Hidden Faults Beneath Morocco’s Essaouira Basin Could Unlock New Groundwater

Beneath the semi-arid coastal plains of western Morocco lies a hidden architecture of faults, salt walls and buried basins that may determine whether millions of people will have reliable access to groundwater in the decades ahead. A new study published in Natural Resources Research has produced the first integrated gravity-seismic hydrostructural framework of the Essaouira Basin, revealing how deep geological structures control the geometry and hydraulic connectivity of the region’s most important aquifer systems. The work, led by Anas Zbiri of Cadi Ayyad University together with colleagues from Morocco, the Czech Republic, Sweden, Poland and France, combines 2D seismic reflection profiles, borehole data, Bouguer gravity analysis and satellite-based lineament mapping into a single coherent picture of the subsurface.

The Essaouira Basin sits along Morocco’s Atlantic margin, where sedimentary layers deposited over hundreds of millions of years record the opening of the central Atlantic and the subsequent uplift of the High Atlas mountains. Groundwater in the region is under mounting stress from climate change and growing demand, yet the deep structural controls on aquifer geometry have remained poorly constrained. Understanding where water-bearing layers thicken, where faults cut them apart, and where impermeable bodies seal them off is essential for managing a resource that recharges slowly in a semi-arid climate.

To build that understanding, the team processed Bouguer gravity anomaly data ranging from −65 to +35 milligals, using polynomial regression to strip away deep regional trends and isolate a residual gravity field between −38 and +19 milligals. These residual anomalies reflect density contrasts in the subsurface caused by the topography of the Paleozoic basement, the rise of Triassic salt diapirs, and lateral variations in the thickness of the sedimentary cover. Gravity data of this kind act as a large-scale density map: masses of dense basement rock pull slightly harder on a gravimeter than lighter sediments or salt, and the resulting patterns can be inverted into structural information.

Extracting usable structure from gravity data requires sharpening the edges of buried bodies. The researchers applied an improved logistic filter, a total horizontal gradient transform and the UC technique to the residual field, and together these methods resolved 13 gravity fault contacts, labelled FG1 through FG13. The mapped faults follow dominant northeast-southwest, east-west and north-south trends, echoing the tectonic history of the Atlantic rift and the later Atlas compression. Each contact marks a place where rock density changes abruptly, typically across a fault plane, and their orientations provide a first-order template for how the basin is fractured.

Gravity alone, however, cannot tell which layer is which. For that, the team turned to five seismic reflection profiles, which image subsurface layering by recording the echoes of sound waves sent into the ground. Calibrated against four boreholes, the seismic data allowed the researchers to constrain five key regional reflectors: the top of the Paleozoic basement, the Triassic-Jurassic boundary, the top of the Jurassic aquifer of Oxfordian-Kimmeridgian age, the top of the Lower Cretaceous aquifer of Barremian-Albian age, and the base of the Upper Cretaceous aquifer at the Albian-Cenomanian transition. These five surfaces define the main water-bearing packages of the basin and anchor the structural interpretation in real stratigraphy.

Integrating the gravity and seismic results revealed five negative gravity anomalies, designated N1 to N5, and three structural sub-basins, SB1 to SB3, bounded by steeply dipping faults. These sub-basins are the deep depressions where sedimentary layers accumulated to their greatest thicknesses, and they correspond closely to the zones where the Upper Jurassic, Lower Cretaceous and Upper Cretaceous aquifer units reach their maximum development. In other words, the places where the basin sank deepest over geological time are precisely the places where the largest volumes of groundwater storage now reside.

One of the most striking findings concerns salt. Seismic profiles imaged salt diapirs, walls of Triassic evaporites that have flowed upward through the overlying sediments, and these structures connect with the well-known Jbel Hadid and Jbel Amsittene salt anticlines at the surface. Because salt is essentially impermeable to water, these diapirs and anticlines act as lateral seals that compartmentalize the aquifer units into hydraulically discrete cells. Water in one cell cannot easily flow into its neighbour, which means that pumping in one compartment will not be buffered by storage in another. For water managers, this is a crucial insight: the basin is not a single connected reservoir but a mosaic of partially isolated blocks.

The structural framework also points to where the resource can best be replenished. Fault intersections, where fracture networks are densest, enhance permeability and maximize recharge potential, and the study identifies these zones as priority targets for artificial recharge schemes and strategic borehole siting. Rather than drilling blindly, planners can now use the structural synthesis map to place wells where fault-bounded structural lows coincide with thick aquifer sections and fracture-enhanced connectivity to the surface. The authors emphasize that this gravity-seismic integrated approach is transferable to comparable semi-arid basins across Morocco and North Africa, where similar structural controls likely govern hidden groundwater systems.

The significance of the work extends beyond the Essaouira Basin itself. Across North Africa, sedimentary basins host deep aquifers that sustain agriculture and urban supply, yet their structural frameworks are often known only from sparse wells. By demonstrating how residual gravity processing, edge-detection filters, seismic stratigraphy and borehole calibration can be woven into a quantitative hydrostructural model, the study offers a template for groundwater exploration under data scarcity. As climate pressures intensify across the semi-arid belt of the southern Mediterranean, mapping the deep structures that store, seal and channel groundwater may prove as important to regional water security as any new dam or desalination plant.

Subject of Research: Integrated geological and geophysical characterization of deep aquifer systems in the Essaouira Basin, western Morocco

Article Title: Hydrogeological Characterization of Deep Aquifer Systems in the Essaouira Basin, Western Morocco: Insights from Integrated Geological and Geophysical Approaches

Article References: Zbiri, A., Kchikach, A., Jaffal, M., Mrlina, J., Zappalá, S., Guernouche, M., Radwan, A. E., & Azaroual, M. (2026). Hydrogeological Characterization of Deep Aquifer Systems in the Essaouira Basin, Western Morocco: Insights from Integrated Geological and Geophysical Approaches. Natural Resources Research. https://doi.org/10.1007/s11053-026-10766-0

Image Credits: AI Generated

DOI: 10.1007/s11053-026-10766-0

Keywords: Essaouira Basin, hydrogeology, groundwater, gravity data, seismic reflection, salt diapirs, faults, aquifer geometry, Morocco, semi-arid regions, structural geology, artificial recharge