Regional Geophysics Research Articles

Regional-scale fault-to-structure earthquake simulations with the EQSIM framework: Workflow maturation and computational performance on GPU-accelerated exascale platforms

Continuous advancements in scientific and engineering understanding of earthquake phenomena, combined with the associated development of representative physics-based models, is providing a foundation for high-performance, fault-to-structure earthquake simulations. However, regional-scale applications of high-performance models have been challenged by the computational requirements at the resolutions required for engineering risk assessments. The EarthQuake SIMulation (EQSIM) framework, a software application development under the US Department of Energy (DOE) Exascale Computing Project, is focused on overcoming the existing computational barriers and enabling routine regional-scale simulations at resolutions relevant to a breadth of engineered systems. This multidisciplinary software development—drawing upon expertise in geophysics, engineering, applied math and computer science—is preparing the advanced computational workflow necessary to fully exploit the DOE’s exaflop computer platforms coming online in the 2023 to 2024 timeframe. Achievement of the computational performance required for high-resolution regional models containing upward of hundreds of billions to trillions of model grid points requires numerical efficiency in every phase of a regional simulation. This includes run time start-up and regional model generation, effective distribution of the computational workload across thousands of computer nodes, efficient coupling of regional geophysics and local engineering models, and application-tailored highly efficient transfer, storage, and interrogation of very large volumes of simulation data. This article summarizes the most recent advancements and refinements incorporated in the workflow design for the EQSIM integrated fault-to-structure framework, which are based on extensive numerical testing across multiple graphics processing unit (GPU)-accelerated platforms, and demonstrates the computational performance achieved on the world’s first exaflop computer platform through representative regional-scale earthquake simulations for the San Francisco Bay Area in California, USA.

Cenozoic tectonic transition within the western segment of the Longmenshan fault, southeast margin of the Tibetan Plateau: Insights from geological and geophysical data

Thrust fault zones around the Tibetan Plateau (TP) record the tectonic evolution between the Plateau and its surrounding terranes, which is helpful for understanding the uplift mechanism and deformational processes of the TP. The Longmenshan fault (LMSF) is the tectonic boundary (TB) between the Yangtze terrane (YT) and Songpan-Garze terrane (SGT), while the TB of its western segment, either the Lijiang-Xiaojinhe fault (LXF) or Jinhe-Qinghe fault (JQF), is controversial. Therefore, we conducted magnetotelluric (MT) imaging, surface structure surveys, and petrologic analysis to further determine the deep–shallow structural relationship of the western LMSF segment. Resistors R1 and R2 revealed by MT imaging may have originated from different magmatism. Among them, R2 may have originated from plume underplating, which is consistent with previous studies, while R1 may have originated from the residue of episodic mafic magma intrusion along the JQF over a broader period. Based on regional geophysics, surface structural patterns and petrologic mineralogy, it is suggested that the JQF may have deformed deep into the lower crust or upper mantle, accommodating the southeast expansion of the TP by thrusting and acting as the TB between the YT and SGT before ∼15 Ma. After ∼15 Ma, due to the activation of the large-scale strike-slip faults, the LXF gradually replaced the JQF to dominate the structural deformation of the western LMSF segment. Our results indicate that the above tectonic transition might be associated with the geodynamic process from centralized deformation to diffuse deformation within the southeast TP during the late Cenozoic.

