the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Geophysical downhole logging analysis within the shallow depth ICDP STAR drilling project (Central Italy)
Abstract. The ICDP STAR drilling project aims to study the seismic and aseismic fault slip behaviour of the active low-angle Alto Tiberina normal Fault (ATF) in the Northern Apennines, Central Italy, drilling and instrumenting six shallow boreholes with seismometers and strainmeters. During the STAR field work, a geophysical downhole logging campaign was carried on defining the optimal target depth for instrument deployment and formation rock characterization. In particular, the main objectives of this study were to define in situ physical properties of the rocks and the tectonic discontinuity geometry along the boreholes. The downhole logging data provide new findings and knowledge especially with regards to the physical properties such as resistivity, gamma ray and wave velocity. The collected parameters were compared to the results of literature data collected in similar lithologies, as well as with the results of logging performed in deeper wells drilled for commercial purposes. The physical properties of the Mesozoic-Early Tertiary calcareous formations show low Gamma Ray values and high compressional (Vp) and shear wave (Vs) velocities (up to 5.3 km/s and 2.9 km/s, respectively), whereas the overlying clay-rich Late Tertiary formations exhibit high Gamma Ray and low resistivity and relatively low Vp and Vs values (up to 3.5 km/s and 2.0 km/s, respectively). The results obtained from the analysis of the orientations of the tectonic structures, measured along the six boreholes, show a good agreement with the orientations of the present-day extensional stress field, NE-SW oriented. Our study allowed to bridge the gap between the physical properties obtained from literature data and those obtained from the deep wells measurements, representing a possible case history for future projects. These new data will contribute to the advancement of knowledge of the physical properties of the rocks at shallow depths, typically overlooked.
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RC1: 'Comment on egusphere-2024-1249', Maria Beatrice Magnani, 10 Jun 2024
The paper by Dr. Montone and colleagues presents new geophysical log data and analysis from six shallow (max depth 160 m) boreholes drilled as part of an ongoing project (STAR project) focused on monitoring the seismic and aseismic deformation on one of the most active low angle normal faults in the Northern Apennines, the Alto Tiberina Fault (ATF). The boreholes are located in the northwest portion of the actively extending region of the Umbria-Marche Apennines. This region of the Apennines has been the location of several, significant, deadly earthquake sequences, the last one of which began in 2016, culminating with a Mw 6.6.
The new data provide insights into the petrophysical properties and tectonic structure orientation at shallow depth of the lithostratigraphic units that make up the northern Apennines, from the late Mesozoic limestones of the Umbria-Marche pelagic units through the more recent siliciclastic turbiditic deposits. The Umbria-Marche carbonate multilayer is capable of hosting sustained levels of seismicity, and any direct information of the physical properties and fractures of these rocks is therefore relevant to understand their mechanical behavior. Additionally, these data are important because little is known about the lithologies at these shallow depths, and the petrophysical properties measured in situ can be then compared with other analogue measurements acquired in deeper boreholes or in lab experiments, allowing estimates of the effects of confining pressures and different stress conditions.The analysis of the orientation of the tectonic structures from the boreholes shows that they are favorably oriented for formation/reactivation in the present stress field, and the authors show that the stress field orientation at shallow depths persists unchanged throughout the crust in this sector of the Apennines.
The paper is well structured, written and illustrated, with an exhaustive explanation of downhole logging processing and analysis, and a clear description of results, and it is a relevant contribution to the Solid Earth community.
Minimal edits can improve the readability and clarity of the paper. Suggestions are listed briefly below. I suggest that the physical properties of the different formations of the Umbria-Marche multilayer investigated by the boreholes be summarized in a table (for example by adding this information in Table 1). In addition, it would be useful to know how the location of the boreholes was selected (which criteria guided the site selection and identification).
Technical corrections:
Line 34 - EPOS initiative: Please spell out EPOS when using it for the first time.
Line 50 - "monitor and record THE seismicity of the low-angle...." (insert THE)
Line 52 - "measure small creep events": what is intended here by "small"? short duration? slow?
Line 93 - replace "rock MASSES" with "rock VOLUMES"
Line 165 - replace "conductibility" with "conductivity"
Line 205 - This section refers repeatedly to images (TT images, acoustic images, AMPL images) but there is no reference to a specific figure to better follow the text. I suggest the authors select a clear figure from one of the boreholes and use it for this purpose here.
Line 275 - add "velocity" in "shear and compressional sonic logs VELOCITY to compare it to fracture porosity"
Line 368 - remove "accurately" and replace with "conduct ACCURATE geophysical downhole logs"
Line 414 - remove IN and UP in "these values help IN building UP"
Line 417 - remove INTO in "reflect INTO their"
Line 419 - rephrase "higher velocitIES COMPARED to less competent"
Fig. 6 - there is some random text on the "Tadpole true" column. Remove
Fig. 9 - Missing rectangle highlighting the portion of the borehole shown on the bottom
Citation: https://doi.org/10.5194/egusphere-2024-1249-RC1 -
AC1: 'Reply on RC1', P. Montone, 14 Jun 2024
Thank you for the comments and useful suggestions provided to improve our manuscript.
