KTB Borehole Measurements: Gravimetry inside the borehole of the German Continental Deep Drilling Program

KTB Borehole Measurements Data Gravimetry inside the borehole Extensive borehole measurements were performed during the active drilling phase of the KTB pilot and main hole. The data report STR 21/03 KTB Borehole logging data contains the full description of the logging data given here. Please read it thoroughly to avoid inappropriate or wrong use of the data. The KTB borehole measurement data files contain the final processed versions of logging data from the two KTB boreholes: • KTB-Oberpfalz VB (KTB Vorbohrung/Pilot Hole or KTB-VB) • KTB-Oberpfalz HB (KTB Hauptbohrung/Main Hole or KTB-HB). Here only the acronyms KTB-VB and KTB-HB are used. In total there are 145 data files from the KTB-VB and 239 data files from the KTB-HB. All logs were run in open hole unless noted otherwise (see the file header). The maximum logging depth was 4001 m in the KTB-VB and 9085 m in the KTB-HB. The files of the Gravimetry measurements contain data from measurements made with a Lacoste-Romberg gravity meter sonde from the company EDCON, USA. The gravimetry logs are not depth corrected to the reference GR because the sonde had no GR sensor. The data are provided in ASCII format. Detailed descriptions are provided in the associated data report (STR 21/03, Kueck et al., 2021) and the KTB Borehole Measurements Catalog. Acknkowledgements: The GFZ German Research Centre for Geosciences, Potsdam, Germany, as successor of the KTB Project Management provides the logging data, which were obtained under grants RG8604, RG8803 and RG 9001 of the Federal Ministry of Research and Technology of Germany.

General Information about the KTB Program The KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) was the German Continental Deep Drilling Program with drilling operations from 1989 to 1994. Two boreholes, one 4 and the other 9.1 km deep, served as a telescope into the earth's interior to allow for in-situ observation of physical and chemical processes and calibration of surface experiments. One of the major goals of the KTB was the elucidation of structure and evolution of the interior zones in a former mountain chain. The drill site near Windischeschenbach, (NE Bavaria, Germany) is at the structural boundary between the Saxothuringian and Moldanubian, two major tectonostratigraphic units of the Hercynian fold belt of the Variscan Orogen in Central Europe. This boundary was regarded as a suture zone formed by the closure of a former oceanic basin c. 320 million years ago. This process gave way to a continent-continent collision and the formation of a mountain chain comparable to the today's extension of the Himalayan mountain chain. Today, the high mountain relief is eroded and, therefore, once deeply buried rocks are exposed at the surface. Therefore, this area is an ideal place for the study of deep-seated crustal processes. Furthermore, detailed geophysical surface experiments during pre-site survey studies revealed that the drill site is characterized by an anomalous high electrical conductivity and pronounced gravimetric and magnetic anomalies at drillable depth of about 10 km. Key questions to be addressed by continental deep drilling include the evaluation of fundamental processes occurring in the lithosphere, the outer skin of our planet and resource base for mankind. KTB's major research topics were: • Evaluation of geophysical structures and phenomena • Investigation of the thermal structure of the continental crust • In-situ investigation of rock-fluids and their contribution to formation of ore deposits • Elucidation of structure and evolution of the continental crust • Determination of the earth's stress field Furthermore, the establishment of a long-term depth laboratory was an initial goal of the KTB which has been realized by the GFZ – German Research Centre for Geosciences since 1996. The KTB drilling and research activities formed a German key contribution to worldwide efforts on understanding our planet and supported the founding of the International Continental Scientific Drilling Program, ICDP. Surface and sample investigations have shown that the rock pile drilled is a steeply dipping highly deformed orogenic stack whose major amphibolite facies overprint is of Caledonian age. Late-stage Tertiary faulting created the current block structure while the Hercynian impact such as granitic intrusions is minor. Borehole logging, tests and key experiments have revealed a higher than expected geothermal gradient of 28°/km with about 270°C and close to ductile conditions at 9100 m, a variable and close-to-fail stress regime in accord with models, and a large reservoir of saline deep fluids existing in a fracture-dominated reservoir down to at least 9 km depth.