Near Surface Geophysics
SCIE-ISI SCOPUS (2003-2023)
1569-4445
1873-0604
Hà Lan
Cơ quản chủ quản: EAGE Publishing BV , WILEY
Lĩnh vực:
Geophysics
Các bài báo tiêu biểu
Properties and effects of measurement errors on 2D resistivity imaging surveyingABSTRACT Electrode spacing errors and errors correlated with the magnitude of the observed potential are two key factors that affect the data quality for DC resistivity imaging measurements. This paper investigates the properties and effects of these two kinds of error on 2D resistivity imaging or inversion for practical applications. By analytic analysis and numerical simulations, the off‐line and in‐line electrode spacing errors were quantitatively estimated for all common electrode arrays (pole‐pole, pole‐dipole, pole‐bipole, Wenner, Schlumberger, dipole‐dipole, γ‐array, Wenner‐β) in 2D resistivity imaging surveys. Meanwhile, the spreading patterns of the spacing errors in the pseudosection and the possible artefacts in the imaging (inverted model) are evaluated. We show that the magnitude of the spacing errors are quite different with these arrays, being largest for dipole‐dipole, Wenner‐β and γ‐array surveys, for which a 10% in‐line spacing error may cause twice as large an error (>20%) in the observed resistance or apparent resistivity, which in turn will produce some artefacts in the inverted model. The observed potential errors obtained with the reciprocity principle and collected from different sites and with different electrode arrays, were analysed to show the properties of the potential error caused by many aspects in the field. Using logarithmic plots and error pseudosections, we found that with different electrode arrays and at different sites the potential errors demonstrate a general property, which may be regarded as a negative‐power function of potential reading. Power net transients, background telluric variation and instrument malfunction are possible sources that may cause the large errors present as outliers deviating from this function. We reaffirm the fact that the outliers are often correlated with high contact resistances for some of the electrodes used in a measurement, but this may also be caused by an unsatisfactory connection between the electrode and the cable due to, for example, dirt or oxide on the connectors. These outliers are often the main part of the errors affecting the imaging results. Furthermore, a robust inversion and a smoothness‐constrained inversion were applied to the investigation of the effects of the measurement errors. Using two real data sets, we show that the smoothness‐constrained least‐squares inversion is much more sensitive to the potential errors than the robust inversion, but the two inversion schemes produce very similar models with a high data quality. Artefacts or indefinite parts in the inverted models correlate with the distribution zones of the outliers in the potential error pseudosection.
Tập 1 Số 3 - Trang 105-117 - 2003
Weathering variations in a granitic massif and related geotechnical properties through seismic and electrical resistivity methodsABSTRACT Mapping and distribution with depth of alteration in rocks is critical in engineering planning because it has a fundamental impact on the geotechnical properties of the materials. Lateral heterogeneity on a weathered rock massif makes boreholes inadequate for its complete characterization. Geophysical methods increase spatial sampling along the study area and can be related to geotechnical parameters, so subsoil conditions can be better understood. In order to determine its geotechnical qualities and variability along two different profiles, we attempt to characterize a granite massif in north‐west Spain by the integration of results from seismic refraction, multichannel analysis of surface waves (MASW) and electrical resistivity tomography methods (ERT). The study area, the so‐called Carlés granite, shows all the weathering grades from sandy soil to fresh rock. A reference borehole where samples were taken and laboratory measurements were made, serves as a direct check for the results of one of the profiles, the other being interpreted without any direct information. This approach has permitted the evaluation of the advantages and limitations of each geophysical method and created an accurate geotechnical model of the massif, correlating physical and geotechnical parameters such as rock quality designation, weathering grade, or standard penetration test. The field seismic velocities have been compared with the ultrasonic measurements at the laboratory, permitting an evaluation of the field and laboratory elastic constants. The trend in the values of these parameters agrees with the field and laboratory test for the shallow parts of the massif. However, unrealistic elastic constants have been obtained for fresh rock based on the results of the field experiments. This is related to an apparent underestimation of the velocity of seismic S‐waves for the deepest layers. This fact suggests that the methodology followed throughout this work is able to provide a full geotechnical model of an altered rock massif for the first tens of metres, discriminating between different weathered levels. It is also useful and reliable when inferring elastic constants for depths of up to 20 m. However, its validity becomes doubtful with depth, so care must be taken when calculating elastic moduli and trying to extrapolate directly to a rock massif.
