Combination Of Geophysical Methods Research Articles

Tracing the spatial organization and activity zones of an Early Mediaeval homestead at the Pohansko stronghold (Czechia) by combining geophysics and geochemical mapping

AbstractGeoarchaeological prospection techniques were applied to identify activity zones and the inner structure of a homestead at the Early Mediaeval site Pohansko near Břeclav (Czechia). By a combination of geophysical methods, the spatial distribution of microartefacts, geochemical analysis and multivariate statistical analysis, we outlined various manifestations of anthropogenic activity. We examined obtained data by Spearman's correlation coefficient, spatial autocorrelation (Global Moran's I) and robust Principal component analysis to identify the spatial pattern of the area. Recognized joint presence of heavy metals (Pb, Zn and Cu) and elements related mostly to organic matter, waste and ashes (S, P and Ca) as well as a small number of slag fragments probably indicate presence of metalworking zones or mixed zones with domestic and industrial debris at the homestead. Further anthropogenic activities could be connected to manuring, animal management or some kind of production activities based on the presence of Mn, P and Cu. Bone and charcoal concentrations supplement the information of geochemical analysis and may indicate the manner of waste management in the peripheral parts of the homestead. In the middle of the homestead, the location of archaeological features indicates an open space in which no specific activity was detected. By means of magnetic susceptibility and judging from the presence of daub, we defined the potential presence of non‐sunken features, which were not recognized by magnetometry. The outcome of the study is yet to be verified by excavation.

Depositional environments and historical contamination as a framework to reconstruct fluvial sedimentary evolution

In this study, we explore the variability of sedimentation conditions (e.g., grain-size, accumulation rate, contamination) according to fluvial depositional environments. Indeed, sediment cores are commonly used as archives of natural and anthropogenic activities in hydrosystems, but their interpretation is often complex, especially in a fluvial context where many factors may affect the quality, continuity, and resolution of the record. It is therefore critical to thoroughly understand the nature and dynamics of an environment in which a sediment core is sampled to be able to interpret it. To that end, four depositional environments from a bypassed reach of the Rhône River were comparatively investigated through geophysics in order to assess the range of sedimentation conditions: a floodplain, a semi-active secondary channel, an active secondary channel, and a dam reservoir. Sediment cores were retrieved from each environment and thoroughly characterised (e.g., grain-size, Total Organic Carbon, organic contaminants). Robust age-depth models were elaborated for each core based on 137Cs, 210Pbex, and Persistent Organic Pollutants (POPs) trends. The results show that each depositional environment recorded a different time-period, and therefore different contamination levels and trends. In particular, a shift from polychlorinated biphenyls (PCBs) to polybrominated diphenyl ethers (PBDEs) as the predominant POP in the sediments can be observed, the tipping point being set in the 1970s. Two types of infrastructure-induced legacy sediments related to two periods of river engineering in the reach were also identified using grain-size analysis. The combination of geophysical methods (Ground Penetrating Radar) and sediment cores is therefore confirmed as a relevant methodology that should be promoted in fluvial contexts in order to reconstruct the sedimentary evolution of fluvial corridors. The study also highlights the challenges of dating recent fluvial sediments and proposes a multi-proxy dating methodology using POPs contamination trends.

Đánh giá chất lượng đê, đập và cảnh báo biến động bất lợi dựa theo kết quả địa vật lý và mô hình tính cơ học

To evaluate quality of dikes for warning disaster areas based on the combination ofgeophysical methods and mechanical calculating models

Comparison Study to the Use of Geophysical Methods at Archaeological Sites Observed by Various Remote Sensing Techniques in the Czech Republic