Thermal Equations of State of Magnesite: Implication for the Complex Mid‐Lower Mantle Seismic Scatterers

AbstractMagnesite (MgCO3) entering the lower mantle together with the subducted oceanic crust is an important carbon carrier. The reaction between magnesite and mantle minerals has been documented, but its influence on the density and velocity profiles of lower mantle remains unexplored. To decipher the deep carbon transportation and its associated effect, here we determined the thermal equations of state of magnesite up to 120 GPa and 2600 K using X‐ray diffraction in laser‐heated diamond anvil cells. The obtained thermal elastic parameters of magnesite facilitated a comprehensive understanding on the influence of magnesite‐SiO2 reaction, variation of carbon and SiO2 content, and temperature on the origin of lower‐mantle scatterers at 1,000–1,800 km depth. Our modeling revealed that the depth of the lower‐mantle VS scatterers is mainly controlled by the Al2O3 content in SiO2, while its magnitude depends on the SiO2 content. Along normal geotherm, the magnesite‐SiO2 reaction would occur before the post‐stishovite transition, consuming substantial SiO2 in the subducted oceanic crust. Depending on the amount of residual SiO2, the post‐stishovite transition can produce a 2.5–5.2 (2)% VS reduction, compatible with the observed seismic scatterers in Izu‐Bonin and Mariana subduction zones. Along slab geotherm, this reaction occurs after the post‐stishovite transition, generating a greater VS reduction of 4.4–6.4 (4)%. We thus propose that the reaction between sinking MgCO3 and SiO2 in the slab is one of the potential factors influencing the magnitude of the lower‐Vs scatterers at 1,000–1,900 km depth. Our results provide new insights into the deep‐mantle carbonate transportation influencing regional geophysics.

Regional Geophysics of the Caribbean and Northern South America: Implications for Tectonics

AbstractThe Caribbean plate is an enclosed oceanic basin whose formation and evolution are controversial. In the most commonly accepted model, the Caribbean plate is mainly composed of the Caribbean Large Igneous Province (CLIP) and the buoyant characteristic of this oceanic plateau resisted subduction and allowed an eastward migration to its present position north of South America. In this study, we integrate a broad range of geophysical and geomorphological data to define structural elements and present‐day tectonics of the Caribbean plate and the surrounding region. We present a Bouguer gravity anomaly map and a new crustal thickness map that documents large areas of normal‐thickness oceanic crust within the Venezuela and Colombia basins of the Caribbean plate. Selected cross sections of seismicity and P‐wave anomalies from a seismic tomographic model depict the present‐day geometry of subducting oceanic plates within the Caribbean region. We observe that rather than resisting subduction, as expected for the thick crust of a buoyant large igneous province, the subduction of the Caribbean plate can be traced to a depth of 600 km beneath NW South America. This, together with the crustal thickness map, implies that a significant area of the Caribbean plate, including the subducted portion, is composed of normal‐thickness oceanic crust. As proposed by the Pacific origin model, the Caribbean plate likely migrated eastward from the Pacific Ocean as an oceanic plate mostly with normal‐thickness crust and limited portions of the crust thickened by hot spot volcanism (CLIP).

Regional geophysics of the Caribbean and northern South America: Implications for tectonics

Regional geophysics of the Caribbean and northern South America: Implications for tectonics

Data–driven prospectivity modelling of sediment–hosted Zn–Pb mineral systems and their critical raw materials

