3D View of an Underground Water System

An underground water system is basically made up of natural water reservoirs (also called aquifers or water table when these water reservoirs are close to the surface) and of water vessels. Those structures are interconnected and feed each other. The 3D view below illustrates this tree-like network structure of water vessels.

3D View of a water system with water reservoirs and water vessels

Formation of a Water System

The geothermal reservoir is fed with water extracted from oceanic plates undergoing subduction (i.e. a plate is pushed underneath another one and thereby undergoes a "squeezing effect"). The groundwater so extracted from the oceanic plates reaches very high pressure and very high temperature which makes it circulate upward through multiple geological layers.

This feeding of the geothermal reservoir corresponds with a steady incoming flow which causes the geothermal reservoir to leak through one – or sometimes more – points, called exurgence points or more simply exurgence. The hydrodynamics imposes that these leaks are point-like (i.e. they are localized on the water reservoir surface, they do not take the form of a slit or of a filtration area). The geological fluid going through these leaks does not diffuse but typîcally forms a bundle of drainage conduits, called water arteries. These conduits keep their slender shape over their whole path (which does not rule out that they can further divide; this happens typically at the transitions between geological layers).

The stability of the efferent flow (i.e. the outgoing flow) from a geothermal reservoir supposes the stability of its afferent flow (i.e. its incoming flow). The incoming flow feeding geothermal reservoirs and coming from the depths of the earth's crust, cannot be measured by angiogeoscopy. The origin of the afferent flow of geothermal reservoirs can only be postulated (as it cannot be measured) and is justified by the existence of a steady efferent flow. More about it is provided in our presentation of the underground water cycle. The incoming and outgoing flow of aquifers or of the water table can perfectly be measured by angiogeoscopy.

Overall Structure

Each water artery follows a path characterized by a decreasing slope : the closer it comes to the surface, the more horizontal it gets.

Water arteries (which, by definition, originate in a geothermal reservoir) end up (quasi systematically) in an aquifer. This is facilitated by the water artery getting more horizontal and therefore “sweeping” a larger surface which increases its chances to "hit" an aquifer and therefore feed it. This feeding, which takes place at the bottom of the aquifer, causes an overpressure that makes it “leak” in its turn to form, like a geothermal reservoir, a bundle of water vessels called water veins. The water veins coming out of that aquifer have lost a significant amount of heat (that has diffused in the lithospheric context of the aquifer, more on this in the overview of the temperature profile of underground natural flows). The so-produced water veins rising from the aquifer end up, in their turn, in one or more aquifers, closer to the surface. These aquifers undergo an overpressure that also produces outgoing water veins and so on until the water veins ultimately reaches the ground surface as a spring or feed the water table.

The natural leaks of a geothermal reservoir produce a tree-like structure of water vessels that have decreasing flow rates, temperatures and pressures as they come closer to the surface. The number of water veins increases with their proximity to the surface while their flow decreases proportionally. The flow rate of the water vessels decreases in opposite proportion of their density.

Ascending water flow occurs stepwise. The steps (typically 3 or 4) correspond to the transit through water reservoirs such as aquifers or the water table. The "originating" and "intermediate" reservoirs are always saturated; they can be fed by water arteries or by water vessels. The water table might not be permanently saturated. Its level fluctuates according to ground water infiltrations. This downward contribution combines with the upward contribution from water vessels to determine the actual level of the water table.

The water pressure decreases more sharply in water vessels than in the water reservoirs through which it passes while the temperature decreases more sharply in "intermediate" reservoirs than in water vessels.

A dead tree provides a good image of this ascending underground water flow. It is however useful “to adjust” this image according to the following considerations:

• certain junctions - but not all - are associated with water reservoirs; the other junctions can, for example, result from the transition of a water vessel from a compact geological layer to another less compact geological layer,
• underground water typically flows from a depth of about one mile (this is just an order of magnitude) to a depth of a few meters; terminal branches should all go up to about the same height for a more faithful illustration,
• the leak or exurgence point of the geothermal reservoir is associated with the stub of a tree splitting in multiple connate branches.

Even if an underground water system is extremely stable in space (in terms of its geometrical structure) and in time (in terms of its hydrodynamic and thermodynamic characteristics), an earthquake can alter its structure (some thermal springs have dried up after an earthquake, not because their feeding has stopped, but because the path of the water vessels has been modified and now end up in the water table instead of reaching the surface).

What we actually measure

The structure of a whole water system is seldom mapped completely. Through angiogeoscopy we can solely map water vessels, not water reservoirs of any sort. During the geophysical researches that we do, we solely focus on water arteries, not water veins. Water arteries provide all the necessary information to determine the productivity potential of hydrogeological resources.

The 2 D view (from the ground surface) of a water system as well as the elevation profile of water vessels is provided for a better understanding of a complex water system.

The upward flow is initiated by the subduction of oceanic plates. The pressure extracts the water while the magma heats it (directly or indirectly). The groundwater so extracted from the oceanic plate reaches very high pressure and very high temperature which makes it circulate upward through multiple geological layers (which it does since eons)