Subsurface Stratigraphy Research Articles

Data Pre-Processing and Signal Analysis of Tianwen-1 Rover Penetrating Radar

The Rover-mounted Subsurface Penetrating Radar (RoSPR) is one of the scientific payloads onboard China’s first independent Mars exploration mission, Tianwen-1. The radar aims to characterize the thickness of the upper Martian soil and investigate the subsurface stratigraphy by collecting and processing the data. This article is mainly divided into two parts, the introduction of data pre-processing and analysis of pre-processed radar signals, aiming at helping scientists make more effective use of radar data. The first part describes the operating principle of the RoSPR and the procedure of radar data pre-processing at all levels. Data pre-processing is mainly designed to transfer the raw data format to a common PDS (Planetary Data System) and eliminate the influence of the instrument. In the signal analysis part, the performances of both self-check signals and echo signals of low- and high-frequency channels are analyzed, which indicate a stable radar system and are useful for background removal. Phase and time calibration is of great importance for improving data quality and making the radar data more accurate. Moreover, further processing is required to obtain clear radar images, such as filtering, background removal and gain setting.

Development of training image database for subsurface stratigraphy

ABSTRACT Image-based stochastic simulation methods, such as multiple point statistics (MPS), can be viewed as a physics-informed Bayesian learning approach, which samples typical stratigraphic patterns from a single training image for onward conditional modelling of subsurface stratigraphy. A training image is essentially a prior geological model, which comprises representative stratigraphic connectivity at the site of interest. One key difficulty hindering wide application of image-based geological modelling methods is the lack of qualified training images. In this study, a systematic framework is proposed to develop training image databases for conditional simulations of subsurface stratigraphy. Collected training images can be further categorised based on three key factors, namely, geological origin, site location and application scenario. As a pilot study, a total of 54 geological cross-sections, mainly interpreted by experienced engineering practitioners, for weathered granite and tuff slopes in Hong Kong are collected and compiled as two training image databases. To demonstrate value and application of the established training image databases, subsurface stratigraphy for real weathered granite slope examples are used as illustrative examples, and stratigraphic uncertainty is also quantified. Results indicate that training image databases are of great significance for subsurface stratigraphy and uncertainty quantification, particularly when only limited site-specific data are available.

Integrated geophysical interpretation and hydrogeochemistry investigation for groundwater aquifer assessment at El – Farafra depression, Western Desert, Egypt

ABSTRACT The aeromagnetic data is utilised to determine the depth of the crystalline (basement) rocksand subsurface structures where the basement depth ranges from 1777 - 3778 m .and thearea is dissected by several faults with various trends, especially in the NNW-SSE direction. The well-logging data is used to investigate the subsurface stratigraphy where theinterpretation of well-logging data indicates that the area is composed of seven geologicunits, with the third, fifth, and seventh units representing the main aquifer. The first zone ofthe Nubian sandstone aquifer is represented by the sandstone that makes up the thirdgeologic unit with a low gamma ray, which is sandwiched by clay layers. The Nubiansandstone aquifer's second zone is represented by the fifth geologic unit, which is composedof sandstone with minor clay layers. It exhibits low to modest gamma-ray values andmoderate to high resistivity values. The seventh geologic unit is composed of sandstone andrepresents the third zone of the Nubian sandstone aquifer, which is sandwiched by claylayers. It has high resistivity values and a low gamma-ray. The aquifer in the study area hastransmissivity and belongs to a high-potential aquifer.The aquifer in the study area has transmissivity and belongs to a high potential aquiferV.

Noble gases in aquitard provide insight into underlying subsurface stratigraphy and free gas formation

