Fault interpretation in seismic reflection data: an experiment analyzing the impact of conceptual model anchoring and vertical exaggeration

[EN]This paper presents an analysis on the limitations and advantages that some types of seismic data display present on the interpretation of the dataset

Fault interpretation in seismic reflection data : an experiment analysing the impact of conceptual model anchoring and vertical exaggeration

Juan Alcalde has been supported by the Natural Environment Research Council (grant no. NE/M007251/1) and the H2020 European Institute of Innovation and Technology (SIT4ME (grant no. 17024)). Clare E. Bond is currently funded through a Royal Society of Edinburgh research sabbatical on uncertainty in seismic image interpretation. Gareth Johnson is funded by the University of Strathclyde Faculty of Engineering. Oriol Ferrer has been supported by the SALCONBELT Project (grant no. CGL2017-85532-P), the Geomodels Research Institute and the Grup de Geodinàmica i Anàlisi de Conques (grant no. 2017SGR-596). Puy Ayarza is funded by the Regional Government of Castile and León (project SA065P17). The seismic image used in the experiment is available on the Virtual Seismic Atlas (https://www.seismicatlas.org, last access: 30 September 2019). The questionnaire presented to the participants is available in the Supplement. Interpretations and statistical analyses are available upon request.Peer reviewedPublisher PD

Lithospheric image of the Central Iberian Zone(Iberian Massif) using global-phase seismic interferometry

This paper presents the lithospheric structure of Central Iberia on the basis of seismic interferometry of teleseismic dat

Caracterización gravimétrica y magnética de controles estructurales en depósitos de Sn-W. Ejemplo del yacimiento de San Finx (A Coruña)

Critical raw materials are essential for the development of our society. However, most shallow ores have already been exploited and only deep targets remain unexplored. This work aims to apply indi-rect geophysical techniques to the San Finx Sn-W deposit (A Coruña) in order to get further constrains of its deep structure and geometry. Accordingly, a magnetic and a gravity survey have been carried out in the area (SE of Noia, A Coruña), at the southern part of the Mal-pica-Tui Complex. The resulting absolute magnetic anomaly and the relative gravity anomaly have been studied analytically and through 2-2.75D forward modeling. Results indicate that, the sampling inter-val (~1 km) is too high to characterize the anomalies related to the Sn-W mineralization. Contrarily, they show the potential field imprint of the regional Variscan tectonics. To better assess the relationship between the regional Variscan tectonics and the mineralization, a higher resolution survey should be acquired to detect this depositLas materias primas críticas son fundamentales para el desarro-llo de nuestra sociedad. Sin embargo, la mayoría de los yacimientos poco profundos ya han sido explotados, quedando únicamente sin explorar los objetivos profundos. En este trabajo se presentan los re-sultados de la aplicación de técnicas geofísicas indirectas al estudio del yacimiento de Sn-W de San Finx (A Coruña). Su objetivo es cons-treñir la geometría y estructura del yacimiento en profundidad. En este sentido, se ha realizado una campaña de adquisición de datos magnéticos y gravimétricos en dicha zona (SE de Noia, A Coruña) en la parte sur del Complejo Malpica-Tui. La anomalía magnética absoluta y la anomalía gravimétrica relativa resultante se han estu-diado analíticamente y mediante modelización directa 2-2,75D. Los resultados indican que el intervalo de muestreo usado (~1 km) es muy alto para individualizar la respuesta magnética y gravimétrica del yacimiento. Sin embargo, los mapas obtenidos muestran que la signatura de campos potenciales está ligada a la tectónica Varisca a gran escala. Encontrar la relación entre esta última y la mineraliza-ción y hacer un muestreo geofísico de alta resolución en la zona son la clave para entender este depósit

Fault interpretation in seismic reflection data: an experiment analysing the impact of conceptual model anchoring and vertical exaggeration

