Surface Geological Observations Research Articles

Structural evolution of the North Himalaya domes as revealed by crustal-scale seismic-reflection surveying

As the typical products of collisional orogeny, gneiss domes are important geological units with which to decipher the crustal deformation and evolutionary history of continental collision. However, their formation mechanisms remain poorly understood. This issue is well illustrated by the debate surrounding the origin of the North Himalaya gneiss dome zone, which has been attributed to middle-crustal channel flow, thrust-duplex development, extensional detachment faulting, or diapiric flow related to partial crustal melting. These models predict different internal structures within individual domes that can be tested by high-resolution seismic imaging. Here, we present newly acquired seismic-reflection data collected along an ∼120-km-long north-south traverse across the central segment of the North Himalaya gneiss dome zone. Analysis and interpretation of the seismic data constrained by surface geology observations imply that (1) the subducting Indian lower crust is decoupled from the deformed middle and upper crust in the North Himalaya, (2) a crustal-scale stack of antiformal duplexes with a structural thickness of ∼35 km defines the cores of the gneiss domes imaged by the seismic survey, and (3) highly reflective, sheetlike bodies imaged in our seismic profile are best interpreted as leucocratic intrusions developed synchronously during gneiss dome development. As a whole, our work suggests that the North Himalaya gneiss dome zone was created by coeval crustal shortening and partial melting of orogenic crust.

New constraints on the shear wave velocity structure of the Ivrea geophysical body from seismic ambient noise tomography (Ivrea-Verbano Zone, Alps)

SUMMARY We performed seismic ambient noise tomography to investigate the shallow crustal structure around the Ivrea geophysical body (IGB) in the Ivrea-Verbano Zone (IVZ). We achieved higher resolution with respect to previous tomographic works covering the Western Alps, by processing seismic data collected by both permanent and temporary seismic networks (61 broad-band seismic stations in total). This included IvreaArray, a temporary, passive seismic experiment designed to investigate the IVZ crustal structure. Starting from continuous seismic ambient noise recordings, we measured and inverted the dispersion of the group velocity of surface Rayleigh waves (fundamental mode) in the period range 4–25 s. We obtained a new, 3-D vS model of the IVZ crust via the stochastic neighbourhood algorithm (NA), with the highest resolution between 3 to 40 km depth. The fast and shallow shear wave velocity anomaly associated with the IGB presents velocities of 3.6 km s−1 directly at the surface, in remarkable agreement with the location of the exposed lower-to-middle crustal and mantle outcrops. This suggests a continuity between the surface geological observations and the subsurface geophysical anomalies. The fast IGB structure reaches vS of 4 km s−1 at 20–25 km depth, at the boundary between the European and Adriatic tectonic plates, and in correspondence with the earlier identified Moho jump in the same area. The interpretation of a very shallow reaching IGB is further supported by the comparison of our new results with recent geophysical investigations, based on receiver functions and gravity anomaly data. By combining the new geophysical constraints and the geological observations at the surface, we provide a new structural interpretation of the IGB, which features lower crustal and mantle rocks at upper crustal depths. The comparison of the obtained vS values with the physical properties from laboratory analysis of local rock samples suggests that the bulk of the IGB consists of a combination of mantle peridotite, ultramafic and lower crustal rocks, bound in a heterogeneous structure. These new findings, based on vS tomography, corroborate the recent interpretation for which the Balmuccia peridotite outcrops are continuously linked to the IGB structure beneath. The new outcomes contribute to a multidisciplinary framework for the interpretation of the forthcoming results of the scientific drilling project DIVE. DIVE aims at probing the lower continental crust and its transition to the mantle, with two ongoing and one future boreholes (down to 4 km depth) in the IVZ area, providing new, complementary information on rock structure and composition across scales. In this framework, we constrain the upper crustal IGB geometries and lithology based on new evidence for vS, connecting prior crustal knowledge to recent active seismic investigations.