AbstractBiogeochemical gas production resulting in free gas phase formation can severely affect groundwater and solute transport in aquifers. Such gas–water interactions are important in aquifers affected by geogenic As, which are commonly associated with biogeochemical CH4 production. Additionally, the influence of aquitards on As concentrations in contaminated aquifers has recently been challenged. These observations prompted the analysis through a heterogeneous aquitard overlying a high CH4−gas‐producing zone of an As‐contaminated aquifer. A sediment core taken through the aquitard was analyzed for noble gases to assess how the aquitard physically contributes to the underlying gas production. Results reveal that the aquitard pore space is unsaturated in two separate layers resulting in hanging pore water constrained by an air‐like gas phase. This interlayering of unsaturated and saturated zones identifies the aquitard's stratigraphy as key in determining hydrostatic pressure—a main control of free gas formation (i.e., CH4) in the underlying aquifer. The partly unsaturated conditions reduce the hydrostatic pressure by 30% compared with fully saturated conditions. To our knowledge, this is the first study applying noble gases to examine the influence of an aquitards physical state on gas production in an underlying aquifer. Further, such partly unsaturated sediment layers of low conductivity might provide preferential pathways for periodic water flow, fostering aquitard–aquifer solute transport. Groundwater samples additionally collected throughout the study site confirm more widespread degassing than previously reported. Up to 90% of the expected atmospheric noble gas concentrations is lost from groundwater immediately below the investigated sediment core.

Regional-scale assessment of earthquake-induced slope displacement considering uncertainties in subsurface soils and hydrogeological condition

Realistic prediction of coseismic landslides is crucial for the design of civil infrastructures and geotechnical systems in seismically active regions. Coseismic landslides are complicated nonlinear and large-deformation processes triggered by ground shaking. To date, many gaps still remain in the scientific understanding of the landslides, particularly in areas lacking information on the subsurface stratigraphy. Analytical methods for estimating coseismic landslides have been based on highly simplified models, where many key influential factors have been overlooked, leading to large uncertainty in using empirical models for landslide prediction. In addition, more evidence recently underscores the necessity of modeling coupled effects between topography and soil amplification, which gives rise to complex wave propagation patterns due to scattering and diffracting of waves within the low-velocity near-surface layers. Neglecting topographic amplification and soil layers could result in significantly underpredicted regional-scale hazards of landslides. In this study, a regional-scale coseismic landslide simulation is carried out using the integral Spectral Element Method (SEM)-Newmark method for Hong Kong Island to quantify the coupled effect of 3D topography, subsurface soil condition as well as hydrogeological conditions. By leveraging extensive geological borehole data, a rigorous statistical model that characterizes uncertainties associated with subsurface soils is established. Influence of key factors that are not well considered in previous coseismic landslide studies, such as variation in irregular rockhead and spatially distributed shear-wave velocity are quantitatively analyzed. Numerical results demonstrate that by rigorously incorporating multiple sources of subsurface soil uncertainties, the total area of landslides in the study region can increase up to 30% compared with benchmark model as evidenced in the physics-based numerical simulation. These case studies will provide a valuable opportunity to revisit key factors that influence coseismic landslides using measured field data.

Chemostratigraphic and mineralogical examination of the Kilwa Group claystones, coastal Tanzania: An alternative approach to refine the lithostratigraphy

The Cretaceous and Paleogene marine sedimentary rocks that crop out along southern coastal Tanzania have been the focus of the Tanzanian Drilling Project (TDP) since 2001. The comprehensive lithological and chronostratigraphic examination of over forty shallow cores by the TDP culminated in the formal definition of the Kilwa Group: a claystone-dominated succession comprising five formations deposited in middle to outer shelf and upper slope marine environments along a passive continental margin. Onshore, the TDP has cored important palaeoclimatic events within the Kilwa Group. Offshore, the group forms the reservoir and seal of several gas fields discovered in southern Tanzania.The formations of the Kilwa Group cored onshore, have been differentiated from each other largely by variations in subsidiary lithologies (sandstones and limestones), rather than by diagnostic characteristics of their dominant lithology (olive grey claystone). To test and refine the lithostratigraphy of the Kilwa Group, a forensic examination of the claystones using whole-rock inorganic geochemistry, mineralogical analysis and detailed biostratigraphy, is employed in this study.1210 core samples collected from 20 onshore TDP boreholes and 185 cutting samples acquired from three wells located in offshore in Blocks 1 and 4 are examined by inductively-coupled plasma optical emission spectroscopy and mass spectrometry, whole-rock and clay fraction X-ray diffraction analysis and heavy mineral analysis using Raman spectroscopy. The different methodologies are used to produce a claystone-based chemostratigraphic framework for the Kilwa Group that comprises three sequences, five packages and six units, and links the shallow subsurface rocks onshore to the deep subsurface stratigraphy offshore.The multidisciplinary geochemical and mineralogical approach reveals that variations in detrital quartz, feldspars (K and Na), heavy minerals, phosphatic minerals and clay minerals (particularly illite, smectite and kaolinite) are key for defining the claystone-based stratigraphy of the Kilwa Group. The variation in the abundance of these mineral through time highlight mostly temporal changes in depositional environment, chemical weathering and sediment provenance that occurred in Tanzania during the Cretaceous and Paleogene.Integration of the chemostratigraphic framework with detailed biostratigraphic information from the study sections and comparison with the published lithostratigraphy of the Kilwa Group onshore reveals that all three stratigraphic schemes are in broad agreement. Nevertheless, refinements are proposed based on the new chemostratigraphic results. It is suggested here that the top and base of the Kilwa Group is older than previously reported (base–Albian and intra-Rupelian, respectively, rather than the end of both stages) and in most cases, the geochemical data suggests that most of the Kilwa Group formations, as cored onshore, are thinner than formerly proposed. Only the Masoko and Lindi Formations are interpreted to be thicker than previously defined by the TDP.