Tập 8 Số 6 - Trang 585-599 - 2010
Effective medium theories for modelling the relationships between electromagnetic properties and hydrological variables in geomaterials: a reviewABSTRACT The paper reviews the effective medium theories used for modelling the relationships between electromagnetic properties (low‐frequency conductivity and high‐frequency permittivity) and hydrological variables (water content, salinity, suction, permeability) in soils and rocks. It aims a) to provide a simple presentation of these theoretical approaches, b) to present their theoretical and practical limitations and c) to establish some connections with empirical equations usually used in hydrogeophysics (i.e., Archie relationships, Topp equation and complex refractive index model). This review demonstrates that two groups of effective medium theories can be clearly identified. The first group constituted by the Maxwell‐Wagner and Symmetric‐Bruggeman rules is characterized by easy‐to‐use models. When the volumetric water content and the texture are known, they allow to obtain fast estimates of effective electromagnetic properties. In the second group, the differential effective medium schemes that are more complex from a mathematical point of view are preferred when frequency‐dependent properties are studied. In particular, differential effective medium schemes are used for providing some insights into the physical basis of spectral induced polarization measurements in hydrogeophysical applications.
Tập 7 Số 5-6 - Trang 563-578 - 2009
Combination of 1D laterally constrained inversion and 2D smooth inversion of resistivity data with a priori data from boreholesABSTRACT Resistivity imaging in combination with borehole information is a powerful tool for site investigation. We show that the combination of 1D laterally constrained inversion (1D‐LCI) with the use of a priori information from borehole data and 2D smooth inversion adds significant value to the interpretation of continuous vertical electrical sounding (CVES) data. The 1D‐LCI offers an analysis of the resolution of the model parameters. This is helpful when evaluating the integrity of the model. Furthermore, with the 1D‐LCI it is possible to constrain model parameters with a priori information, e.g. depth‐to‐layer interfaces, based on borehole information. We show that 2D smooth inversion resolves lateral changes well, while 1D‐LCI results in well‐defined horizontal layer interfaces. In geological environments where the lateral variations are not too pronounced, the 1D‐LCI contributes to a geological interpretation of the resistivity measurements. Depths to layers can be interpreted with greater certainty than if using results from 2D smooth inversion only. The inclusion of a priori information in the inversion reveals further details and enhances the geological interpretation significantly.
Tập 3 Số 2 - Trang 71-79 - 2005
Spectral Induced Polarization (SIP) signatures of clayey soils containing tolueneABSTRACT We performed laboratory experiments to investigate the sensitivity of the Spectral Induced Polarization (SIP) method to toluene contamination in clayey soils. We used mixtures of quartzitic sand and montmorillonite as soil samples, artificially contaminated with varying amounts of toluene. Care was taken to quantify the experimental uncertainty resulting from packing since such effects must be quantified if variations in SIP signatures between samples are to be reliably interpreted in terms of the effects of hydrocarbon concentration. The SIP response of all samples following addition of toluene was monitored for a period of 40 days following sample preparation. Stepwise regression was used to examine the statistical significance of correlations between (i) clay content and (ii) toluene concentration and SIP parameters. Both single‐frequency real and imaginary conductivity measurements, along with the integral chargeability, normalized chargeability, DC conductivity and time constant obtained from a Debye decomposition fitting, were examined in this regression analysis. The SIP measurements show a clear time dependence following sample preparation, indicating that samples containing toluene may take significant time to reach an equilibrium electrical response. SIP measurements are significantly related to toluene content shortly after sample preparation, when the expected dependence of SIP on clay concentration is apparently suppressed. However, for the state of electrical equilibrium after 40 days (interpreted to indicate surface chemistry at equilibrium) there is no significant relation between SIP measurements and toluene content; instead SIP measurements are then significantly correlated with clay concentration. The total chargeability, normalized chargeability and relaxation time obtained from the Debye decomposition show no correlation with toluene content, indicating that this procedure, which likely integrates over multiple mechanisms, may not be suitable for understanding relationships between SIP and hydrocarbon contamination. We find only small low‐frequency polarization signals observed in relation to toluene concentration (2 mrad at 0.01 Hz), which initially decreases the interfacial polarization. Unlike earlier works, our results do not support the use of the SIP method as a tool for monitoring toluene contamination in clay soils.