Structural style of Taq Taq Anticline in the Zagros Fold-Thrust Belt in the Iraqi Kurdistan Region from the Integrated Surface and Subsurface Data

Taq Taq is a prolific oilfield in the Kurdistan Region of Iraq. It consists of a longitudinal double plunged anticline that is located within the Foothill Zone of the Zagros Fold-Thrust Belt. We studied the structural style of the Taq Taq Anticline from three balanced and retro-deformed structural cross-sections constrained by the integration of intensive surface geological observations, seismic sections and well data. The sections were constructed to understand the variation in the structural architecture of the anticline both along strike and up-section, and to describe the kinematic evolution and plausible fold model of the structure. The anticline exhibits symmetrical to slightly reverse asymmetry, and bounded by two main blind thrust faults (fore- and back-thrusts) that dip toward each other. The thrusts are interpreted to be detached along a deep-lying décollement in the KurraChine Formation (Upper Triassic) without involving basement fault. Thus, the structure can be considered as a faulted-detachment fold anticline with pop-up geometry. The average horizontal shortening value ranges between 5.6% and 8.1% with an average value of ca. 6.7%. Generally, the shortening values decrease gradually to the up section due to slight decrease in the fault offset value up-section. The increase in deformation intensity, asymmetry, shortening rate and geometry variation toward SE is mainly related to displacement variation on individual thrusts, fold wavelength and amplitude, or may be related to the effect of strike-slip fault. Therefore, the middle and NW parts of the structure can be considered as a better well location for the Taq Taq crestal reservoir.

Linkage between the India–Asia collision and far-field reactivation of the Altai mountains

The present northwest-trending Altai mountains, as far as ∼2000 km north of the Indian–Asian collision front, are dominated by a transpressional strike-slip fault system, and are generally considered as one of distant tectonic effects of the ongoing India–Asia collision. However, the Cenozoic reactivation of the Altai and its linkage with the tectonic evolution of the Tibet plateau remain mechanically challenging. Here, we constructed large-scale 2D numerical models with inherited lithospheric heterogeneities from India to Altai representing the onset of the India–Asia collision. The sensitivity of model results to variable thermo-mechanical and geometric properties of the orogen-like Tibet and Altai as well as variable thermal state of the craton-like India and Junggar reveals that both the particular lithospheric architecture of western Tibet and basement structural inheritance of central Asia regulate the long-distance deformation propagation from the India–Asia plate boundary to the Altai–Sayan region. The model results indicate that the direct collision of the underthrusting Indian lithosphere with the rigid Tarim block beneath western Tibet plays a dominant role in the ultra-long-distance deformation propagation to central Asia. As less-deformed rigid blocks, the Tarim and Junggar basins with Precambrian basements serve as secondary indenters that transfer the compressive stresses extensively to the Altai–Sayan region. Constrained by substantial surface geology and geophysical observations, this numerical study recognizes the linkage between the evolving lithospheric structure of western Tibet with the northward younging trend of reactivation of the orogenic belts in central Asia. By means of the rigid Indian–Tarim lithospheric mantle collision, the plate-convergence stress can be effectively transmitted through Tarim to Tian Shan and subsequently through Junggar to Altai, leading to successive reactivation of the Tian Shan and Altai mountains. The locations of reactivation are determined by the inherited structural zones of lithospheric weakness that formed during the Paleozoic assembly of the Central Asian Orogenic Belt.

Fold formation in Kurdish Zagros (N Iraq)