Active seismic exploration along a human lunar mission traverse analogue in the San Francisco volcanic field

Lunar surface activities during Apollo and terrestrial analogue lunar mission simulations have commonly focused on traverses that prioritize surface observations and sample collection activities. Along the way, geophysical measurements are often made. However, they are not necessarily made in a way that optimizes information about the physical subsurface properties, which is something that geophysics can provide. In 2010, NASA simulated a high-quality multiweek human lunar rover traverse analogue mission in the San Francisco volcanic field in Arizona. The traverse route and associated science station locations were selected based on addressing surface observation and sampling tasks. Geophysical studies were not included in the simulation. We returned to the same field area and obtained data on 19 active seismic refraction geophone lines from the science station locations accessed during the simulation. We analyzed the data to calculate 1D seismic velocity profiles for each of the lines. Results revealed up to seven distinct seismically defined material types, including a nearly ubiquitous veneer of regolith of variable thickness at the surface. Results also provided depth and thickness of the seven material types in the first 60 m of the subsurface at each of the science station locations. These cannot be obtained by geologic observations of the outcrops. Systematic interpretation of the area's overall subsurface stratigraphy was not feasible due to the geophysically nonsystematic nature of the original traverse's prioritization of the science station locations. The added geophysical understanding of a region could drive additional geologic investigations to locate samples of otherwise unknown material through the location of surface exposures or coring. This emphasizes the importance of synchronizing geologic and geophysical research requirements during lunar traverse planning and execution to optimize addressing scientific and utilization questions.

Radar Backscatter and Emissivity Models of Proposed Pyroclastic Density Current Deposits on Venus

AbstractMagellan synthetic aperture radar observations of Venus revealed a small number of deposits in the highland regions that were suggested to have formed from pyroclastic density currents. Studying these deposits is useful for understanding the nature of pyroclastic activity and eruptive history on Venus. The proposed pyroclastic deposits occupy the uppermost unit in local stratigraphy and are found near exceptionally high reflectivity (∼0.6) units in the highlands. Their radar properties include high copolarized backscatter (∼−8 to −15 dB) and moderate emissivity values (∼0.70–0.88) in the 12.6 cm wavelength Magellan data acquired at incidence angles between ∼15° and 45°. We aim to characterize the structure of these deposits by modeling the observed backscatter and emissivity as a function of different physical and dielectric properties and shallow subsurface stratigraphy. Three different physical scenarios focusing on three different scattering mechanisms—surface scattering, subsurface scattering from buried dielectric horizons, and volume scattering from buried, distributed scatterers—are considered. By comparing the model results to Magellan observations, we narrow down likely pyroclastic deposit structures. We show that the deposits are likely analogous to dense, welded ignimbrites with high surface roughness. We also investigate other possible but less likely scenarios of a thin, low‐density, low‐loss mantling pyroclastic deposit on top of high reflectivity units and a thick, low‐density, low‐loss deposit with ∼5–10 volume % of scatterers of sub‐wavelength size. Future multiwavelength, multipolarization radar observations from VERITAS and EnVision may enable unambiguous characterization of these deposits.