Demand for critical raw materials is expected to accelerate over the next few decades due to continued population growth and the shifting consumption patterns of the global economy. Sedimentary basins are important sources for critical raw materials and new discoveries of sediment–hosted Mississippi Valley–type (MVT) and/or clastic–dominated (CD) Zn–Pb deposits are likely required to mitigate future supply chain disruptions for Zn, Pb, Ag, Cd, Ga, Ge, Sb, and In. Herein we integrate public geoscience datasets using a discrete global grid to system to model the mineral potential for MVT and CD deposits across Canada, the United States of America, and Australia. Statistical analysis of the model results demonstrates that surface–wave tomography and derivative products from satellite gravity datasets can be used to map the most favourable paleo–tectonic settings of MVT and CD deposits inboard of orogenic belts and at the rifted edges of cratonic lithosphere, respectively. Basin development at pre–existing crustal boundaries was likely important for maintaining the low geothermal–gradients that are favourable for metal transport and generating the crustal fluid pathways that were reactivated during ore–formation, as suggested by the statistical association of both sediment–hosted mineral deposit types with the edges of upward–continued gravity and long–wavelength magnetic anomalies. Multivariate statistical analysis demonstrates that the most prospective combination of these geophysical datasets varies for each geological region and deposit type. We further demonstrate that maximum and minimum geological ages, coupled with Phanerozoic paleogeographic reconstructions, represent mappable proxies for the availability of oxidized, brine–generating regions that are the most likely source of ore–forming fluids (e.g., low– to mid–latitude carbonate platforms and evaporites). Ore deposition was likely controlled by interaction between oxidized, low–temperature brines and sulfidic and/or carbonaceous rocks, which, in some cases, can be mapped at the exposed surface or identified using the available rock descriptions. Baseline weights–of–evidence models are based on regional geophysics and are the least impacted by missing surface information but yield relatively poor results, as demonstrated by the low area–under–the–curve (AUC) for the spatially independent test set on the success–rate plot (AUC = 0.787 for MVT and AUC = 0.870 for CD). Model performance can be improved by: (1) using advanced methods that were trained and validated during a series of semi–automated machine learning competitions; and/or (2) incorporating geological and geophysical datasets that are proxies for each component of the mineral system. The best–performing gradient boosting machine models yield higher AUC for the test set (AUC = 0.983 for MVT and AUC = 0.991 for CD) and reduce the search space by >94%. The model results highlight the potential benefits of mapping sediment–hosted mineral systems at continental scale to improve mineral exploration targeting for critical raw materials.

Coupling of regional geophysics and local soil-structure models in the EQSIM fault-to-structure earthquake simulation framework

Accurate understanding and quantification of the risk to critical infrastructure posed by future large earthquakes continues to be a very challenging problem. Earthquake phenomena are quite complex and traditional approaches to predicting ground motions for future earthquake events have historically been empirically based whereby measured ground motion data from historical earthquakes are homogenized into a common data set and the ground motions for future postulated earthquakes are probabilistically derived based on the historical observations. This procedure has recognized significant limitations, principally due to the fact that earthquake ground motions tend to be dictated by the particular earthquake fault rupture and geologic conditions at a given site and are thus very site-specific. Historical earthquakes recorded at different locations are often only marginally representative. There has been strong and increasing interest in utilizing large-scale, physics-based regional simulations to advance the ability to accurately predict ground motions and associated infrastructure response. However, the computational requirements for simulations at frequencies of engineering interest have proven a major barrier to employing regional scale simulations. In a U.S. Department of Energy Exascale Computing Initiative project, the EQSIM application development is underway to create a framework for fault-to-structure simulations. This framework is being prepared to exploit emerging exascale platforms in order to overcome computational limitations. This article presents the essential methodology and computational workflow employed in EQSIM to couple regional-scale geophysics models with local soil-structure models to achieve a fully integrated, complete fault-to-structure simulation framework. The computational workflow, accuracy and performance of the coupling methodology are illustrated through example fault-to-structure simulations.

Coastal and offshore provinces of Timor-Leste — Geophysics exploration and drilling

Regional geophysics research provides for prospect assessment of Timor-Leste, part of the Southeast Asia Archipelago in a region embracing the Banda Arc, Timor Island, and the northwest Australia Gondwana continental margin edge. Timor Island is a microcontinent with several distinct tectonic provinces that developed initially by rifting and drifting away from the Australian Plate. A compressive convergence began in the Miocene whereby the continental edge of the large craton collided with the microcontinent, forming a subduction zone under the island. The bulk of Timor Island consists of a complex mélange of Tertiary, Cretaceous, Jurassic, Triassic, Permian, and volcanic features over a basal Gondwana craton. Toward the north, the offshore consists of a Tertiary minibasin facing the Banda Arc Archipelago, with volcanics interspersed onshore with the basal Gondwana pre-Permian. A prominent central overthrust nappe of Jurassic and younger layers makes up the mountains of Timor-Leste, terminating south against an accretionary wedge formed by this ongoing collision of Timor and Australia. The northern coast of the island is part of the Indonesian back arc, whereas the southern littoral onshore plus shallow waters are part of the accretionary prism. Deepwater provinces embrace the Timor Trough and the slope of the Australian continental margin being the most prospective region of Timor-Leste. Overall crust and mantle tectonic structuring of Timor-Leste is interpreted from seismic and potential field data, focusing mostly on its southern offshore geology where hydrocarbon prospectivity has been established with interpretation of regional seismic data and analyses of gravity, magnetic, and earthquake data. Well data tied to seismic provides focal points for stratigraphic correlation. Although all the known producing hydrocarbon reservoirs of the offshore are Jurassic sands, interpretation of Permian and Triassic stratigraphy provides knowledge for future prospect drilling risk assessment, both onshore and offshore.