Analysis of seismic sections and wells near the frontal folds of Iraqi Kurdistan (Low- and Highly folded zone) document the interplay of folds, faults and possible mechanisms of their generation.Serial sections from 3D seismic volume in the Low folded zone are interpreted based on calibration wells. Sections display fold evolution from low amplitude kinked monocline to high amplitude ramp folding. The fold continues towards the exposed Safeen anticline with a subvertical limb. These lowland sections can also be considered as the lateral growth segment of Safeen anticline.Three geoseismic sections through frontal folds were constructed with variable quality seismic and dips, formation tops from surface and from mountains wells. These sections show tight anticlines with thrust faults cutting to near surface on both limbs; remnant normal faults; a subvertical southern limb crossed by flat thrusts; and a Triassic ductile core.Sections suggest that this part of Zagros is built of two ‘rigid’ units: an upper, Mesozoic-Neogene and a lower, Paleozoic one, separated by a ductile detachment along the lower Triassic. The upper unit is prone to folding and faulting, the lower one to faulting. The detachment in-between comprises the Kurrachine C, lower Triassic Beduh and Mirga Mir formations, and potentially the top members of Permian Chia Zairi formation. Detachment is possibly dominated by evaporites. Our detachment is thicker than only Beduh-Mirga Mir detachment proposed by recent reconstructions of others.Folds are initiated as detachment folds above thickened parts of the Lower Triassic, then evolve into ramp folds. Ramp is formed by the merger of flat thrusts with pre-existing steep earlier normal faults that affect the Mesozoic section. Shear and drag beneath these ramps lead to steepening of the southern limb and to fault propagation. A basal flat thrust is the final element of this fault propagation stage. Underlying rigid Paleozoic can form imbricates that push the Triassic detachment upwards, creating passive folds. These Paleozoic imbricates dip to the south and not to the north, as suggested by reconstructions of others. Finally, strike slip shear documented at outcrops also contributes to the final fold form.Own and others' surface geological observations, indications from seismic and radiometric data suggest that fold generation started in Late Cretaceous, continued in the Paleogene in internal Highly folded zone. In our study area only Late Neogene fold growth can be documented, which is in harmony with 5–8 Ma uplift dates of others. Others’ geomorphologic work suggests fold lateral growth is a quick process. We propose that regional folds, such as Safeen anticline grow in distinct short periods separated by longer quiet intervals.

Seismic Anisotropy and Mantle Deformation Beneath the Central Sunda Plate

AbstractThe central Sunda plate, which forms the core of Southeast Asia, has been extensively studied based on analyses of data mainly from surface geological observations. In contrast, largely due to the limited coverage by seismic stations in the area, a number of key issues associated with mantle structure and dynamics remain enigmatic. These can possibly be constrained by investigating seismic azimuthal anisotropy in the upper mantle. Here we employ the shear wave splitting technique on three P‐to‐S converted phases from the core‐mantle boundary (PKS, SKKS, and SKS) recorded by 11 stations to systematically explore the spatial variation of azimuthal anisotropy beneath the central Sunda plate. Most of the Malay Peninsula is revealed to possess mostly trench‐perpendicular fast orientations that can be attributed to mantle flow induced by the Indo‐Australian subduction. In addition, the central part of the Malay Peninsula is characterized by a 2‐layered model of anisotropy, which is possibly associated with the joint effects of lithospheric fabrics and a slab tear‐induced toroidal flow. Absolute plate motion (APM)‐parallel anisotropy is observed in northern Borneo and the Nansha Block, where APM‐driven simple shear in the transitional layer between the partially coupled lithosphere and asthenosphere is mostly responsible for the observed anisotropy. The APM‐induced flow may be locally modified by a fossil slab segment beneath Sabah.

Bayesian geological and geophysical data fusion for the construction and uncertainty quantification of 3D geological models

Traditional approaches to develop 3D geological models employ a mix of quantitative and qualitative scientific techniques, which do not fully provide quantification of uncertainty in the constructed models and fail to optimally weight geological field observations against constraints from geophysical data. Here, using the Bayesian Obsidian software package, we develop a methodology to fuse lithostratigraphic field observations with aeromagnetic and gravity data to build a 3D model in a small (13.5 ​km ​× ​13.5 ​km) region of the Gascoyne Province, Western Australia. Our approach is validated by comparing 3D model results to independently-constrained geological maps and cross-sections produced by the Geological Survey of Western Australia. By fusing geological field data with aeromagnetic and gravity surveys, we show that 89% of the modelled region has >95% certainty for a particular geological unit for the given model and data. The boundaries between geological units are characterized by narrow regions with <95% certainty, which are typically 400–1000 ​m wide at the Earth’s surface and 500–2000 m wide at depth. Beyond ~4 ​km depth, the model requires geophysical survey data with longer wavelengths (e.g., active seismic) to constrain the deeper subsurface. Although Obsidian was originally built for sedimentary basin problems, there is reasonable applicability to deformed terranes such as the Gascoyne Province. Ultimately, modification of the Bayesian engine to incorporate structural data will aid in developing more robust 3D models. Nevertheless, our results show that surface geological observations fused with geophysical survey data can yield reasonable 3D geological models with narrow uncertainty regions at the surface and shallow subsurface, which will be especially valuable for mineral exploration and the development of 3D geological models under cover.