The Paleozoic petroleum system of the Molla-Bismil area (Diyarbakır Basin, Turkey) and source rock properties of Lower Silurian (Llandovery) organic-rich shale

The Molla-Bismil area is a petroleum province, which hosts predominantly Paleozoic oil fields in the foreland area of the Diyarbakır Basin. The Arpatepe, Bahar, Yeniev, and Çalıktepe fields are Paleozoic oil field discoveries in the Molla-Bismil area to-date and have consequently been the focus of considerable research. This significant breakthrough reveals that the deep-wells in the Molla-Bismil area have proven the existence of a Paleozoic petroleum system. Prospective source rocks are located stratigraphically above the upper part of the Bedinan Formation (Upper Ordovician) reservoir, near the base of the Dadaş Formation (Llandovery, Dadaş-1 member). The lithology of the lower part of the Dadaş-1 member, also known as hot shales in the Molla-Bismil area, are typified by organic-rich brownish-black colored shale containing bitumen and thin limestone beds. New well data assists with the regional definition and correlation of typical source rock properties within the Diyarbakır Basin. Subsurface stratigraphy was derived for several deep wells from the Molla-Bismil area and core or drill cuttings from the Çatak-1, Bahar-1, Göksu-1R and Çalıktepe-2 wells were studied and reviewed for lithological and geochemical information. The understanding and prediction of the geochemical characterization of the Lower Silurian (Llandovery) organic-rich shales relevant for hydrocarbon production are crucial for Turkey's fossil fuel supply. Based on new drilling data, we re-examined the properties of the Llandovery organic-rich shales in the Molla-Bismil region. This study brings together new and previously unpublished knowledge about the Paleozoic plays of SE Turkey.

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

AbstractThe Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20–3.52 g/cm3. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion.

Application of machine learning in the identification of fluvial-lacustrine lithofacies from well logs: A case study from Sichuan Basin, China

The lithofacies identification is critical for forecasting sweet spots of hydrocarbon explorations. Well logs are widely used in lithofacies identifications because they are petrophysical measurements of subsurface stratigraphy which reflect lithological successions and depositional processes. The traditional lithofacies identification from well logs is a manual work that is time-consuming and bias-prone. An automated and bias-free method is in demand. To this end, we created a lithofacies dataset of eleven wells with well log records and lithofacies descriptions that were interpreted manually based on facies analysis of drilling cutting descriptions and well logs. Then we developed machine learning models that were trained using the lithofacies dataset of the fluvial-lacustrine Upper Triassic Xujiahe and Lower Jurassic Ziliujing formations in Yuanba Area, northern Sichuan Basin of southwestern China. By employing extreme gradient boosting and resampling algorithms, this machine learning model is efficient and outperforms support vector machine and multiple-layer perceptron, as indicated by its highest accuracy and F1-score of 0.90, the highest AUC of 0.94, as well as the shortest training time. Moreover, the result suggests that resampling is necessary for lithofacies identification with the imbalanced dataset. A combined method of oversampling and undersampling is better than a single resampling method. This study presents a successful application of machine learning in fluvial-lacustrine lithofacies identification from well logs and suggests the great potentiality of machine learning in subsurface hydrocarbon explorations.

Data-driven construction of Three-dimensional subsurface geological models from limited Site-specific boreholes and prior geological knowledge for underground digital twin

A digital twin is a digital replica or virtual representations of 3D physical entities in the real world. In practice, it is challenging for 3D modelling of subsurface stratigraphy in an underground digital twin due to insufficient site-specific measurements and a lack of efficient 3D spatial prediction tools. In this study, a data-driven and deep learning method, called IC-XGBoost3D, is proposed to build a 3D geological model from limited site-specific boreholes and 2D training images reflecting prior geological knowledge. Anisotropic stratigraphic relationships are learned from two perpendicular 2D training images, and the extracted stratigraphic statistics serve as the input for pre-training a 2D simulation slice. A sequence of 2D simulation slices is then simulated with constraints by site-specific boreholes and subsequently reassembled into a 3D geological model. Note that the parameters of the deep learning method are calibrated with site-specific data and prior training images before being applied to develop the geological model. The model performance is demonstrated and validated using an illustrative example. The proposed method can efficiently generate an anisotropic 3D geological model as a point cloud from two perpendicular training images and sparse boreholes with a high prediction accuracy. More importantly, the proposed method not only infers the most probable 3D geological domain, but also provides a quantitative evaluation of associated 3D stratigraphic uncertainty. Effects of irregular borehole spacing and single training image on the simulation performance are also investigated.