Primary gold mineralization in the Lisungwe Valley area, Kirk Range, southern Malawi

Abstract A conspicuous paucity in gold resources in Malawi’s mineral inventory may be partially attributed to a general low resolution in the knowledge of Malawi’s geology, coupled with historical under-exploration for this important economic commodity. To address this knowledge gap, the present study focusses on developing a regional, local and microscopic understanding of the characteristics of gold occurrences in the mineralized Little Chisumbwiti river valley, located in the Kirk Range region which forms part of Southern Irumide mobile belt in southern Malawi. Regional scale interpretations are afforded by in-depth investigation of airborne geophysical data, which are then supported by ground geological mapping and by microscopic observations using optical and electron microscopy, and X-ray computed tomography (XCT). Structural mapping and regional geophysics indicate that the area has been affected by at least two periods of deformation (D1 to D2). The D1 deformation event is characterized by northwest – southeast directed crustal shortening (tentatively associated with the Southern Irumide orogeny) which produced northeast – southwest structures. Gold mineralization in the Little Chisumbwiti river valley is hosted in northeast – southwest trending quartz—sulphide vein sets, where it occurs as flakes ranging in size between 0.24 and 4 mm. The gold is associated with a paragenetically-late pyrite-chalcopyrite-sphalerite generation of sulphide precipitation, and to a lesser extent, with sericitised biotite schist wall rock. Exploration in the Kirk Range region should focus on the northeast-southwest structures, which represent potential conduits for fluid flow during the D1-related hypogene Au mineralization event, however the impacts of subsequent D2 deformation must also be considered. Locally, wall rock lithology (e.g., Fe (II)-rich biotite schists) may have an additional second order control on the siting of gold mineralization. The new insights reported in this study contribute meaningfully towards advancing the status of Malawi’s regional geology and should serve as a useful resource for future exploration efforts undertaken in the Kirk Range, and in other parts of south/central Malawi occurring within the Southern Irumide tectonic block.

Geological investigations with high spatial resolution WV-3 satellite imagery and regional geophysics at the Haib Cu porphyry, Namibia

SummaryRecent improvements in available multi-spectral satelliteborne shortwave infrared sensors and their spatial resolution opens up the opportunity for furthering surface mineralogy mapping. Their image interpretation can be augmented with regional geophysics, e.g. subsurface structural information via magnetics or fine tuning the interpretation of mineral chemistry using radioelement data. The new higher spatial resolution satellite WorldView-3 sensor is compared in this study with the ASTER satellite imagery over the Haib copper prospect in southern Namibia using published geology and airborne hyperspectral imagery control.The results show an improvement using higher spatial resolution combined with improvements in SWIR imagery for the mapping of different AlOH clays, potentially related phyllic and argillic alteration that may be associated with structurally controlled alteration and mineralisation.