New gravity data and 3-D density model constraints on the Ivrea Geophysical Body (Western Alps)

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

From mountain summits to roots: Crustal structure of the Eastern Alps and Bohemian Massif along longitude 13.3°E

The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearance, is clearly identified along most of the swath. The Variscan lithospheric blocks beneath the Bohemian Massif are imaged with sub-vertical boundaries. Beneath the Eastern Alps, the shape of the Moho is consistent with bi-vergent orogenic thickening, with a steeper and deeper-reaching Adriatic plate plunging northwards beneath the European plate in the north. At the junction of these plates at depth, around the previously proposed Moho gap, the root of the Eastern Alps is a broad trough characterized by a zone of low velocity-gradient that is up to 20 km thick, transitioning between crust and mantle. Our receiver-function results corroborate earlier lithosphere-upper mantle seismic tomography images, and highlight the Adriatic affinity of the Eastern Alpine slab. The zigzag deployment pattern of stations in the EASI experiment also allows distinction of short-wavelength variations perpendicular to the profile, both within the shallow and the deep crust. This underlines the importance of applying 3D imaging in complex geodynamic systems.

Electrical imaging of the geoenvironment around water infrastructures in Istanbul city

Subsurface structures from two different districts of Istanbul, hosting waste and freshwater transmission lines, were imaged by geoelectrical method. The environmental impact on Kagithane-Terkos freshwater transmission line is one of the issues. That waterline underwent a substantial landslide damage. The previous site selection of Kagithane-Terkos line was only based on surface geological observations. Even though the pipeline was positioned away from the surface scarps of landslides, the pipes were damaged. To find out the reason, we made some vertical electrical sounding measurements using Schlumberger array in the region. We inverted the electrical sounding data using 2D inversion technique. The final geoelectrical images show main landslide failures, at about 10–30 m depth, which are overlain by debris with a resistivity value of with <6 Ω m. The geoelectrical findings reveal that a buried major failure surrounds the surface landslides behind. Consequently, the water infrastructure remains under the influence of landslide. Our second application site was the area of the Sazlidere tunnel, which will transfer wastewater, polluting Sazlidere dam, to the treatment plant. The geoelectrical images along Sazlidere tunnel route show resistive (<100 Ω m) and moderately conductive (≤50 Ω m) structures along the tunnel axis, representing unaltered to highly weathered rocks, respectively. Furthermore, consecutive hidden fault zones which severely affect the construction process of the tunnel are detected and located.

Seismic reflection survey at Llancanelo region (Mendoza, Argentina) and preliminary interpretation of Neogene stratigraphic features

A shallow multichannel seismic survey reaching depths of 700/800 m was performed for the first time in the Llancanelo Lake region (Southern Mendoza Province, Argentina), in order to depict the major Neogene sedimentary-volcanic sequences that form the final infilling of the tectonic-volcanic basin where the lake is located. The seismic survey advances on the results of previous geoelectric and electromagnetic surveys carried out at early stages of the research that reached the uppermost 80–100 m of the sequences (Quaternary), and therefore they go deeper in the subsoil. All the surveys were supported by surface and subsoil geological observations. After explaining the details of the performed seismic methodology, the obtained results are discussed, which indicate the presence of three major sedimentary units with increasing volcanic (basaltic layers) intercalations with depth, that accommodate to the geometry of the depocenter. The entire sequence encompasses most of the Neogene. This research sets the methodological basis for future, more detailed shallow seismic surveys in the region.

Petrogenesis and geochronology of Cretaceous adakitic, I- and A-type granitoids in the NE Yangtze block: Constraints on the eastern subsurface boundary between the North and South China blocks

The position of the subsurface boundary between the North China block (NCB) and South China block (SCB) has been debated, mainly using evidence from surface geology and geophysical observations. Here, petrochemical and geochronological data on four early Cretaceous granitic plutons from a focused area east of the Tan–Lu fault, NE Yangtze block, are reported to provide constraints on their petrogenesis and tectonic affinity. The Chuzhou intermediate intrusions consist mainly of quartz monzonite and granodiorite, and formed at ca. 125–127Ma. They have high MgO, Al2O3, Sr, and low Rb, Y and Yb contents, together with high Sr/Y and La/Yb ratios, indicating an adakitic affinity. They also show strongly negative whole-rock εNd(t) and zircon εHf(t) (−26 to −16) values, and old Hf crustal model ages (2.2–2.8Ga). Coupled with their negative Nb–Ta anomalies, high K2O/Na2O (0.91–1.18) and low Ce/Pb (1.08–5.40) ratios, these data suggest that the Chuzhou adakites were derived from thickened Archean to Paleoproterozoic lower crust. In contrast, the Fanchang, Qingyang and Huangshan intermediate–silicic complexes are dominantly composed of I-type granodiorite and quartz monzonite and A-type monzogranite and alkali feldspar granite. The I-type intrusions (126–138Ma) were emplaced slightly earlier than the A-type intrusions (121–129Ma), and distributed around A-type intrusions with clear intrusive boundaries. The I- and A-type rocks have lower MgO, CaO, Co, Sr, and higher Rb, Nb, Th and HREE contents than the adakitic rocks, with strong depletion in Ba, Sr, P, Eu and Ti for the A-type rocks. They have higher whole-rock εNd(t) (−9 to −5) and zircon εHf(t) (−13–0) values, and younger Nd model ages (TDM2=1.1–1.4Ga) and zircon Hf model ages (Tcrust=1.2–2.0Ga) than the adakitic rocks. These features indicate that the I- and A-type granites originated by partial melting of Mesoprotorozoic–Neoproterozoic lower crust, followed by fractional crystallization of plagioclase, K-feldspar and ferromagnesian minerals. The completely different isotopic compositions suggest that the Chuzhou adakites and three I- and A-type complexes were derived from different lower crustal domains, the former with NCB affinity and the latter with SCB affinity. Thus, a subsurface boundary between the NCB and SCB in the area east of the Tan–Lu fault can be constrained to a position between the Chuzhou and Ningwu–Fanchang area, possibly near Nanjing city. The association of Cretaceous bimodal volcanic rocks, within-plate mafic rocks and rift basins, suggests that the early Cretaceous rocks in the NE Yangtze block were produced in a lithospheric extensional setting, probably related to mantle-derived magma underplating and lithospheric thinning.

What drove continued continent-continent convergence after ocean closure? Insights from high-resolution seismic-reflection profiling across the Daba Shan in central China

We conducted deep seismic-reflection surveying across the Jurassic Daba Shan thrust belt of central China to investigate how and why the continued convergence between north and south China lasted ∼50 m.y. after the Triassic closure of their intervening oceans. Our study, together with surface geology, gravity surveying, and magnetic observations, indicates widespread occurrence of mafic plutons below the Daba Shan thrust belt. We propose that subduction of dominantly eclogitized mafic crust of northern south China provided the driving force for continued convergence between north and south China after ocean closure.

Ground‐penetrating radar investigation inside a karstified limestone reservoir

In this paper, we present ground‐penetrating radar (GPR) investigations performed along a 3.7 km long tunnel located inside a lower Cretaceous limestone massif of south‐eastern France. This fractured massif is mainly characterized by water circulation and karstic structures. This kind of geological formation contains a large part of the fresh underground water resources of the world and is also considered as an analogue of Middle East oil reservoirs. Since tunnel walls are covered by thick reinforced concrete, direct geological observations are impossible.After some preliminary tests, the entire tunnel was investigated using 250 MHz shielded antennas. Data are generally of very good quality, with reflection time up to 400 ns (down to 18 m under the tunnel floor with a velocity of 9 cm/ns). We correlate the GPR signal along the tunnel with surface geological observations: the upper part of the investigated formation (Bedoulian) displays prominent stratigraphic reflectors while the lower part (Barremian) does not. Numerous diffractions are observed in both formations and can be related to karstic features.These investigations allow to better constrain the geological context along the tunnel, necessary for future hydrogeological studies. We conclude that this tunnel offers a unique opportunity of performing GPR measurements within a karstified limestone massif.

Seismic reflection imaging of the mafic-ultramafic Windimurra Igneous Complex, Yilgarn Craton, Western Australia

A deep seismic reflection survey was carried out over the Meso-Neoarchean Youanmi Terrane, Yilgarn Craton in Western Australia, including 3 seismic lines across the Windimurra Igneous Complex. This gabbroic intrusion is the largest exposed single mafic-ultramafic intrusion in Australia, currently mined for magnetite-hosted vanadium in its upper zone. Acquisition was carried out with 3 Hemi 60 vibrators, 80 or 40 m VP interval, 40 m group interval, and a 300 channel symmetric split spread, resulting in 75 to 150 fold high quality data. Key processing steps included refraction and residual automatic statics, spectral equalisation, detailed stacking velocity analysis, dip moveout (DMO) correction and post stack Kirchhoff migration. The Windimurra Igneous Complex exhibits high seismic velocity (approximately 6.5 km/s), both in interval velocity from stacking velocity and bedrock velocity from refraction statics analysis. Seismic attenuation is low, resulting in high frequency and good resolution to the base of the complex, seen at a maximum two way time of around 2.5 seconds. Gross structure on the seismic sections is also consistent with the gravity and magnetic data. An inward dipping conical structure is evident and consistent with surface geological observations. The base of the complex is marked by distinctive high amplitude multi-layered reflectors, which may represent significant thicknesses of ultramafic zone material unexposed at the surface. This zone has been a long sought after target for Cr-PGE exploration. The complex internal structure of the intrusion, possibly including discordant igneous layering features, has been well imaged up to the base of regolith.

Controlled-source electromagnetics for reservoir monitoring on land

The main goal of exploration geophysics is to obtain information about the subsurface that is not directly available from surface geological observations. The results are primarily used for finding potential reservoirs that contain commercial quantities of hydrocarbons. A number of possible geophysical methods exists these days to achieve such a goal. One of them is the controlled-source electromagnetic (CSEM) method. CSEM data can provide resistivity maps of the subsurface. Because the bulk resistivity depends on the resistivity of the pore fluid, these maps may enable us to estimate the nature of the fluid content in the reservoir. The CSEM method exploits electromagnetic fields to remotely characterize the nature of the fluid content in the pores. When a dipole current source is stuck into the ground or placed in the seawater, current flows from one pole to the other through the sediments, creating an electrical field in the subsurface. If highly resistive bodies are present in the subsurface, the electrical field measured at some distance from the source will be larger in amplitude than the field in the absence of these bodies. As hydrocarbon-bearing rock is highly resistive, one may link the larger amplitude to the presence of hydrocarbon reservoirs. A logical consequence of this phenomenon is that the CSEM method may also be suited for monitoring a hydrocarbon reservoir during production. The reason is that water flooding or steam injection for oil production creates resistivity changes in the reservoir, and if those changes are large enough, we can expect differences in the CSEM response with time-lapse surveys. This consideration led us to further investigate the EM monitoring problem. We tried to answer two questions: are the time-lapse changes in the reservoir detectable, particularly in the presence of noise, and if so, could we use timelapse signals to locate where the time-lapse changes happened in the subsurface? In this thesis, we considered land CSEM and found that the resistivity change due to displacement of oil by brine can produce a small but measurable difference in the CSEM response. Interestingly, those response differences at the surface are confined to the lateral extent of resistivity changes in the subsurface, even in the presence of various kinds of repeatability noise. We found a simple and effective method to remove the repeatability noise due to the airwave. Finally, results obtained when incorporating nonlinear EM inversion into the monitoring problem suggest that this application of the CSEM method has the potential to play a significant role in the oil and gas industry.

GEOPHYSICAL INVESTIGATION OF THE TRANSITION ZONE BETWEEN THE CONGO CRATON AND THE KRIBI-CAMPO SEDIMENTARY BASIN (SOUTHWESTERN CAMEROON)