A combination of geophysical methods could be very a useful and a practical way of verifying the origin and precise localisation of archaeological situations identified by different remote sensing techniques. The results of different methods (and scales) of monitoring these fully non-destructive methods provide distinct data and often complement each other. The presented examples of combinations of these methods/techniques in this study (aerial survey, LIDAR-ALS and surface magnetometer or resistivity survey) could provide information on some specifics and may also be limitations in surveying different archaeological terrains, types of archaeological situations and activities. The archaeological site in this contribution is considered to be a material of this study. In case of Neolithic ditch enclosure near Kolín were compared aerial prospection data, magnetometer survey and aerial photo-documentation of excavated site. In the case of hillforts near Levousy we compared LIDAR data with aerial photography and large-scale magnetometer survey. In the case of the medieval castle Liběhrad we compared LIDAR data with geoelectric resistivity measurement. In case of a burial mound cemetery we combined LIDAR data with magnetometer survey. In the case of the production area near Rynartice we combined LIDAR data with magnetometer and resistivity measurements and result of archaeological excavation. Fortunately for successful combination of geophysical and remote sensing results, their conditions and factors for efficient use in archaeology are not the same. On the other hand, the quality and state of many prehistoric, early medieval, medieval and also modern archaeological sites is rapidly changing over time and both groups of techniques represent important support for their comprehensive and precise documentation and protection.

Magnetotelluric Imaging of a Carbonatite Terrane in the Southeast Mojave Desert, California and Nevada

The southeast Mojave Desert hosts one of the world’s largest rare earth element (REE) deposits at Mountain Pass, California. Although surface geology has been studied, a full understanding of the carbonatite and associated intrusive suite complex requires subsurface geophysical characterization. In this study, a combination of geophysical methods, including magnetotelluric (MT), magnetics, and gravity are used to create a two-dimensional (2D) geophysical model to a depth of about 10 km. An electrically conductive body is found 2–3 km below and west of the deposit that is associated with a magnetic high that could be connected to a deeper (10 km) conductive body related to possible intrusions or hydrothermal systems. The carbonatite body coincides with a steep magnetic gradient and a bench or terrace in the gravity data that may reflect relative lower-density intrusive rocks. Although carbonatite rocks are typically magnetic, the carbonatite rocks, associated intrusive suite, and host rocks in this area are essentially non-magnetic. Combined geophysical data indicate that the enriched REE deposit may be related to a regional extensive hydrothermal alteration event.

Reflections of Prehistoric and Medieval human activities in floodplain deposits of the Únanovka Stream, South Moravia, Czech Republic

Floodplain sediments of the Unanovka Stream's catchment were investigated utilizing the methods of sedimentology and paleoecology. The development of human settlements in the stream catchment and dating from the Prehistoric through the Medieval, have been compared with floodplain evolution. Paleoecological data were obtained from the pollen record and plant macroremains. Bulk samples of the sediments were analyzed for phosphates, magnetic susceptibility, and grain size; macroremains were dated using the radiocarbon method. Floodplain structure were explored by combination of geophysical methods and drilling. The floodplain base was formed during the beginning of the Neolithic, where a sandy, organic silt layer was formed upon a sandy gravel basement, under a deciduous forest environment. From approximately the late neolithic, when a silty loam sediment started accumulation, direct paleoecological evidence of agriculture has been recorded. Since the beginning of the Middle Ages, the dominant vegetation was of a ruderal character, with Alder. From the High Middle Ages, the processes of accumulation and erosion in the neighboring region accelerated.

Geophysical investigation for the detection of liquefaction phenomena in an archaeological site, Lechaion, Greece

