6 Key Facts of Hydrogeology

Except for - extremely rare - fossil aquifers, all aquifers, all geothermal or hydrothermal reservoirs are fed by an incoming flow and therefore also produce an outgoing flow. These flows are globally equal on a “sufficiently large” period of time. The deeper the reservoir the more stable in time the flow.

Exploiting a water bearing site does not consist in emptying a reservoir - as it is, for example, the case for oilfields and gas-bearing deposits - but consists in "diverting" an underground flow to the surface (depending on the pressure, pumping might - or not - be necessary). Gas and oil are never integrated to a natural flow, water [almost] always is.

Consequence: The sustainable production capacity of a water bearing site is determined by its total outgoing natural flow, not by the size or capacity of the reservoir from which it is issued.

The downward flow is associated with the infiltration of surface water and rainfalls. Typically the downward flow does not pass the phreatic (contrary to a widespread preconceived idea) – at least not to a relevant extent.

The upward flow rises from the depth of the earth crust. It is initiated by the subduction of oceanic plates. The geologic pressure caused by subduction extracts the water from the plate 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 "squeezes" upward through multiple geological layers (which it does since "eons"). This high enthalpy flow travels large distances across the crust to feed geothermal reservoirs, then rise to the surface and feed aquifers which themselves feed the phreatic zone.

The hydrodynamics of the upward flow is such that this flow is concentrated in conduits called water vessels.

From the phreatic zone up, the upward flow balances the downward flow (over "long enough" periods of time).

Consequence : Only the upward groundwater flow is of interest to and exploited by angiogeology which focusses on water vessels.

Saturated water reservoirs (which is the case of all water reservoirs of interest) do not ooze or seep out but leak at very localized spots. Water vessels — i.e. natural water conduits or slender structures through which underground water flows naturally - arise from these leakage points. The content of a water reservoir does not diffuse through a wide filtration area but leaks through those water vessels. This outgoing flow balances an incoming flow from deeper reservoirs or aquifers. The flow through water vessels increases with their depth whereas their overall density (per ground surface) decreases with their depth. All in all, the average flow and the average density of water vessels (over a large enough area) evolve in opposite proportion in function of their depth.

Outgoing flows are ascending. They are initiated in the deepest reservoirs, called geothermal or hydrothermal reservoirs.

The probability that an exploration drilling hits, by chance, the path of a water vessel issued from a geothermal reservoir is tiny. This is why "traditional" hydrogeology and the "standard" techniques of geophysical exploration ignore water vessels (as they cannot localize them and do not exploit them).

Flow entering the phreatic zone is made up of both infiltrating water (flowing downward) and of water vessels coming from below (flowing upward).

Consequence: Water vessels leaving a water reservoir are the true representatives of the yield potential of that reservoir rather than its size or capacity (on which traditional search techniques typically focus).

In order to divert a groundwater flow to the surface one obviously needs to draw it where its flow rate is maximum. This maximum flow rate occurs at locations called exurgence points, namely locations where water reservoirs initiate their natural outgoing flow (also called efferent flow). Collecting groundwater at that precise location prevents the flow leaving naturally the water reservoirs from competing with the artificial collecting point.

Geothermal or hydrothermal reservoirs have seldom more than one leakage point. It is that specific point that must be targeted for drilling.

Consequence : Drawing a groundwater flow where its flow rate is maximum is helped by the fact that upward groundwater flows (i.e. exploitable flow) are highly concentrated at precise locations called "exurgence points".

The potential of a site is underestimated because of the incapacity of current geophysics to precisely locate productive collecting spots. The surveys currently performed are almost always made dependent on geothermal surface manifestations. However, only a negligible fraction of the geothermal reservoirs produce surface geothermal manifestations. The geothermal potential of a site is therefore always substantially higher than what traditional geophysical survey techniques would suggest.

Consequence : Quantum geophysics developed in the context of angiogeology enable to quickly and reliably find the optimum drilling location.

Contrary to a widely shared belief, geothermal or hydrothermal reservoirs are not located in specific areas : they are ubiquitous. Depending on the regio, they are situated at more or less greater depths.

Consequence : exploiting those reservoirs can take place everywhere on earth (knowing that in some regios one will need to drill deeper and therefore be more precise in locating the extraction point).