Hydrogeophysical Characterization of Fractured Aquifers for Groundwater Exploration in the Federal District of Brazil

The present study applies a geophysical approach to the Federal district of Brazil, a challenging hydrogeologic setting that requires improved investigation to enhance groundwater prospecting to meet the rising water demand. The geophysical characterization of a complex hard-rock aquifer sub-system was conducted using direct current (DC) electrical resistivity tomography (ERT) integrated with surface geological information. With a total of twenty-seven ERT profiles, the resistivity acquisition was carried out using a dipole-dipole array of electrodes with an inter-electrode spacing of 10 m. Based on resistivity ranges, the interpretation of the inverted resistivity values indicated a ground profile consisting of upper dry soil, saprolite, weathered, and fresh bedrock. Along with this layered subsurface stratigraphy, the approach allowed us to map the presence of significant hydrogeological features sharp contrasting anomalies that may suggest structural controls separating high-resistivity (≥7000 Ω m) and low-resistivity (<7000 Ω m) conducting zones in the uppermost 10 m of the ground. The assumed impacts of these features on groundwater development are discussed in light of the Brasilia aquifer settings.

Soil and Groundwater Investigation for Sustainable Agricultural Development: A Case Study from Brunei Darussalam

Brunei has made significant progress in agricultural development over the past four decades. However, crop production, such as rice, is still insufficient to meet the demands of the growing population, mainly due to the scarcity of water resources, acidic soils, and advancing environmental deterioration. This study conducted soil and groundwater investigations at three selected rice cultivation sites in Brunei Darussalam. The geoelectrical resistivity method using the vertical electrical sounding (VES) technique delineated subsurface stratigraphy with resistivities ranging from 1 to 180 Ωm in the investigated area. A potential aquifer zone was inferred in one of the surveyed areas with resistivities ranging from 10 to 150 Ωm at depths of about 40 to 60 m below ground level. Groundwater levels were also shallower in irrigated sites compared to non-irrigated sites. Groundwater physicochemical properties revealed high alkalinity and mineralisation, as evident from high magnesium, bicarbonate, ammoniacal nitrogen, and iron, as well as pH values in the water samples, believed to be the effects of leaching of salt ions into the shallow aquifer through infiltration recharge. In addition, soil physicochemical properties showed that the soils near a pumping well were more acidic, had lower moisture, and total nutrients, likely due to the effects of groundwater pumping. Our results demonstrate that future irrigation activities need to consider local aquifer characteristics, groundwater flow paths, and nutrient transport to ensure sustainable development in agricultural areas.

Nonlinear Dynamic Analyses of Perris Dam Using Transition Probability to Model Interbedded Alluvial Strata

AbstractThis case study presents an application of a conditional transition probability method for interpreting subsurface stratigraphy for the interbedded alluvium underlying Perris Dam, and evalu...

Tectono-stratigraphic evolution of the Jurassic-Cretaceous Laguna Merin Basin (Uruguay): New insights from magnetotelluric transects