The area of Lechaion located in the northeastern part of the Peloponnese (Greece) is of great archaeological interest because it represents the western harbor of ancient Corinth and includes the remains of an early Christian basilica destroyed during a series of earthquakes in the sixth century AD. Numerous depressions and buckling structures of circular and linear geometry observed on the decorated floor of the basilica are indicative of ground deformation processes possibly related to earthquake-induced ground liquefaction. Electromagnetic (with a conductivity meter), ground penetrating radar (GPR) (with 250 MHz shielded antenna), and electric (ERT) methods have been applied in an effort to study the properties of the substratum and to identify indicative features related to ground liquefaction phenomena. The electromagnetic survey that was used as a reconnaissance method with both coil orientations provided valuable information through data processing, identifying zones of higher conductivity that favor liquefaction, or even the detection of the features themselves. The GPR method detected vertical zones that could represent sand vents either directly with multiple diffractions or indirectly through severe wave attenuation and reduction of the anomaly intensity due to finer sand and increased water saturation. Finally, the ERT method detected layering and the geologic status of the survey area but more importantly, it successfully detected narrow vertical zones of lower resistivity at shallow depths corresponding to sand vents via distinct variations in grain size and permeability. This combination of geophysical methods has successfully detected the dominant trend of east-northeastern–west-southwestern direction and a minor vertical one of north-northwestern–south-southeastern direction of surface depressions caused by liquefaction and generally can successfully provide valuable information on the extent and characteristics of ground liquefaction features in areas of geotechnical or archaeological interest located in regions of intense seismic activity.

Integrated Geophysical and Hydrochemical Approach for Locating Fresh Water Locations in a Coastal Terrain

An integrated geophysical investigation viz., Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) were carried out in a coastal terrain at Vizag Steel Plant (VSP), Visakhapatnam, Andhra Pradesh., India for locating fresh water locations. The entire area was underlain by recent alluvial of varying thickness consists of sand, clay and silt with basement as Kondalite rock formation consisting of saline water in some parts of the area. Locating favorable groundwater locations in such a terrain is a difficult task. The area consists of saturated clay, saturated silt and saline water has the same resistivity signature and misleads resistivity data interpretation for identifying and recommending the suitable groundwater locations in a coastal terrain. To overcome these uncertainties, a second geophysical approach or combinations of geophysical methods become necessary to resolve these resistivity ranges in a more reliable and better fruitful interpretation of sub-surface layers. In order to pin-point the groundwater locations at VSP; VES, ERT and GPR investigations were carried out. Interpreted results of VES, ERT pseudo-section and GPR images (radargram) were correlated with each other to ascertain confidently the geophysical signature of the sub-surface. Combination of geophysical methods gives better resolution or interpretation of sub-surface information before made any recommendations for drilling of borewell. Based on integrated geophysical investigation few borewell sites were recommended and drilled. The observed drilled lithologs was well correlated with the VES, ERT and GPR data. Besides, this hydrochemical analysis of water samples was carried out from the existing borewells and dugwells. Total Dissolved Solids (TDS) and chloride concentration were ranges from 189 to 3398 mg/l and 5 to 1610 mg/l. The eastern and north-eastern part of the area has elevated concentration of TDS and chloride indicating intrusion of saline water. TDS concen- tration of drilled borewell was found to range from 400 to 500 mg/l. The observed yield of drilled borewells ranges from 104 to 3623 gal/hr.

Cross-well 4-D resistivity tomography localizes the oil–water encroachment front during water flooding

The early detection of the oil–water encroachment front is of prime interest during the water flooding of an oil reservoir to maximize the production of oil and to avoid the oil–water encroachment front to come too close to production wells. We propose a new 4-D inversion approach based on the Gauss–Newton approach to invert cross-well resistance data. The goal of this study is to image the position of the oil–water encroachment front in a heterogeneous clayey sand reservoir. This approach is based on explicitly connecting the change of resistivity to the petrophysical properties controlling the position of the front (porosity and permeability) and to the saturation of the water phase through a petrophysical resistivity model accounting for bulk and surface conductivity contributions and saturation. The distributions of the permeability and porosity are also inverted using the time-lapse resistivity data in order to better reconstruct the position of the oil water encroachment front. In our synthetic test case, we get a better position of the front with the by-products of porosity and permeability inferences near the flow trajectory and close to the wells. The numerical simulations show that the position of the front is recovered well but the distribution of the recovered porosity and permeability is only fair. A comparison with a commercial code based on a classical Gauss–Newton approach with no information provided by the two-phase flow model fails to recover the position of the front. The new approach could be used for the time-lapse monitoring of various processes in both geothermal fields and oil and gas reservoirs using a combination of geophysical methods.

