GeologicUnit

[mbeaufils]

Definition source : from IUGS and OGC GeoSciML, also candidate for IFC.

Conceptually, a GeologicUnit may represent a body of material in the Earth whose complete and precise extent is inferred to exist (e.g., North American Data Model GeologicUnit, Stratigraphic unit in the sense of NACSN, or International Stratigraphic Code ), or a classifier used to characterize parts of the Earth (e.g. lithologic map unit like 'granitic rock' or 'alluvial deposit', surficial units like 'till' or 'old alluvium'). It includes both formal units (i.e. formally adopted and named in an official lexicon) and informal units (i.e. named but not promoted to a lexicon) and unnamed units (i.e., recognizable, described and delineable in the field but not otherwise formalised). In simpler terms, a geologic unit is a package of earth material (generally rock).

[jjkaelin]

This comments addresses the use of the terms Unit and Formation in engineering geology and geotechnical practice. The discussion applies however equally to other uses of Unit, e.g. GeotechnicalUnit and HydrogeoUnit.

1. I see a basic conflict between using Unit as an abstraction for the many terms used in practice, which is a valid data engineering approach, and geotechnical practice. I question that these "unit" terms will be understood "at first glance" by practitioners (which In my opinion is important).

2. In engineering geology reports, I have generally found Formation to be more used than Unit to describe stratigraphic units (but not exclusively).

Discussion of the terminology Units vs Formation should agree the vocabulary to be used here.

As part of the semantic context, hierarchies should also be considered and allowed, e.g. geological members of a geological formation. The same applies to e.g. aquifers and groundwater basins.

Other uses of Unit (put here for simplicity):

GeotechnicalUnit -> I find this term confusing. Different practitioners and project groups will understand this term differently. Generally (Europe + international practice) the term Homogeneous Zone is being used to describe what I understand as the intended meaning of Geotechical Unit.

HydrogeoUnit: the use of Aquifer and GroundwaterBasin should be considered, being more understandable.

[mbeaufils]

Additionnal details on the GeologicUnit concept. The CGI vocabularies introduce sub-types:.

[dponti]

I'm satisfied with the definition here. I think the question for us to resolve is with the definition of the geotechnical unit with respect to a geologic unit - does it stand alone or is it best defined as a specialization of geologic unit based on properties of interest, much like a geophysical unit is considered by CGI as a type of geological unit.

[neilchadwick-dg]

@jjkaelin I sympathise with your concern on terminology. Unit vs 'stratum' is something I researched for AGSi. I found that unit was probably the correct term over stratum, but I see that many people (mainly engineers) always use stratum. 'Formation' I thought was reserved as part of the 'official' geological classification system for a country, which may not always correspond one to one with the 'unit' that the project team happens to be using (it is acceptable for them to differ). But I should probably defer to the geologists on this!

Generally, I'm happy with geologicunit. My recollection is that geologicunit in the CGI vocab was a high level catch-all term. In practice, for most uses cases that we are dealing with geologicunit will be sufficient. If very detailed geological work is done then some of the other unit types may come into play, but I see those as a minority use cases (more relevant to GeoSciML).

My (and the AGSi) view is that a geologicunit is part of an observational model. It is usually the 'best guess' of what is going on in the ground. The classification will normally be driven by the engineering geology. Uncertainty is inherent, of course.

In the majority of cases, the geologicunit will not be directly used in design.

It is the geotechnicalunit that will be used in analysis/design. This is normally derived from the geologic units. The classification system may be the same, or it may differ to suit to ground and/or analysis method. The extents are likely to differ to take account of uncertainty, i.e. assessing 'characteristic' extends. Parameters for design can be assigned.

A geological model with geological units may give birth to several different geotechnical models with different geotechnical units (or at least different parameters).

.

[jjkaelin]

neilchadwick-dg wrote:

In the majority of cases, the geologicunit will not be directly used in design

This is what is bothering me. In my view it seems obvious that all our terminology should be usable in design. It may not be the local "dialect" or the project "lingo". However it should be terminology that one can use in a design report without hesitation.

Am I missing something here? I believer that I am very familiar with the differences between geological 'units' and geotechnical 'units', so that is not my issue here.

[PieGARNIER]

Personnaly, I like the "Unit" term. I think it reflects at some point a geological or geotechnical interpretation, where "stratum" of "formation" may be understood as "closed" objects (commonly associated to a horizontal layer of homogenous soil properties, but what soil & what proprerties?). "Unit" also gives flexibility, since it can refer to various "scopes" : for example a same soil volume may be an overlay of various geotechnical unit, depending on what focus is given (mechanical, granulometric, etc.).

The way I see it, since each "unit" will refer to a specific model (geotechnical, geological, design model), I think it will be understandable.

[mbeaufils]

Having a deeper look to the type of GeologicUnit as defined by CGI and reused in the INSPIRE Directive.

Also considering that the GeologicalUnit is a generic term, do we target a specific GeologicUnit for the geological model? eg. LithostratigraphicUnit?

[Didymograptus]

I suggest that at the moment we don't target a specific type. Let's define what we /the industry needs first and then see if it fits with an established definition. There is no need to constrain our thoughts at the moment.