The use of conceptual models is essential in the in- terpretation of reflection seismic data. It allows interpreters to make geological sense of seismic data, which carries inherent uncertainty. However, conceptual models can create powerful anchors that prevent interpreters from reassessing and adapting their interpretations as part of the interpretation process, which can subsequently lead to flawed or erroneous outcomes. It is therefore critical to understand how conceptual models are generated and applied to reduce unwanted effects in interpretation results. Here we have tested how interpretation of vertically exaggerated seismic data influenced the creation and adoption of the conceptual models of 161 participants in a paper-based interpretation experiment. Participants were asked to interpret a series of faults and a horizon, offset by those faults, in a seismic section. The seismic section was randomly presented to the participants with different horizontal-vertical exaggeration (1 : 4 or 1 : 2). Statistical analysis of the results indicates that early anchoring to specific conceptual models had the most impact on interpretation outcome, with the degree of vertical exaggeration having a subdued influence. Three different conceptual models were adopted by participants, constrained by initial observations of the seismic data. Interpreted fault dip angles show no evidence of other constraints (e.g. from the application of accepted fault dip models). Our results provide evidence of biases in interpretation of uncertain geological and geophysical data, including the use of heuristics to form initial conceptual models and anchoring to these models, confirming the need for increased understanding and mitigation of these biases to improve interpretation outcomes

Potential field signature (gravity and magnetism) of the Otero de Herreros deposit (Segovia). Comparison with existing data and models

Este trabajo presenta nuevos mapas de anomalía magnética y gravimétrica absoluta (Bouguer) del yacimiento polimetálico del Ce-rro de Almadenes (Otero de Herreros, Segovia). Los datos han sido adquiridos y procesados con equipamiento y técnicas modernas (eli-minando todas las posibles fuentes de ruido), y los mapas obtenidos se han comparado con los mapas de anomalías ya existentes. La adquisición de datos detallados de campos potenciales proporciona nuevos criterios para controlar la geometría de las capas mineraliza-das, restringiendo su potencia y profundidad, a través de la modeli-zación directa en 2D de perfiles perpendiculares a las estructuras. Los resultados muestran que mientras que la anomalía magnética ayu-da a constreñir la geometría de la capa M mineralizada, la anomalía gravimétrica puede indicar las zonas donde la mineralización está más concentrada. Este trabajo muestra que el avance metodológico e instrumental en las técnicas de adquisición de datos de campos potenciales da lugar a mapas de anomalías mejor constreñidas, que permiten una modelización más detallada de las mismasMagnetic and gravity data have been acquired at the polyme-tallic deposit of Cerro de Almadenes (Otero de Herreros, Segovia), using modern equipment and techniques, resulting in new anomaly maps that are compared with existing anomaly maps. The acquisi-tion of this new data provides additional criteria to control the geo-metry of the mineralized layers, partly constraining their width and depth through forward 2D modeling techniques. The new datasets and models are not influenced by surface magnetic noise or by fac-tors related to data processing, as previous ones. The resulting mag-netic anomaly allows to constrain the geometry of the mineralized M-layer while the gravity anomaly indicates the interval where the ore is concentrated. Recent methodological and instrumental advan-ces in potential field data acquisition techniques has enabled us to obtain better data and more constrained anomaly maps, which can be modeled with more detailed techniques

Seismic imaging and modelling of the lithosphere of SW-Iberia.

[EN]Data from a closely spaced wide-angle transect has been used to study the middle-to-lower crust and the Moho in SW-Iberia. A low-fold wide-angle stack image reveals a highly heterogeneous seismic signature at lower-crustal levels changing laterally along the profile. The lower crust features an irregular distribution of the reflectivity that can be explained by a heterogeneous distribution of physical properties. The Moho discontinuity also features a high variability in its seismic character that correlates with the different tectonic terranes in the area. A 2D finite difference code was used for solving the elastic wave equation and to provide synthetic wide-angle shots. Relatively simple layer cake model derived from conventional refraction interpretation generates the main events of the shot records. However, these models cannot account for the lateral variability of the seismic signature. In order to obtain more realistic simulations, the velocity model was modified introducing stochastic lensing at different levels within the crust. The Moho was modelled as a 3 km thick layered structure. The resulting average velocity models include a high velocity layer at mid-crustal depth, a highly reflective lower crust and a relatively thin horizontal Moho. This heterogeneous model can be achieved by lensing within the crust, a layered mafic intrusion and a strongly laminated lower crust and Moho

Gravity and magnetic anomalies in the allochthonous Órdenes Complex (Variscan belt, northwest Spain): Assessing its internal structure and thickness

