20]. Current calculations based on averaged aquifer properties on worldwide continental shelves estimate that the volume of brackish (<10 g/L) water stored in passive margins worldwide is approximately 5 105 km3; this number decreases to × 3 105 km3 if considering water of <1 g/L[4,16]. However, this estimate includes only 16% of the × present-day coastline. The presence of low salinity water in the shallow offshore might produce an economic advantage for countries that rely heavily on desalinization as their main source of freshwater. The economic costs of this expensive process could considerably decrease by the use of extracted water with lower salt content. Therefore, there is a mounting interest in producing accurate maps of offshore groundwater resources, determine their likely responses to climate and sea-level changes and assess how much water they could sustainably supply, especially in proximity of water-stressed coastal cities [21]. Geosciences 2020, 10, 299; doi:10.3390/geosciences10080299 www.mdpi.com/journal/geosciences GeosciencesGeosciences 20202020, ,1010, ,x 299 FOR PEER REVIEW 22 of of 34 34 I FigureFigure 1. 1.WorldWorld map map showing showing the distribution the distribution of vast of mete vastoric meteoric offshore o groundwaffshore groundwaterter reserves (modified reserves from(modified Reference from [4]), Reference with case [4]), studies with caseshown studies in this shown paper. inIn thisred: paper.reserves In proven red: reserves by observational proven data,by observational in green: reserves data, with in brackish green: reserves component with highlighted brackish by component pore water highlighted analysis, in byblue: pore significant water meteoricanalysis, offshore in blue: groundwater significant meteoricwith indirect offshore evidence groundwater based on withonshore indirect paleo-groundwater evidence based [4]. on In blackonshore dashed paleo-groundwater circles: case studies [4]. that In black use offshore dashed circles:geophysical case studiesdata for thatgroundwater use offshore analysis geophysical and are describeddata for groundwaterin this paper. The analysis yellow and transparent are described overlay in thisshows paper. the highest The yellow density transparent coverage of overlay seismic reflectionshows the data, highest mostly density from coverageindustry-funded of seismic and reflection confidentiality data, mostly protected from sources. industry-funded This map andonly providesconfidentiality an approximation protected sources. of data Thisdistribution map only and provides it represents an approximation a sub-set of data of dataglobally distribution acquired (source:and it represents https://www.globalseis a sub-set of datamiclibrary.com/, globally acquired https://pwdat (source: https:alibraries.maps.arcgis.com/).//www.globalseismiclibrary.com /, https://pwdatalibraries.maps.arcgis.com/). 1.1. Distribution of Offshore Groundwater 1.1. Distribution of Offshore Groundwater Offshore groundwater is composed of meteoric-derived water, compaction-driven fluids and thermobaric-derivedOffshore groundwater fluids [22]. is composed Shallow and of meteoric-derivedproximal sediments water, (at 10 compaction-driven s to few 100 s m of fluidsseabed and and burialthermobaric-derived depths) on continental fluids [22 shel]. Shallowves contain and proximalpore fluids sediments that are (atdominated 10 s to few by 100water s m of of meteoric seabed origin,and burial flowing depths) in a onbasinward continental direction shelves [23]. contain Deeper pore sediments fluids that up are to dominated few 1000 bys of water meters of meteoric of burial depthorigin, contain flowing pore in a basinwardfluids, of higher direction salinity, [23]. Deeperexpelled sediments by compaction up to few in 1000a landward s of meters and of upward burial direction.depth contain Occasionally, pore fluids, the presence of higher of salinity,fresh or expelledlow salinity by fluids compaction in the marine in a landward subsurface and is upwardlinked to thedirection. expulsion Occasionally, of diagenetic the presence fluids from of fresh sediment or lows salinity such fluidsas evaporites, in the marine clays subsurface and silica is [24–28]. linked Thermobaricto the expulsion derived of diageneticfluids are present fluids fromat high sedimentser depths such(see summary as evaporites, in Reference clays and [22]). silica [24–28]. ThermobaricOf these offshore derived fluidsgroundwater are present systems, at higher topographically depths (see summarydriven meteoric in Reference recharge [22 (TDM),]). that is, a combinationOf these oofff shorerainfall groundwater recharge and systems, topographica topographicallylly driven drivenflow, is meteoric the most recharge important (TDM), driver that for theis, aemplacement combination of of fresh rainfall or low recharge salinity and (also topographically defined as ‘freshened’) driven flow, water is the in mostcontinental important shelves driver and upperfor the slopes emplacement of passive of margins fresh or [29–31]. low salinity In these (also syst definedems, the as ‘freshened’)offshore reach water of modern in continental coastal shelvesaquifers intoand the upper submarine slopesof realm passive is constrained margins [29 by–31 a]. flow In these cell system systems, with the recharge offshore reachfrom land. of modern This depends coastal onaquifers whether into the the onshore submarine water table realm is is high constrained enough to by provide a flow the cell necessary system withdriving recharge force and from whether land. thereThis dependsis hydraulic on whetherconnectivity the onshorebetweenwater the offshore table is aquifer high enough and the to onshore provide recharge the necessary area driving(see e.g., Referencesforce and [4,32]). whether Land-derived there is hydraulic groundwater connectivity can be betweentransported the by off conduitsshore aquifer in sub-sea and thesediments onshore or byrecharge pore-water area (seeflow e.g., in Referencessediments [of4, 32sufficient]). Land-derived porosity groundwaterand permeability can be (see transported e.g., Reference by conduits [33]). Groundwaterin sub-sea sediments may discharge or by directly pore-water on the flow seafl inoor sediments through submarine of sufficient groundwater porosity and discharge permeability (SGD) [30,34–39].(see e.g., Reference [33]). Groundwater may discharge directly on the seafloor through submarine groundwaterThese processes discharge alone, (SGD) however, [30,34– 39do]. not explain the large volumes of low-salinity groundwater that areThese found processes across alone,continental however, shelves do not[4]. explainOffshore the low-salinity large volumes groundwater of low-salinity accumulations, groundwater not necessarilythat are found connected across to continental onshore rech shelvesarge areas, [4]. Ocanffshore also be low-salinity stored in confined groundwater paleo-aquifers. accumulations, These arenot generally necessarily paleo-freshwater connected to onshoreof meteoric recharge origin, areas, trapped can in also sediments be stored during in confined past times paleo-aquifers. of lower sea levelThese (see are e.g.,