The importance of site selection, sediment supply, and hydrodynamics: A case study of submarine paleoseismology on the northern Cascadia margin, Washington USA

The Washington continental margin presents both a classic test of submarine paleoseismology, and an opportunity to explore advancement of the field through analysis of sediment dispersal in a heterogeneous system. New and archive core, bathymetric, backscatter and seismic reflection data from the Washington Cascadia margin show that during high-stand conditions, the northernmost canyons, Barkley, Nitinat, Juan de Fuca (JDF), and to some extent Quillayute are relict systems, with little Holocene recharge. The remaining canyons, Quinault, Grays, Guide, and Willapa, are recharged to a varying degrees by northward transport of Columbia River derived sediment. All systems are nonetheless active conduits for turbidity currents during the Holocene, which are weaker and more restricted than their Pleistocene counterparts. Sedimentologic and CT analysis, supported by radiocarbon ages, micropaleontology, and the Mazama ash datum show that the Holocene sedimentary sequence consists of a series of sand to mud turbidites in the active portions of all systems, interbedded with hemipelagic sediment. However, the Pleistocene Astoria and Nitinat fans are largely inactive in the Holocene, with turbidity current activity limited to the proximal parts of the main channels. Within active systems, the turbidite record is modulated by local landsliding and growth of active folds and faults.In the relict systems, the turbidite record begins in the middle canyon reaches, as opposed to the actively recharged systems which have a turbidite record in the more proximal canyon reaches. Deposits in the relict systems increase in thickness and modal grain size downstream, suggesting multiple sources and or entrainment of material along the canyon walls.Hydrodynamic models using Monte Carlo model searches for best fitting flow parameters show the most likely pathways by which Holocene and Pleistocene currents flowed through the complex accretionary wedge to the abyssal plain. Model results, supported by heavy mineral distributions, show that the northern canyon systems (Barkley, Nitinat, JDF, Quillayute) are independent of the southern systems, (Quinault, Guide, Grays, Willapa) during the Holocene, supporting the classic “confluence test”. Balancing of sediment volumes required suggests sediment supplied to the slope canyons and abyssal channels in turbidity current events is 2–5 times greater than that supplied by recharge to the canyon heads by external sources. This implies a process that is independent of recharge that scavenges the plentiful relict and hemipelagic sediment supply on the continental slope.Lithostratigraphic correlation and age models suggest similar sequences were deposited in most systems independently, and likely synchronously. The correlated turbidite sequence suggests deposition of ~23 Holocene and 16 post Mazama turbidites in most parts of the canyon systems, with only modest variation, mostly attributable to evidence of bypassing and erosion. The best candidate for triggering of these sequences remains great earthquakes, and the sequence correlates well to onshore and other offshore paleoseismologic evidence.The new data, including new cores in northern Oregon, suggest revisions to segmented rupture boundaries. Several partial ruptures (Segment B) likely extended further north to Juan de Fuca Canyon; one event was resolved into two, and one Washington only event was added. Most “Segment C” ruptures were found to extend further northward to the latitude of Astoria Canyon. Mean recurrence times are therefore revised to ~434years for the Washington Coast, and ~340years for the central and northern Oregon coast. Conditional probabilities in the next 50years also are revised to 10–17% for the Washington coast, and 16–22% for the central and northern Oregon coast.In Cascadia, Sumatra and other locales globally, core siting, spatial sample density, high-resolution analysis of core and regional geophysics and understanding of flow dynamics are critical to capturing a paleoseismic record. Sedimentary records vary across turbidity current systems of varying sediment supply, physiography and sediment sources. Variations in the record will reflect these factors, yet may still present a consistent paleoseismic history. This analysis highlights the importance of a thorough understanding of flow paths, hydrodynamics and the interplay of sedimentation and local tectonics when testing for evidence of earthquakes in the sedimentary record of active margins.