To understand the subsurface stratigraphy and structure of the Laguna Merín Basin (LMB), data from 42 magnetotelluric stations were interpreted by performing 1-D and 2-D inversions along three profiles perpendicular to the main basin borders. The Aiguá-India Muerta-Chuy Lineament (AICL) constitutes the southern boundary of the LMB and is characterised by a subvertical sinistral transtensional fault with a general orientation of N85°E. The basin's northern boundary, represented by the Cebollatí-Merín Lineament (CML), is characterised by a set of echelon dextral transpressional faults with a general orientation of N50°. Both faulting sets are associated with maximal horizontal stress oriented NE-SW. The infill of the LMB comprises thick Jurassic-Cretaceous igneous rocks (average of ~1500 m) overlying a crystalline Precambrian basement and overlain by ≤500 m-thick Cretaceous-Cenozoic sedimentary sequences. Likewise, a very deep (> 3500 m) conductive body was identified in the basement between the CML and AICL, coincident with the highest values of gravimetric Bouguer anomalies recorded in Uruguay. Shallow Cretaceous-Cenozoic sedimentary sequences show resistivities between 1 and 25 Ω.m, an average thickness of 250 m, and a maximum thickness of 500 m. The deeper Jurassic/Cretaceous igneous rocks show average resistivities between 40 and 252 Ω.m, and a maximum thickness of nearly 2000 m with intercalations of conductive layers (< 25 Ω.m) of unknown nature in the lower half of the basin infill. A tentative 3-D model is proposed to represent the early stage of magmatic emplacement during the opening of the South Atlantic Ocean.

Interactive web mapping tools and custom subsurface cross-sections for interdisciplinary geologic investigation

Using Python-based geospatial analytics, open-source web mapping technologies, geophysical data models, and subsurface stratigraphy models from the Regional Geology Geologic Framework Model database assembled by Los Alamos National Laboratory, we developed a suite of web-based geologic investigation tools to identify and understand subsurface structures and geophysical properties concerning salt and shale formations within the contiguous United States. Coupled with a web map interface, these tools allow for the interactive visualization of various geologic data and demonstrate the ability to quickly generate custom subsurface cross-sections, borehole charts, and diagrams for azimuthal orientation data. These capabilities were developed for stakeholder and researcher use to facilitate informed decision making for spent nuclear waste disposition. However, these capabilities provide a flexible model for a variety of subsurface investigation needs, and we have demonstrated this flexibility by adapting these tools to meet visualization needs for various subsurface models within a web-based platform.

Geoelectrical properties of saline permafrost soil in the Adventdalen valley of Svalbard (Norway), constrained with in-situ well data

Direct Current (DC) Resistivity and Induced Polarization (IP) response of six profiles were measured using the Gradient electrode configuration in Adventdalen, Svalbard, to characterise the near-surface stratigraphy of the soil and to account for geotechnical and environmental aspects of global warming in the arctic region. In addition, Wenner array data was collected for the selected profiles to examine its effectiveness as compared to the Gradient array, given the characteristics of the study site. Two commercial inversion software programs, Res2DINV and AarhusINV, were used for the inversion of the DC resistivity and IP data, to compare the software. Physical soil properties, including porosity, water saturation, water salinity, freezing temperature and grain size distribution, previously measured from samples retrieved from wells along the studied profiles, were integrated in this study to investigate the correlation with geoelectrical properties of the sediments inferred from the DC resistivity and IP data.Results from processing of the Wenner array DC resistivity data provided higher resolution as compared to the Gradient array data, especially from deeper parts of the models, due to its higher signal-to-noise ratio. The Wenner array data also indicated better inversion result for the IP data as distinctive anomalies were better indicated in data from Wenner array survey. The Wenner array data also provided a realistic trend for the anomalies, thanks to the symmetrical geometry of the electrodes during the survey, although at the cost of time and higher expenses. Inversion results proved that AarhusINV resolved the geometry of the subsurface layers with higher resolution compared with the Res2DINV. However, the two inversion algorithms use slightly different parameters for the processing and for presenting the results, thus only allowing qualitative comparison. Based on the interpretations of the DC resistivity and IP data, four distinctive zones were identified from the surface to the maximum depth of 26 m, consisting of (i) unfrozen active-layer-(silts and sands), with intermediate resistivity values 200–300 Ω·m; (ii) frozen soil with 3–10 m thickness and resistivity values between 2500 and 5000 Ω·m; (iii) unfrozen soil (cryopeg) with high salinity and low resistivity of 40 Ω·m; and finally (iv) clayey-unfrozen soil sediments with low resistivity ranging 10–20 Ω·m, at depths between 13 and 26 m. The IP data allowed for the delineation of a low chargeability zone near the surface and a high chargeability zone at greater depth which denote the active layer, lower parts of unfrozen soil sediments and cryopeg respectively, within the top 10 m of the subsurface.The 3D subsurface model of the study area was created based on interpretations of the DC resistivity and IP data and was constrained by the description of the subsurface stratigraphy from nearby wells, which provided detailed information about the vertical stratigraphy of the study area. In addition, a good correlation was observed between the studied physical properties of the sediments and the DC resistivity data for the intersecting profile SVAER04, as the interface between high and low resistivity data at ca. 10 m depth coincided the sedimentary formation with intermediate-fine grain size, high porosity, high water saturation and high salt content. Our findings show that joint application of the geoelectrical surveys and laboratory analysis of soil samples are an efficient complement to each other. These methods can be used as an alternative to each other to investigate larger areas where achieving high resolution data is not necessary.Our study results underline the importance of choosing the right survey design for collecting the DC resistivity and IP data. Contribution of the IP data in this study mostly concerns the shallow parts of the subsurface due to the low signal-to-noise ratio at greater depths, and frozen ground which limited the ion mobility and hence the IP response. An appropriate inversion program coupled with integration of the physical properties of the sediments, can be successfully applied to characterise the near-surface morphology of the sedimentary formations to address the geotechnical and environmental challenges related to the permafrost in the Arctic.

A two-dimensional temperature field simulation of the La Primavera geothermal area, México

A reasonable estimation of the geothermal potential of any geothermal system depends on the reliable knowledge of its geological, geophysical and geochemical parameters. The heat flow is an important geophysical criterion used to describe the subsurface temperature profiles in prospecting areas. These profiles are helpful tools to locate high temperatures and thermal disturbances (heating or cooling) that could indicate permeable or fractured zones in the subsurface. A good area to apply this approach is the La Primavera caldera (LP), México, with an estimated geothermal potential high enough to install a 75 MW power plant. A conductive model of La Primavera would provide vital information to understand the heat flow during future work on and eventual exploitation of the geothermal field. We developed a 2D conceptualized numerical model of La Primavera based on the heat transfer equations. Then, we applied the Tri-Diagonal Matrix Algorithm (TDMA) method to discretize the equations. The resulting model described the temperature distribution as a function of depth in 10 geothermal wellbores drilled within an area of 4 km2 inside the caldera. The lithology of these exploration wellbores (subsurface stratigraphy) was used to correlate the proposed 2D geological profiles and to construct the model geometry and thermal conductivity at depth. The temperature profiles were used as boundary conditions to calibrate and validate the numerical model. The results indicate that estimated temperatures (450-500 °C) occur at depths of 3 km hosted within the granitic basement beneath La Primavera. These high temperatures may be linked to remnant intrusive bodies located in depth. These results suggest that La Primavera is a promising target for future geothermal exploitation.

Stratigraphic identification with airborne electromagnetic methods at the Hanford Site, Washington

Stratigraphic units can influence the fate and transport of subsurface contaminants within groundwater. Units having coarse-grained sediments act as preferential flow pathways, and therefore can accelerate the transport of contaminants to reach human and ecological receptors. At legacy waste sites, detailed knowledge of subsurface stratigraphy can be used for effective monitoring and remediation planning to help minimize risk to human health and the environment. Airborne electromagnetic (AEM) methods can non-invasively provide information on kilometer-scale or larger subsurface stratigraphic features and fill informational gaps in directly sampled data from sparsely located boreholes. In this paper, we present inversion results of a 412 line-km frequency-domain AEM survey to delineate subsurface stratigraphic features at the Hanford Site, located in southeastern Washington State. The inversion was performed using a massively parallel 3D electromagnetic modeling and inversion code, where the modeling is based on solving frequency-domain Maxwell's equations using an unstructured-mesh finite-element method and the inversion employs a Gauss-Newton optimization scheme. The results are compared to an underlying geologic framework model (GFM), built by interpolating contact depths of stratigraphic units interpreted from site borehole datasets. In areas with good borehole coverage, the inversion results show a good match with the GFM to a depth of about 60 m. Outside of these areas, the inversion results exhibit inconsistencies from the assumptions made to create the GFM, demonstrating that the AEM survey results can be used to improve the understanding of the geological conceptual model.