The application of non-destructive methods to assess the stability of the national nature monument of the Pravčická Brána Rock Arch, Czech Republic

The study contains the results of non-destructive research of the Pravčicka Brana Rock Arch which focuses on the structure and natural dynamics of this rock formation and its current level of stability. The main results include a description of the block fabric of the rock body and the nature of the contact zone between the arch beam and the southern pillar, discovery of relatively fresh secondary fissures and identification of zones with weakened strength within the sandstone massif. Also the local hydrodynamic regime was determined by a combination of geophysical methods. Long-term monitoring has demonstrated slow and irreversible body movements and reversible quasicyclical movements associated with changes in temperature on a scale of days up to years. The collected information was used to develop a structural deformation model of the arch body, including a description of the nature of the disintegration. The work was designed to fully respect the protective conditions of the site, to facilitate future follow-up activities and to monitor any possible negative changes in the rock massif.

Noninvasive characterization of the Trecate (Italy) crude-oil contaminated site: links between contamination and geophysical signals

The characterization of contaminated sites can benefit from the supplementation of direct investigations with a set of less invasive and more extensive measurements. A combination of geophysical methods and direct push techniques for contaminated land characterization has been proposed within the EU FP7 project ModelPROBE and the affiliated project SoilCAM. In this paper, we present results of the investigations conducted at the Trecate field site (NW Italy), which was affected in 1994 by crude oil contamination. The less invasive investigations include ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and electromagnetic induction (EMI) surveys, together with direct push sampling and soil electrical conductivity (EC) logs. Many of the geophysical measurements were conducted in time-lapse mode in order to separate static and dynamic signals, the latter being linked to strong seasonal changes in water table elevations. The main challenge was to extract significant geophysical signals linked to contamination from the mix of geological and hydrological signals present at the site. The most significant aspects of this characterization are: (a) the geometrical link between the distribution of contamination and the site's heterogeneity, with particular regard to the presence of less permeable layers, as evidenced by the extensive surface geophysical measurements; and (b) the link between contamination and specific geophysical signals, particularly evident from cross-hole measurements. The extensive work conducted at the Trecate site shows how a combination of direct (e.g., chemical) and indirect (e.g., geophysical) investigations can lead to a comprehensive and solid understanding of a contaminated site's mechanisms.

Joint interpretation of geoelectrical and soil‐gas measurements for monitoring CO2 releases at a natural analogue

ABSTRACTThe development and validation of hierarchic monitoring concepts is essential for detecting and assessing possible leakages from storage formations, especially for carbon capture and storage (CCS) applications. Joint interpretation of various techniques (such as carbon dioxide (CO2) concentration and flux measurements, self‐potential (SP) and geoelectrical surveys) showed that the combination of geophysical methods with soil‐gas analysis for mesoscale monitoring of the shallow subsurface above geologic CO2 storages can be a valuable tool for mapping and monitoring potential CO2 spread in the subsurface. Three measurement campaigns were undertaken – May 2011, July 2011 and April 2012 – at an analogue site in the Cheb Basin, Czech Republic, with the aim of studying CO2 leakages and their temporal and spatial behaviour. Results of geoelectrical investigations give an insight into the structural features of the subsurface. CO2 discharge into the atmosphere is mostly impeded by shallow, clay‐rich, partly water‐saturated zones, which can be seen in the electrical resistivity tomography (ERT) results. Several transport processes can be identified based on SP measurements. The SP results highlight the complex behaviour of temporal variations for the flow patterns. In particular, coupled migration of gas and water plays an important influencing role in this process. Site‐specific, near surface geological features and meteorological conditions seem to exert great influence on the degassing pattern and measured CO2 values. Therefore, soil‐gas measurements represent a snapshot which illustrates both a distinct typical pattern of the soil‐gas distribution in the near subsurface and certain differences caused by soil and meteorological conditions. Observed CO2 soil‐gas anomalies and modelled results suggest that the occurrence of gas discharge is much more localized around restricted areas, often controlled by local permeability contrasts. Hence, our results show that a proposed monitoring concept should integrate SP, time‐lapse ERT, meteorological parameters and soil‐gas measurements to provide a comprehensive insight into the subsurface structures and processes.

