the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 May 2024
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
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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
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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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