Tập 10 Số 6 - Trang 503-515 - 2012
Electrical properties of clay in relation to moisture lossABSTRACT Subsidence of buildings situated on clay soils can be a serious insurance problem in dry summers. In order to investigate subsidence and monitor its progress, non‐invasive techniques that provide information on soil properties have enormous potential. To help assess the application of new electrical tomographic monitoring techniques, the electrical properties of seven geographically extensive clays and their relationships with moisture content have been determined. It is found that resistivity increases slowly as moisture content decreases to between 10–15% of the total saturation, beyond which resistivity increases rapidly. As electrical resistivity of clay is also dependent on ambient temperature, this must be monitored and corrected for. The results suggest that electrical resistivity could be a useful indicator of moisture content in clay soils.
Tập 8 Số 2 - Trang 173-180 - 2010
S‐wave velocity structure of Chlef City, Algeria, by inversion of Rayleigh wave ellipticityABSTRACT Chlef City, Algeria, which is located in the Lower Cheliff Basin, is vulnerable to seismic hazards. Since there is no constrained velocity model for the Lower Cheliff Basin, particularly at greater depths, we conducted an ambient vibration investigation to map the shear‐wave velocity structure beneath the city, with the primary goal of supplementing the existing microzonation studies. Here, we inverted the Rayleigh wave ellipticity measurement curve of ambient vibrations measurements from 120 sites in Chlef City to estimate the shear‐wave velocity structure. The study area was subdivided into six zones based on similarities between the observed horizontal‐to‐vertical spectral ratio of the ambient vibrations curves. Our resultant shear‐wave velocity models show that the observed fundamental frequencies (0.3–1.6 Hz) are related to a thick layer (~800 m) of upper Miocene deposits, where on average. Generally, the Mesozoic basement shows . Moreover, the clear peaks observed at higher frequencies and only in the northwest part of the city are related to a thin layer of Quaternary deposits ().
Tập 16 Số 3 - Trang 328-339 - 2018
Coupling hydrogeophysics with hydrodynamic modelling to infer subsurface hydraulic architecture of an alluvial floodplainABSTRACT This paper underscores the importance of spatially dense geophysical data sets for making informed decisions in water management strategies. Such decisions may require understanding how site‐specific subsurface architecture – especially hydraulic connectivity – impacts the response of a shallow aquifer to anthropogenic hydrologic disturbances (e.g. over‐pumping of a shallow aquifer). At a 0.2‐km2 alluvial floodplain site characterized by thick clay over fine sand to gravel and shale bedrock in the subtropical, sub‐humid belt of the Gulf Coast of the United States, we image an asymmetrically shaped, compartmentalized, sand‐dominated channel‐belt using electrical resistivity tomography and 31 time‐domain electromagnetic soundings probing to depths of ∼40 m and ∼90 m, respectively. Lithological interpretation and a hydrological model are developed based on the geophysical data and nearby sediment cores, where the resistivity of the groundwater is 9.1 Ωm. In a modelling scenario wherein the compartmentalized sand channel‐belt starts out dry (i.e. an over‐pumped shallow aquifer), we simulate 26 weeks of infiltration due to flooding of the surface. Preferential filling of the channel‐belt occurs through its sides rather than from above, generating a new understanding of the hydraulic connectivity around and into asymmetrically shaped, sand‐dominated channel‐belts. This insight can inform decisions about the optimal placement of shallow water wells in a heterogeneous alluvial floodplain aquifer system and also highlights the dangers of over‐pumping.
Tập 19 Số 3 - Trang 335-352 - 2021