Limits of the angiogeology

From its nature, the angiogeoscopy, or survey angiogeological, present of the limits of which it is useful to be conscious. Those are however only slightly handicapping compared to the key objectives of a hydrogeological survey: to find the collecting locations optimal of the water bearing sites. They are mentioned by preoccupation of objectivity and to support the comprehension of the angiogeology and with its applications.

The stagnant water reservoirs are invisible for him

If the angiogeoscopy is extremely precise and sensitive to the groundwater circulation in the water vessels, it is, on the other hand, blind man with the underground stagnant water reservoirs (or rather quasi-stagnant, because there is well a circulation rising from the food and the leakages” of these reservoirs). The angiogeoscopy is unable to determine contours, the structure or the volume of a water reservoir or a groundwater sheet. It cannot infer the existence - or the advantage - of a water reservoir only starting from its vessels of food and evacuation (“leakages”) whose it can locate the ends precisely (thus also limits of the reservoir, in these points only).

This gap of the angiogeology remains however without consequence, because:

• in fact the flows determine the advantage of a collecting well, as indicated in the heuristic justification of the angiogeology
• they are the natural leakage points of a geothermal reservoir or aquifer which constitute the best possible points of collecting well.

Limited range of the angiogeological “visibility”

The angiogeology cannot carry out detection beyond a certain depth which is substantially lower than the thickness of the nonoceanic earth's crust. It cannot detect the origin of flows upstream of the geothermal reservoirs, but well flows upstreams of all the other types of water reservoirs (more surface, because fed, directly or indirectly, by the water reservoirs). It is rare that the exurgence points (i.e. leakages located typically at the place nearest to the surface of the geothermal reservoirs) are at a depth higher than 2,000 meters (which remains well in-on this side perimeter of detection by angiogeoscopy).

What appears a scientific disadvantage - by the limitation of the capacity to radiograph the underground water system beyond the range of “angiogeological visibility” - is not a practical disadvantage, because this depth of visibility is of the same order as that beyond which the costs of drilling become prohibitory. Those grow in a quasi exponential way with the depth. To Visualize the presence of water to more a great depth which the current angiogeological limit is thus scientifically interesting, but not very likely to lead to a profitable industrial exploitation, especially within sight of the proportion of potentially exploitable layers (see the cycle of water).

Transparency of the geological layers

The angiogeological location does not provide any information on the nature of the geological layers which must be crossed to reach the specified collecting location.

The angiogeology provides, at best, a certain idea of the homogeneity and compact character of the geological layers. These characteristics infer way of the water vessels, formation of junctions and differences between water capillaries which are formed by ramification of the water arteries.

This ignorance is mitigated by the analysis of the geological maps. If a drilling fails - for example because of crossed cavities - it is sometimes possible to be folded back on another collecting well on the same site which will not escape the angiogeoscopy.

The angiogeoscopy does not remove the need for a exploratory drilling

A exploratory drilling by collecting location specified by angiogeoscopy remains essential, initially to confirm the existence of the water bearing site, then to precisely measure some of its essential characteristics like:

• Depth,
• Flow,
• Pressure,
• The temperature,
• Physico-chemical characteristics like:

• the acidity of the collected fluids,
• the water-steam proportion (very important in the case of a geothermal exploitation),
• quantity of not-condensable gases (which must, in any event, being extracted),
• mineralization, i.e. the proportion of the principal ions in solution (especially if a hydrothermal exploitation is considered)

The angiogeology can consider all the properties hydrogeological useful starting from 3 D structure of a network of water vessels. It cannot however not measure them precisely. If it determines that a water bearing site is commercially exploitable with such or such end, it will be the case. While risking oneself with a comparison, one could say that to see a tree on foot a good idea of the quantity of wood usable for joinery gives, but this quantity can be precisely measured only after cutting up.

The industrial exploitation of a water bearing site requires, in any event, always the control of a exploratory drilling to plan the details of execution of the project.

Still insufficient Comprehension of the water cycle going down

The angiogeology studies the hydrogeological characteristics which concern the ascending water cycle, since the leakages angifères which appear within geothermal reservoirs until the tangled up networks of water veins which go through the subsoil close to surface

The water cycle descendant is consisted:

• Flows of infiltration of rainwater in the ground. These flows are not saturated - thus not measurable by angiogeoscopy - as is the flow within a water vessel. They contribute to feed the aquifers - just like ascending flow - in a proportion which varies with the depth (see our page on the infiltration).
• The feeding flows of the geothermal reservoirs whose origin can only be conjectured (see our page devoted to the cycle of water). The geological fluids are not detectable by angiogeoscopy with these depths raised because of characteristic of their circulation: they are known as “unstable” because they are not confronted with pressure losses comparable with those which they undergo downstream from the geothermal reservoirs (what enables them to circulate at very long distances).

Conclusion

The advantages of the angiogeoscopy carry it largely on its limits developed above and make a technique intended of it to replace the techniques of geophysical research used in hydrogeological survey. In Spite Of these limits, our capacity of detection and our knowledge of the hydrodynamics of the water vessels make it possible to exploit in an optimal way a site equipped with geothermal reservoirs, hydrothermal or mineral-water.