The Órdenes Complex is the largest Variscan allochthonous structure of NW Iberia, and preserves the suture of a long-standing Paleozoic ocean, probably the Rheic. Gravity and magnetic data, the latter specifically acquired on land for this study, show that the complex occupies the core of an open synform with a maximum depth of 9–10 km, which contrasts with the flat geometry of the lower crust and Moho discontinuity beneath. The maximum depth reached by the ophiolitic rocks marking the suture is around 7 km. The allochthonous units formed by basic and ultrabasic rocks are lens-shaped in section, and occur not only at the periphery of the complex, but form wide ribbons trending NE-SW to N-S. The Bouguer anomaly related with the longest of them, the Fornás Unit, previously used to support an autochthonous interpretation of the complexes, is modeled as a rootless, massive amphibolite body with a maximum thickness of 6 km located at the downthrown block of a large normal fault cutting across previous thrust faults and extensional detachments. The main magnetic anomalies are associated with ultrabasic rocks cropping out in the NW and SE, but a weak, wide anomaly in the central part of the complex is related with one or more thin layers of amphibolite partly mineralized with massive sulphides. The weakly arcuate geometry of this anomaly and of the Bouguer anomaly caused by the Fornás Unit may reflect the NE flank of the Central Iberian arc, an orocline whose core is occupied by the allochthonous complexes.FEDER, Ministerio de Ciencia e Innovación (Ramón y Cajal

Is it possible to retrieve Moho reflections from high-frequency autocorrelations of ambient noise?

[EN]This presentation describes the process to retrieve Moho reflections from autocorrelation of seismic noise dat

Seismic structure and composition of the southern central Iberian crust: The ALCUDIA wide angle seismic reflection transect

The authors thank the Associated Editor and the anonymous reviewers for their thoroughly valuable suggestions and comments that improved the manuscript. Seismic data were collected in 2012 with funding provided by the Spanish Ministry of Science and Innovation (grants: CGL2004-04623/BTE, CGL2007-63101/BTE, CGL2011-24101, CSD2006-00041). Instrumentation was provided by the IRIS-PASSCAL instrument center, Socorro, New Mexico, USA. The seismic data, including experiment geometry are stored in the IRIS-PASSCAL facilities and can be accessed through the IRIS-PASSCAL data management center. I.P. is funded by the Spanish Government and the Universidad de Salamanca with a Beatriz Galindo grant (BEGAL 18/00090). S.A. Ehsan is funded by the European Commission grant Marie Curie Actions (264517-TOPOMOD-FP7-PEOPLE-2010-ITN). We thank Instituto Geologico y Minero de Espana for providing the logistic help and an academic crew for data acquisition. GMT was used to prepare some of the figures shown in the paper.The nature of the crust beneath central Iberia was estimated by a wide-angle seismic reflection/refraction transect, ALCUDIA-WA, which sampled the southern half of the Variscan Central Iberian Zone, covered in the north by the Cenozoic Tajo Basin. The shot gathers recorded by vertical component sensors revealed well defined P- and S-wave phases. These arrivals were modeled by an iterative forward approach providing 2D crustal models showing variations in the velocity distribution with upper crustal P- and S-wave velocities increasing northwards. The lower crust P-wave velocities are homogeneous along the profile while the S-wave velocities slightly increase northwards. The Moho is placed at 32 km depth in the southern edge of the profile, deepening northward down to 35 km beneath the Tajo Basin. The Poisson's ratio, calculated from P- and S-wave velocities, varies along the profile at upper crustal depths. The highest values are located below the Mora and Pedroches batholiths. These resulting physical properties can serve to constrain the crustal composition by comparing them with laboratory measurements on rock samples. Our results suggest that the upper crust in the southern and central segments of the ALCUDIA profile is made up of low-grade metasedimentary rocks, while the northern segment is dominated by igneous rocks, in agreement with the surface geology. Separated by a sharp boundary located between 12 km (south) and 18 km (north) depth, the lower crust is more homogeneous and shows low Poisson’ ratios compatible with a rather felsic composition. However, outstanding lamination described in coincident vertical incidence data indicates some degree of intercalation with mafic components.Spanish Government CGL2004-04623/BTE CGL2007-63101/BTE CGL2011-24101 CSD2006-00041Spanish GovernmentEuropean Commission BEGAL 18/00090Universidad de Salamanca BEGAL 18/00090European Commission grant Marie Curie Actions 264517-TOPOMOD-FP7-PEOPLE-2010-IT