The natural gamma radiation (NGR) data were acquired each 10 cm (see details below) on fluvio-lacustrine and palustrine sediment materials, from the continental CDB1 drill-core located in the Rennes Basin (Chartres de Bretagne, France, geographic coordinates: 48.04412074N; 1.71899076W, Elevation: +37m NGF, Nivellement Général de France). The studied geological time interval covers the Eocene-Oligocene transition interval, roughly from 31 to 38.5 Ma.
NGR measurements were conducted by the Semm Logging company during the drilling campaign of the CINERGY project (CDB1 borehole) during summer 2010, under the supervision of BRGM (Bureau de Recherches Géologiques et Minières, France), on behalf the contracting authority of Chartres-de Bretagne. The main objective of the CINERGY project was to explore the Cenozoic cover of the Armorican shield preserved in a small strike-slip basin. The CINERGY Project was funded by the BRGM, the AELB (Agence de l'Eau Loire-Bretagne) and the ADEME (Agence de l'Environnement et de la Maitrise de l'Energie) French agencies.
The NGR probe used during the CDB1 well logging consists of a scintillation detector (sodium iodide crystal coupled to a photo-multiplication tube), which converts gamma rays into electric pulses. The measured values are expressed in API (American Petroleum Institute) unit. This unit was calculated using a linear interpolation of raw data expressed in pulses per second (pps) according to a calibration set with a very stable source, the radioactivity of which is known in API unit. Because disintegration is a random phenomenon, raw measurements taken every 2 cm are integrated over a 25 cm long window and restored at a 10 cm step, which is consistent both with the crystal size and the average dip of the strata, nearly 15 degrees.
