Dear Mr. Shaw:

It may happen to find in this publication an answer to your question:

DILL, H.G., BUZATU A., and MAFTEI A.E. (2017) Capturing digital data with handheld devices to determine the redox regime, lithology and provenance of siliciclastisediments and residual deposits - A review and field manual.- Arabian Journal of Geosciences 10: 188-217 (plus supplementary data).

I wish you much success

H.G.Dill

Dear Ashok,

At the Geological Survey of the Netherlands we routinely use spectral gamma ray logs in a number of our deeper boreholes (20-500 m) for the purpose of geological mapping and characterisation of Tertiary and Quaternary deposits. These consist mainly of sandy and clayey marine and fluvial sediments, not just clay as in your case. Moreover, in the Dutch lowlands, the groundwater table is in general very close to the surface, so I cannot comment on the influence of drainage conditions on gamma ray values. However, I do not think that groundwater will be the most important factor influencing the gamma ray values, although one might assume that water decreases gamma ray transmission and thus cause (slightly?) lower values. Other and more important factors explaining variations in gamma ray values are a.o.:

1.       Physical characteristics of the borehole itself: diameter (inc. variations like cavities made during drilling) and presence and thickness of casings. Everything else being equal, larger diameter boreholes and greater thickness of casings will decrease gamma ray values. Often boreholes are wider and/or have thicker or more casings in the uppermost part (~tens of meters), and thus will register a lower gamma ray value in that zone. Thus, this is not to be confused with a ‘groundwater’ or drainage effect in the upper part below the surface! In our experience, the physical features of boreholes are often difficult to reconstruct, especially for older ‘archived’ boreholes, as this kind of data is frequently overlooked and not recorded. If you make your own drillings, this is very important primary information that should be recorded in detail for the proper interpretation of a.o. gamma ray logs.

2.       Grain size and the presence of clay: the smaller the grain size and the higher the clay content, the higher the gamma ray values. This is self-evident, as mapping/correlating fine-grained/clayey layers is one of the main goals of gamma ray logging.

3.       Organic matter content: in general, the more organic matter, the higher the gamma ray values. In spectral gamma logs, this will best be seen in the much higher U gamma ray values.

4.       Mineralogy and geochemistry: These are often influenced by the provenance of the sediments. E.g. everything else being equal we see that sediments originating from deeply weathered Tertiairy deposits have much lower gamma ray values than those from more freshly eroded material from the Alpine region, due to the presence of a.o. micas in the latter, which can also be seen visually in sediment samples. The presence of glauconite also increases gamma values. As glauconite occurs only in marine to coastal/estuarine deposits, this will probably not be relevant for your ‘fluvial-only’ environments. Using spectral gamma logs, different minerals can often be determined by their K-U-Th ratios, and there is a lot of information on this subject in textbooks, papers and the Internet (see below).

The Internet yields a lot of information on the subject. Some interesting links/examples:

http://www.epgeology.com/sedimentology-f19/fluvial-sequence-stratigraphy-using-thorium-potassium-t461.html

https://water.usgs.gov/ogw/bgas/spectral_gamma/

https://agilescientific.com/blog/2013/2/26/interpreting-spectral-gamma-ray-logs.html

https://www.spec2000.net/07-grlog.htm

http://petrowiki.org/Gamma_ray_logs

http://www.pe.tamu.edu/blasingame/data/z_zCourse_Archive/P663_10B/P663_Schechter_Notes/GR%20Log.PDF

http://archiwum.inig.pl/INST/nafta-gaz/nafta-gaz/Nafta-Gaz-2016-01-01.pdf

Succes with your research!

Patrick Kiden

Spectral GR is commonly used in well bore logging to define clay type, as well as mineralogy. From this, all sorts of interpretations are made as to depositional environments and constituents. There are a number of very good texts on this, as Patrick referred to above. You may also want to refer to "Fundamentals of Well-log Interpretation, 2. The Interpretation of Logging Data" by O Serra, ISBN 0-444-42620-5  which deals with the sort of questions you refer to.

http://homepages.see.leeds.ac.uk/~earpwjg/PG_EN/CD%20Contents/GGL-66565%20Petrophysics%20English/Chapter%2012.PDF

https://agilescientific.com/blog/2013/2/26/interpreting-spectral-gamma-ray-logs.html

Although it is oxidized floodplain samples but you may have a detailed look for minute clay content variation or organic content. It is not related to drainage condition only, it may relate to state of oxidation of floodplain samples, if silt content increases gamma value will decrease automatically. Even potassium or feldspar content or mud cake on borehole wall, caving on wall may change the gamma values. You may consult the following texts:

"Fundamentals of Well-log Interpretation, 2. The Interpretation of Logging Data" by O Serra

http://www.epgeology.com/sedimentology-f19/fluvial-sequence-stratigraphy-using-thorium-potassium-t461.html

https://agilescientific.com/blog/2013/2/26/interpreting-spectral-gamma-ray-logs.html

http://petrowiki.org/Gamma_ray_logs

Thanks a lot Harald Dill, Patrick Kiden, Harald Bolt, Hilmi Salem and Prof. Islam for your valuable suggestions.

This is a good and extremely interesting question, myself and others have noted similar signals in the Cretaceous fluvial strata of Utah. This sounds like it may be a product of potassium leaching from an early stage diagenetic process caused by the interaction of palaeo-ground waters and K bearing clay rich material. This may then indicate that your deposits are of a leached palaeosol, below is an appropriate reference.

An indication of such a process would be in your spectral data by comparing the K/Th ratio. This will compare the relative enrichment (or in this case depletion!) of mobile potassium to immobile thorium.

Gary J. Hampson, William Davies, Sarah J. Davies, John A. Howell, Keith R. Adamson; Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA. Journal of the Geological Society ; 162 (4): 603–621. doi: https://doi.org/10.1144/0016-764904-059

Many thanks for an interesting topic and one pertinent to my work. Thanks also to Harald Dill, Patrick Kiden, Harald Bolt, Hilmi Salem and Prof. Islam your insights have been extremely helpful to me.

Thanks

Andy

Thanks a lot Andy.