A Multi‐Scale Scratch Analysis Method for Quantitative Interpretation of Regional Gravity Fields

AbstractThis paper presents new systematic methods of regional gravity data processing that combine theories based on multi‐scale wavelet analysis, spectral analysis of potential fields, geophysical inversion, and information extraction. We call this data processing system as the multi‐scale scratch analysis for delineation of crustal structures, deformation belts and division of continental tectonic units. The multi‐scale scratch analysis contains four modules; they are spectral analysis for division of density layers, decomposition of the field by using wavelet transformation and multi‐scale analysis, depth estimation and density inversion of decomposed gravity anomalies, and scratch analysis. The basic principles, application techniques and examples for each module are explained. As a complicated and sophisticated process, the multi‐discipline research on regional geophysics from geophysical investigation to tectonic results requires combination of new methods and techniques coming from different disciplines, to build a wide and thick theoretic base for supporting the multi‐discipline research. The multi‐scale scratch analysis combines supporting bases coming from applied mathematics, geophysics, and information science respectively.

Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area, Western Australia

Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains.

GOCE gravity gradient data for lithospheric modeling

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) is the European Space Agency's (ESA) satellite gravity mission to determine the Earth's mean gravity field. GOCE delivers gravity gradients, a new type of satellite data. We study how these data can improve modeling of the Earth's lithosphere. We discuss the use of the original GOCE gravity gradients versus the use of gravity gradients in grids at satellite altitude or close the Earth's surface and conclude that grids are easier to handle than the original data because one does not have to deal with very different error characteristics of the different gradients, given in a rotating frame at varying heights. The downward continuation to the surface enhances signal and better reflects the near-surface geology. But this does not outweigh the amplification of noise and omission errors, which is why we recommend using the field at mean satellite altitude for lithospheric modeling. The North-East Atlantic region is ideal to analyze the additional value of GOCE gravity gradients because it is a well-studied region in terms of regional geophysics. We calculated the gradient sensitivity for crustal depth slices using a 3D lithospheric model. This reveals that especially interfaces with large density contrasts have a distinct signal in the gravity gradients, but that they are quite insensitive to intra-crustal density sources, which can have quite a large effect on surface gravity data. We also show that the satellite gradients have a depth sensitivity well suited to study the upper mantle density structure, making them complementary to gravity and seismic tomography. In the underexplored Rub’al-Khali area the GOCE vertical gradient was used to invert for crustal thickness. The updated Moho model gives a good fit to four of the six gradients and independent depths from seismic stations. The Moho model was used to update the heat flow model and source rock maturity maps, which are generally consistent with known source rock maturity trends in the surrounding regions. GOCE gradients are therefore useful to map crustal thickness and deep regional structures for frontier areas. In combination with other data, heat flow can be modeled which is essential for basin maturity evaluation.

Imaging a potential geothermal target using Mt Isa regional seismic reflection and potential field geophysics, Queensland, Australia

A regional seismic survey in north Queensland, with acquisition parameters set for deep crustal imaging, shows a potential geothermal target beneath about 2 km of sediments. Beneath the sedimentary structure there appears to be an area of low seismic reflection signal from about 1 s to 4 s. Combined with the relatively low gravity signature over this location, this area of low seismic reflection signal could be interpreted as a large granite body, overlain by sediments. This body lies near an area of high crustal temperature and suggests a potential geothermal energy target.

J. P. Busby, A. S. D. Walker & K. E. Rollin 2006. Regional Geophysics of South-east England. Version 1.0 on CD-ROM. Keyworth: British Geological Survey. Price £25.00. - G. S. Kimbell, R. M. Carruthers, A. S. D. Walker & J. P. Williamson 2006. Regional Geophysics of Southern Scotland and Northern England. Version 1.0 on CD-ROM. Keyworth: British Geological Survey. Price

J. P. Busby, A. S. D. Walker & K. E. Rollin 2006. Regional Geophysics of South-east England. Version 1.0 on CD-ROM. Keyworth: British Geological Survey. Price £25.00. - G. S. Kimbell, R. M. Carruthers, A. S. D. Walker & J. P. Williamson 2006. Regional Geophysics of Southern Scotland and Northern England. Version 1.0 on CD-ROM. Keyworth: British Geological Survey. Price £25.00. - Volume 145 Issue 1