Three different sets of geophysical data are used to investigate the structure of the transition zone between the north-western boundary of the Congo Craton (CC) and the Kribi-Campo sedimentary basin. These are shear wave velocity, gravity and magnetotelluric (MT) data, respectively. The combined use of these data helps to reduce the non-uniqueness of solutions inherent to forward models, leading to a better inference of the structure of the transition zone. The interpretation of shear wave velocity models enhanced the difference in composition of upper crustal layers against the similar composition (densities of ~3.0 g/cm 3 ) of the lower crust, with different thicknesses beneath both tectonic regions. The analysis of gravity maps obtained from the area shows the signature of the main geological units and particularly the north-south gradient correlating with the strike direction of the so-called Kribi-Campo fault (KCF). Using constraints from shear wave velocity models together with surface geology observations, a 2D1/2 gravity model was obtained along a profile crossing the CC margin and the Kribi-Campo sedimentary basin. The new model is consistent with the presence of a thick mafic layer (>10 km) at depth below 18 km beneath both geological units, and also with crustal thicknesses of 28 km and 45 km beneath the basin and CC, respectively. The model also suggests that KCF resulted from the intrusion of magmatic rocks during the continental collision, which were later metamorphosed into granulites. The resulting suture may be interpreted from the model as the thrusting of the Panafrican Mobile Belt rocks onto the CC. Similar conclusions are inferred from the MT model and the signature of the above mentioned presence of granulites is interpreted in this case as low resistive rocks emplaced into high resistive materials. This provides additional support for the gravity model.

Reflection seismic imaging of the end-glacial Pärvie Fault system, northern Sweden

Reflection seismic data were acquired along a c. 23 km long profile over the Pärvie Fault system with a nominal receiver and source spacing of 20 m. An hydraulic breaking hammer was used as a source, generating signals with a penetration depth of about 5–6 km. Steeply dipping reflections from the end-glacial faults are observed, as well as sub-horizontal reflections. The location and orientation of the reflections from the faults agree well with surface geological observations of fault geometries. Reflections from a potential fourth end-glacial fault is observed further to the east along the profile. The more sub-horizontal reflections may originate from gabbroic bodies within the granitic basement or from deeper lying greenstones. Our results indicate that the end-glacial faults dip at moderate to steep dips down to at least 2–3 km depth, and possibly continue at this dip to depths of 6 km. This result has significant implications for determining the state of stress required to activate the faults in the past and in the future.

Using a 3D geological mapping framework to integrate AEM, gravity and magnetic modelling - San Nicolas case history

We present an example of the use of a software framework that combines 3D geological mapping and geophysical modelling. This approach would be relevant in a wide range of applications, from regional geological mapping, groundwater studies, through to mineral or geothermal prospecting. In this instance, we illustrate a typical workflow with a case study involving the San Nicolas VMS deposit and surrounding region. An initial 3D geological map, based on sparse surface geological observations, is progressively refined using AEM and potential field modelling results. At each stage, the geological map is used to capture and communicate the inferred distribution of map units. During the latter stages of interpretation, the geological map is also used to provide constraints for potential field modelling. In addition to the opportunity for direct detection of highly conductive basement-hosted sulphide mineralisation, the availability of AEM data proved crucial in the San Nicolas example in that we were able to use these data to determine the thickness of a moderately conductive transported cover unit. This significantly reduced the ambiguity of subsequent potential field interpretations.

On the potential of Ground Penetrating Radar to help rock fall hazard assessment: A case study of a limestone slab, Gorges de la Bourne (French Alps)

The stability of a potentially unstable rock mass is highly sensitive to the continuity of the joints cutting it. The rock fall hazard assessment, generally based on surface geological observations and on a simple geomechanical model, suffers from the lack of information on the 3D geometry and the properties of the joints. This case study investigates the potential of Ground Penetrating Radar (GPR) measurements regarding hazard assessment in hard rock. GPR was used to detect and characterize joints that could cause the failure of an overhanging rock mass located above a road in the “Gorges de la Bourne” (France). GPR data were acquired using high frequency shielded antennae (500 and 800 MHz) to assess the extension of these joints and to detect other possible fractures. Combined with a velocity profile deduced from a Common Mid-Point (CMP) analysis, these measurements provided detailed images of the fracture network, which were locally consistent with turn rate velocity measurements performed within two horizontal boreholes. The rock bridge percentage in the critical joints was estimated from GPR data to be too low to ensure the stability of the overhanging rock mass. Consequently, mining of the rock mass was decided on. Afterwards, a comparison between GPR images and the real joints observed after mining also confirmed the GPR interpretation for the location and the extension of the joints.