We are awaiting the conclusion of the review process to make the requested changes.
Paola Montone and Co-Authors
Citation: https://doi.org/10.5194/egusphere-2024-1249-AC1
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AC1: 'Reply on RC1', P. Montone, 14 Jun 2024
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CC1: 'Comment on egusphere-2024-1249', Giacomo Medici, 19 Jul 2024
General comments
Good and novel research in the field of structural geology/geophysics. I am providing minor comments as researcher working on download borehole logging and petrophysics.
Specific comments
Lines 11-26. Make clear the depth of investigation for the boreholes studied in the abstract. Researchers by seeing wireline and fluid logs might get the impression of a much deeper investigation.
Lines 30-97. I would push the introduction beyond literature on STAR drilling project and the regional geology. I’m only trying to bring the impact out of your novel research.
Line 97. Consider to disclose the 3 to 4 specific objectives of your research by using numbers (e.g., i, ii, and iii).
Lines 156-157. “Data acquisition from downhole logging becomes the key element in determining the rocks physical characterization”. Add recent review that incorporates techniques for downhole logging methods in fractured rocks:
- Review of Discrete Fracture Network Characterization for Geothermal Energy Extraction. Frontiers in Earth Sciences 11, 1328397
Line 292. Can you provide information on the speed of downhole logging for OBI and ABI? It can impact of the quality of the dataset and the structure picking analysis. Other data available have much lower resolution in the same geological formations. Researchers might be interested on finding the reason.
Lines 368-470. Same here. I would extend the introduction beyond literature on STAR drilling project and the regional geology. I’m only trying to bring the impact out of your novel work.
Lines 472-652. Integrate recent and relevant scientific literature on the topic.
Figures and tables
Figure 5. Variability of the thickness / lateral extension of the lithological logs is unclear.
Citation: https://doi.org/10.5194/egusphere-2024-1249-CC1 -
RC2: 'Comment on egusphere-2024-1249', Anonymous Referee #2, 25 Jul 2024
The work presents a set of borehole data acquired as part of STAR, an ICDP project aimed at studying the seismic/aseismic behavior of the Alto Tiberina Fault, one of the most active faults in the northern Apennines, Central Italy. The wells house monitoring instruments that are part of the Near Fault Observatory TABOO, a multidisciplinary research infrastructure that is part of the European Plate Observing System. Well logging drive the positioning of the monitoring instruments in the boreholes.
In the absence of core sampling, well logs are the only means of deciphering the stratigraphy crosscut by the boreholes, and acquiring data on the physical properties of rocks formations. With this in mind, although the boreholes presented here are relatively shallow (max. 133 m depth), the new dataset is certainly relevant for completing the knowledge of the area under investigation.
The well-described and well-presented dataset is, however, a standard dataset, with no elements of technological innovation.
The authors state that the main objective of their work is to bridge the gap between the physical properties in the literature, obtained from rock samples, typically from outcrops, analyzed in laboratory, and geophysical field data or deep borehole data collected a few kilometres deep. Actually, it would be worth discussing differences and similarities of deep and surface boreholes in in more detail. The data from deep and shallow wells discussed here, are comparable in terms of some parameters and not others. Why? For example, the 30% difference between seismic velocity data measured in deep and shallow wells is not correlated with fracture density, but attributed to compaction. Are there data that can confirm that, e.g. a comparison with density increase? Can the authors quantify the role of fractures?
The stress field inferred from breakout data in deep and shallow wells seems to remain the same. This is an important result. How widespread is this observation, and how does it differ from other cases where the stress field varies as one approaches the surface?
My overall impression is that as it stands, the manuscript presents a valuable collection of data, which reconfirms the knowledge already established in the literature but is a bit lacking in scientific novelty. As the manuscript is at present it would perhaps be more suitable for a data journal and not for a research journal. I would recommend a complete revision of the discussion of the results that highlight the novelty brought by this dataset, for example highlighting how the data drives the build up of a near fault observatory (e.g. how were the boreholes locations selected?), or how are the implications in the view of seismic/aseismic behavior, or also how do they close the gap between lab and field data e.g. discuss the scale difference, the upscaling problem, the different constrains.
Punctual observations:
Fig. 2: it would be nice to have the ATF located.
Fig. 4: Legend: the Marnoso-Arenacea formation is reported with 5 different colors. Why? In Fig 5 the Marnoso-Arenacea is only one. which one is it?
Fig. 5: Unit of the GR missing
Fig. 6: in the column Tadpole true there is a text that should be cancelled (Lorem ...)
Fig. 8, 9: what are the N?
L 82: "Giving food for thoughts about the effects" sounds a colloquialism. Better perhaps "that give insights into the effects"
L 546: The title of De Paola et al. 2009 (https://doi.org/10.1029/2008JB005967) is "Brittle versus ductile deformation as the main control on the transport properties of low-porosity anhydrite rocks"
Citation: https://doi.org/10.5194/egusphere-2024-1249-RC2
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