Seismic migration with inverse Q filtering

Taking account the earth Q effect (i.e., frequency‐dependent amplitude decay and velocity dispersion) during seismic migration simultaneously, the resultant seismic image is expected to be high resolution with true amplitude and correct timing. We present here a stabilized algorithm for such a “migration incorporating inverse Q filtering” scheme, so that the imaging technique is applicable to seismographs with long recording time. The earth model is assumed to be 1‐D with continuous variations vertically in velocity and attenuation, and the imaging algorithm is implemented easily and efficiently in the frequency‐wavenumber domain. Although such a 1‐D model suits mostly to the cases in lithospheric‐scale regional geophysics, it is sometimes also suitable to the exploration seismic studies. The method is demonstrated using a real data example from exploration seismics with an attempt to recover a target reflection underneath a group of strong coal‐seam reflections.

Selected short stories on novel applications of near‐surface geophysics

Applications of near‐surface geophysics can be grouped under engineering and geotechnical, environmental, ground water, archaeological, forensic or law enforcement, military, and miscellaneous. However, the distinction is often arbitrary. For example, cavity or tunnel detection might be classified as engineering/geotechnical, archaeological, military, or forensic/law enforcement although the geophysical methods and approach are likely the same in each case. Near‐surface geophysics, for most applications, has three distinguishing characteristics not shared with regional geophysics or oil and gas exploration: (1) very high resolution surveys; (2) public health and safety concerns; (3) “near real‐time” validation or “ground truthing” of the interpretations.

On the Use of the Geoid in Geophysics: a Case Study Over the North West Shelf of Australia

The geoid is the fundamental surface that defines the figure of the Earth. It is approximated by mean sea-level and undulates due to spatial variations in the Earth’s gravity field. The use of the geoid in regional geophysics is illustrated for the North West Shelf of Australia by removing long-wavelength geoid features, due predominantly to deep-Earth mass anomalies, in order to reveal near-surface structure. After this process, the residual geoid anomalies correlate well with known geological structures. Therefore, the geoid can provide information, complementary to other geophysical data, of the Earth’s internal structure.

A New Tectonic Model for the Development of the Eastern Cordillera, Altiplano, and Subandean Zones, Bolivian Central Andes, 20 degrees S Latitude

Construction of a regional transect across the central Andes at 20[degrees]S sheds new light on the relationship between the Altiplano, Eastern Cordillera (EC), and Subandean zones and allows us to refine the two-stage model of Isacks (1988) for the growth of the Central Andes. This new model is based on examination of the regional geology and geophysics, coupled with field investigations, satellite image analysis, and new Ar-Ar geochronology. In this model, widespread Oligocene to mid-Miocene compressional deformation in the Altiplano and EC was followed in the late-Miocene and Pliocene by thrusting localized east of the EC within the Subandean fold-thrust belt. During the first stage of deformation, the Altiplano basin underwent important subsidence and internal deformation. The EC was both deformed internally and thrust westwards over the Altiplano basin, while the present Subandean zone was the site of an early, broad foreland basin which received material eroded from the EC. During the second stage, beginning at [approximately]10 ma, deformation terminated within the EC and became concentrated within the fold-thrust belt in response to large scale overthrusting of the EC above the Brazilian shield; this resulted in major thrusting along the Cabalgamiento Frontal Principal (CFP), which soles into the master Subandeanmore » decollement, and [approximately]100 km of telescoping within the early, broad foreland basin. In the EC, this second stage is marked by the elaboration of a regionally extensive erosion surface, ponding of gravels in shallow basins, and the emplacement of giant ignimbrite sheets. The Eastern Cordillera can thus be thought of as a crustal-scale wedge which has been extruded upward and outward on alternate sides during successive stages of late Cenozoic deformation. This motion has served to drive subsidence in both the Altiplano and Subandean foreland basins, as well as shortening in the fold-thrust belt.« less