Generating soil electrical conductivity maps at regional level by integrating measurements on the ground and remote sensing data

In arid and semi‐arid areas, salinization of soil and water resources is one of the major threats to irrigated agriculture. For management purposes, quantifying both the extent and distribution of salinization is important, but accurate data with sufficient spatial resolution are often not available. Commonly used techniques such as soil sampling and geophysical methods are time‐consuming and yield only point data. A method is described in which multispectral remote sensing images can be used to regionalize point data measured on the field. Field data consist of measurements of electrical conductivity and are obtained by the combination of geophysical methods and the analysis of field soil samples. Uncalibrated salinity maps were calculated with spectral correlation mapping using image‐based reference spectra of saline areas. As an alternative indicator for soil salinity, the NDVI was used. The method was verified in the Yanqi Basin, northwestern China. Correlations between field data and the uncalibrated salinity maps were found over non‐irrigated sites for all images. Good correlations (R 2 up to 0.85) resulted for images collected during the winter months. The high correlation coefficients allow the uncalibrated salinity maps to be scaled to electrical conductivity maps.

Application of integrated methods in mapping waste disposal areas

An integrated suite of environmental methods was used to characterize the hydrogeological, geological and tectonic regime of the largest waste disposal landfill of Crete Island, the Fodele municipal solid waste site (MSW), to determine the geometry of the landfill (depth and spatial extent of electrically conductive anomalies), to define the anisotropy caused by bedrock fabric fractures and to locate potential zones of electrically conductive contamination. A combination of geophysical methods and chemical analysis was implemented for the characterization and management of the landfill. Five different types of geophysical surveys were performed: (1) 2D electrical resistance tomography (ERT), (2) electromagnetic measurements using very low frequencies (VLF), (3) electromagnetic conductivity (EM31), (4) seismic refraction measurements (SR), and (5) ambient noise measurements (HVSR). The above geophysical methods were used with the aim of studying the subsurface properties of the landfill and to define the exact geometrical characteristics of the site under investigation.

Geophysical characterisation of a rockslide in an alpine region

A combination of geophysical methods has been used for the characterisation of a rockslide area, located in the Graie Alps, NW of Italy, where the construction of a roadway tunnel has been planned. Electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and spectral analysis of surface waves (SWM) have been adopted to determine the overburden thickness and the geometry of the sliding surfaces. The geomechanical characterisation of rock mass was performed by means of horizontal seismic profiles (HSP) and sonic log in a horizontal borehole. The results have been calibrated by comparing the geophysical responses with the geological evidence derived from boreholes and with the movements recorded using inclinometric probe. Moreover, borehole geophysics results were validated by the results of laboratory measurements performed on core samples. The integration of geophysical and geotechnical results allowed the definition of a subsoil model for the investigated area and provided the necessary data for the design of the roadway tunnel. The study shows that surface and borehole geophysical techniques could be an efficient and cost-effective tool in rockslide investigations.

Geophysical Monitoring Through a Tidal Cycle at Crescent Beach State Park, Maine

This paper presents results of an experiment in which data from a combination of geophysical methods (resistivity, induced polarization, electromagnetic profiling, and magnetic profiling) were used to identify shallow geologic and hydrogeologic structure, and to enhance understanding of groundwater flow in a dynamic beach environment. Based on this study, well sorted beach sand is underlain by laterally variable glacial till, which overlies bedrock. An extensive patch of glacial till was mapped toward the ocean side of the beach. Bedrock has lithologic variability, as evidenced by a non-uniform magnetic response. In particular, a northeast-southwest trending magnetic body, believed to be a basalt dyke, was detected by its magnetic signature. During the transgressive phase of the tidal cycle, saline groundwater flow appeared to be quite rapid, extending ahead of the surfical water by at least [Formula: see text], as evidenced by the electromagnetic response. During the regressive phase, a slug of saline water remained up beach from the high tide mark for several hours prior to draining. Also during this regression, electromagnetic evidence for preferential groundwater flow pathways, probably bedrock fracture zones, was observed. These fracture zones appear to contain a relatively constant concentration of saltwater, while surficial sediments above and to the side are flushed with fresh groundwater during tidal regression.

Geophysical prospection and archaeological excavation of 13th-15th century deserted medieval settlements in the Czech Republic

This article deals with the present state and character of the investigations of deserted medieval settlements in the Czech Republic. A number of examples are used to show a combination of geophysical methods and subsequent archaeological excavations that are used to determine the overall structure and picture of the settlement, as well as to explain settlement patterns in the regions of Bohemia and Moravia. Copyright © 2001 John Wiley & Sons, Ltd.

The self‐potential method in the geothermal exploration of Greece

Self‐potential (SP) anomalies are generated by flows of fluid, heat, and ions in the earth. SP investigations have been used to locate and delineate sources associated with such flows in three areas of geothermal interest in Greece—Lesvos Island, Loutra Hypatis (central Greece), and Nisyros Island. A combination of geophysical methods, with SP being the primary method, has been applied in these areas. The SP method is adversely influenced by various sources of noise. Field procedures have been suggested to minimize their effects by monitoring electrode polarization and telluric activity. The interpretation of SP contour maps is preferred to using profile data. A procedure was adopted for SP interpretation, and the results were satisfactory. However, this model is based on thermoelectric sources only and is not related directly to hot fluid movement. In all three survey areas, the geothermal zones delimited by the SP interpretation in combination with data acquired by other geophysical methods result in an integrated interpretation of the geothermal system. Since SP and very‐low‐frequency (VLF) anomalies can be generated by the same geological source (i.e., geothermal, highly conductive zone), the corresponding results are compared to provide a strong indication of the presence of geothermal zones. The activity of geothermal zones affects the conductivity of the surrounding medium, which also can be detected by dc resistivity and audio‐magnetotelluric (AMT) methods. In addition, geothermal zones can be related to various interfaces or tectonic features that can be detected by gravity or seismic methods.

PROSPECTING FOR SUBSURFACE WATER IN THE CENTRAL VOLCANIC HIGHLAND OF ARMENIA BY GEOPHYSICAL METHODS*

Work carried out in several volcanic areas in Armenia has led to a combination of geophysical methods for hydrogeological problems. Prospecting of interlava and sublava water flows is done by electrical profiling and vertical electrical sounding. When mapping the relief of sublava water-resisting rocks, the electrical sounding (ES) method is widely used. This is based on the evaluation of lava resistivity. The method discussed allows to determine the most probable value of this parameter, by using statistical methods.

Geophysical prospecting in the Lake Victoria goldfields

Two areas of the Lake Victoria Region in Tanzania, where most of the old gold prospects are situated, were chosen for a comprehensive mineral survey. Since most of the country is covered by tropical soils, geophysical methods were applied. The gold occurs in quartz veins, shear zones and impregnation deposits, sometimes closely associated with sulphides or graphites, mostly in the archean country rocks of the Nyanzian System. The Nyanzian rocks form irregular shaped rafts in the roof of the central granite and have nearly the same physical properties as the gold occurrences. This obstacle was overcome by a special combination of geophysical methods: