Near-surface Investigations Research Articles

Retracted: Induction-free acquisition range in spectral time- and frequency-domain induced polarization at field scale

Spectral induced polarization imaging is a valuable method in near-surface hydrogeological and environmental investigations. Many field applications have been presented during the last 20 years, both in frequency and time domain. Despite the electromagnetic inductive effects always occurring in induced polarization measurements, they are usually disregarded in the spectral inversion of the data. Consequently, in order to invert only data with negligible inductive effects, the frequency and time ranges of both frequency-domain and time-domain data have often to be restricted, limiting the higher invertible frequency and the smaller invertible time. Consequently, this study investigates the induction-free acquisition range of both frequency-domain and time-domain acquisitions, computing the electromagnetic inductive effects of homogeneous media, with theoretical and field examples. It is found that it is much easier to get induction-free early times than induction-free high frequencies. For nested arrays it is difficult to measure a significant spectral range in the frequency domain, but also with the dipole-dipole array the inductive effects are not negligible with the typical settings used in field applications.

The Galperin source: A novel efficient multicomponent seismic source

Multicomponent acquisition enables a more complete observation of the seismic wavefield compared with single-component measurements and has therefore experienced increased interest in the past few decades. Whereas the use of multicomponent receivers is well-established, efficient and effective multicomponent sources are still difficult to realize. One of the main drawbacks of common multicomponent sources is that the source coupling differs for different components. Differences in the source coupling can lead to amplitude variations introducing uncertainties when jointly analyzing data from two or three components (e.g., for multicomponent polarization analysis). To overcome this problem, we have designed a new multicomponent vector source, referred to as the Galperin source. The design is inspired by the Galperin receiver configuration, and it consists of three orthogonal source vectors ([Formula: see text], [Formula: see text], and [Formula: see text]), which all have the same inclination relative to the horizontal. Using the Galperin configuration, three orthogonal source components can be acquired without moving the source and changing the source coupling, which also increases acquisition efficiency. We built a prototype of the Galperin source and compared it with other multicomponent sources used for near-surface investigations. Comparisons in the time and frequency domains indicate a good agreement between data acquired using the Galperin source and other, more conventional, sources. However, because the three source acquisition vectors [Formula: see text], [Formula: see text], and [Formula: see text] and the resulting processing coordinates [Formula: see text], [Formula: see text], and [Formula: see text] are linked by a rotational matrix, errors on one component can project into others. Therefore, precise leveling of the Galperin source and identical source strength for all three source components is essential to minimize errors.

Near-surface study of a hot spring site in Fiji

A geophysical investigation of a hot spring system located in Rabulu, Fiji, was carried out from October 2014 to March 2015. The investigation covered a survey area of 6075 m2. Self-potential (SP), ground temperature and soil carbon dioxide (CO2) concentrations were measured and investigated for their distribution characteristics and inter-linkages. Results indicated obvious anomalous zone at the hot spring discharge site. The SP profile analysis highlighted thermal water upwelling zones and elevation-driven subsurface groundwater pathways. Measurement of subsurface temperatures up to 1 m depths revealed increasing temperatures, indicating potentially high thermal gradients in the area. Surface soil CO2 distributions also agreed with SP and ground temperature results. The overall result of the study demonstrated that synchronised measurements of SP, ground temperature and soil CO2 can be instrumental in identifying anomalous zones near the hot spring sites. Other parameters such as spring water temperature, discharge rate and energy flux estimates from the spring were calculated and analysed. The high-dense multi-parameter data coverage allowed interpretation of geothermal features at a scale never conducted in Fiji before. The near-surface investigations reported in this study corroborate previously suggested steady geothermal activity in the region, deserving further detailed investigation.

Fast 3D multichannel deconvolution of electromagnetic induction loop-loop apparent conductivity data sets acquired at low induction numbers

Electromagnetic induction (EMI) sensors using sufficiently low-frequency harmonic sources and sufficiently small loop separations operate in the low-induction-number (LIN) domain for a relatively wide range of background conductivity. These systems are used in diverse near-surface investigations including applications from soil sciences, hydrology, and archaeology. The special case of portable multiconfiguration EMI sensors operating at frequencies [Formula: see text] offers the possibility of using a fast linear deconvolution method to interpret multichannel data sets in three dimensions. Here, we have developed a fast 3D inversion/deconvolution method regularized with 3D smoothness constraints and formulated in the hybrid spectral-spatial domain. Compared with other linear approaches, the spectral-spatial domain formulation significantly reduces the computational cost of the processing and opens the door for real-time 3D interpretation of large data sets consisting of more than 100,000 data points. First, we test our proposed algorithm on synthetic data sets computed with the full Maxwell theory. Then, we apply our method to a real four-configuration EMI data set acquired to map the thickness of peat layers embedded in a sandy environment. For the synthetic and the field example, we compared our result with the result obtained using a standard point-by-point 1D nonlinear inversion approach. This comparison demonstrates that the proposed methodology provides superior lateral resolution compared with the 1D nonlinear inversion, at the same time significantly reducing the computational cost of the processing.

An inter-disciplinary approach to airborne electromagnetics (AEM) survey design for groundwater exploration using the Australian Geoscience Data Cube and Morphotectonics

Over the past decade, advances in new satellite and airborne sensor technologies provide an opportunity for rapid multi-scale mapping, measurement and monitoring of the physical state of the crust, including resolution of key elements of surface and sub-surface hydrological systems. These advances have been mirrored by the development in advanced computational research infrastructure which is now giving the groundwater research community access to high-resolution (spatial and temporal) biophysical datasets (e.g. climate, ecology, geoscience and geospatial) relevant to broader hydrological systems understanding. This infrastructure facilitates integration of multiple datasets and rapid and improved signal processing, inversion, and sophisticated analysis. These datasets provide a catalyst for collaboration, with inter-disciplinary approaches enabling new discovery science in a 'big data' environment, and enabling the qualitative and quantitative analysis and modelling of landscape and hydrological system processes. In Australian landscapes, airborne electromagnetics (AEM) is widely used in near-surface (<200m) groundwater investigations due to the ability to acquire consistent, spatially coherent information of high quality using calibrated systems, in very short timeframes. This study reports on an evolving inter-disciplinary approach to AEM survey design for groundwater exploration. Recent investigations have employed time series analysis of surface water availability (using the Australian Geoscience Data Cube (AGDC)) combined with morphotectonic analysis of digital elevation datasets, tectonic analysis, and geomorphic analysis of satellite optical data, to help predict preferential recharge zones and shallow groundwater resources. This novel approach has been used successfully for groundwater exploration in the western Murray Basin and Kimberley Region of northern Australia.

Opposing-coils transient electromagnetic method focused near-surface resolution

The transient electromagnetic (TEM) method is a commonly used, nonintrusive, geophysical method, but inherent mutual induction between the transmitter (TX) and receiver (RX) coils strongly influences the measurements. We have developed an opposing-coils configuration to greatly reduce this effect. Three coils are used in this system. The upper opposing coil is physically the same as the lower TX coil, and they are concentric and parallel to the middle RX coil. A pair of currents with equal amplitudes but reverse directions is injected into the opposing and TX coils. Theoretical calculations in free space show that the received magnetic field by the RX coil is zero, which indicates that the mutual induction effect could be largely reduced. Physical experiments prove that an almost-pure secondary field could be acquired using this system. We have studied an optimal separation between the TX and opposing coils to guarantee that the primary magnetic field is powerful and the instrument is compacted for field work. Then, the efficient exploration depth of this system for typical geoelectric models was simulated to be approximately 15–50 m. Comparisons of simulated responses over highly conductive thick plates in free space and a field test over a culvert structure between this system and EM-47 showed that the system has enhanced sensitivity and lateral resolution. This system can be used in near-surface investigations, e.g., groundwater, environmental, and engineering investigations.

Frequency-domain inversion of marine CSEM data in one-dimensional vertically anisotropic structures

The marine controlled-source electromagnetic (CSEM) method has proven to be an important addition to seismic imaging techniques in exploration of offshore hydrocarbon reservoirs and near-surface investigations. In inversion and interpretation of industrial CSEM data sets, this method can be used in a number of situations by assuming isotropy. However, the presence of electric anisotropy in the Earth's crust, due to thin layer interbedding or grain alignments in the sediments, can significantly alter the response measured by the EM receivers. Ignoring anisotropy in interpreting marine CSEM data may lead to distorted images of seabed conductivity structures, even misinterpretation.#br#In this paper, we present an inversion method for frequency domain marine controlled-source electromagnetic data generated by a titled dipole source in vertically anisotropic stratified media. This approach is based on the Gauss-Newton scheme. We extend and generalize the formulation of marine CSEM fields to calculation of the electromagnetic fields excited by arbitrarily oriented dipole sources. The partial derivatives of the electromagnetic fields with respect to both the horizontal and vertical resistivity are analytically calculated. Based on the relationship between the horizontal resistivity (ρh) and the vertical resistivity (ρv) of the inversion model, an adaptive selection method for regularization factors is proposed to balance the effects of the data misfit and the structural constraint.#br#The synthetic and real data inversion tests indicate that our inversion method can reconstruct the anisotropic resistivity of the overburden layer and the basement, and the burial depth, the thickness and the vertical resistivity of the reservoir layer can be well recovered. A better reconstruction can be obtained with multiple frequency and multiple component data sets. Combing inline and broadside geometry data sets can provide high resolution in reconstructing the burial depth of the reservoir layer and resistivity of the anisotropic basement.

Investigation of Near Surface–Mounted Method for Shear Rehabilitation of Reinforced Concrete Beams Using Fiber Reinforced–Polymer Composites

This paper presents the results of an investigation of reinforced concrete (RC) T-beams retrofitted in shear with near-surface mounted (NSM) fiber-reinforced polymer (FRP) rods. Six full-scale 4,520-mm-long RC T-beams were tested to study the effects of important parameters such as the presence of NSM FRP rods, the presence of steel stirrups, and the steel stirrup ratio. This paper provides an insightful and comprehensive description of the behavior of strengthened T-beams under increasing load, from the formation of the first crack to ultimate failure. The results of this study and those gathered in the presented database show that existing steel stirrups and strengthening NSM FRP did not diminish each other’s effect when failure modes unrelated to shear resistance of RC beams were prevented. The experimental results of this study and those in the database were used to verify a newly proposed model to predict the shear contribution of NSM FRP rods and laminates in RC beams strengthened in shear. The proposed model showed an improved accuracy compared with the existing models in the literature.

Reflection waveform inversion of ground-penetrating radar data for characterizing thin and ultrathin layers of nonaqueous phase liquid contaminants in stratified media

Accurately quantifying thin-layer parameters by applying a targeted reflection waveform inversion methodology to ground-penetrating radar (GPR) reflection data may provide a useful tool for near-surface investigation and especially for contaminated site investigation where nonaqueous phase liquid (NAPL) contaminants are present. We implemented a targeted reflection waveform inversion algorithm to quantify thin-layer permittivity, thickness, and conductivity for NAPL thin ([Formula: see text] dominant wavelength [Formula: see text]) and ultrathin ([Formula: see text]) layers using GPR reflection data. The inversion used a nonlinear grid search with a Monte Carlo scheme to initialize starting values to find the global minimum. By taking a targeted approach using a time window around the peak amplitude of the reflection event of interest, our algorithm reduced the complexity in the inverse problem. We tested the inversion on three different synthetic data sets and four field data sets. In all testing, the inversion solved for NAPL-layer properties within 15% of the measured values. This algorithm provides a tool for site managers to prioritize remediation efforts based on quantitative assessments of contaminant quantity and location using GPR.

Effect of near-surface topography on high-frequency Rayleigh-wave propagation

Rayleigh waves, which are formed due to interference of P- and Sv-waves near the free surface, propagate along the free surface and vanish exponentially in the vertical direction. Their propagation is strongly influenced by surface topography. Due to the high resolution and precision requirements of near-surface investigations, the high-frequency Rayleigh waves are usually used for near-surface structural detecting. Although there are some numerical studies on high-frequency Rayleigh-wave propagation on topographic free surface, detailed analysis of characters of high-frequency Rayleigh-wave propagation on topographic free surface remains untouched. Hence, research of propagation of Rayleigh waves on complex topographic surface becomes critical for Rayleigh-wave methods in near-surface applications. To study the propagation of high-frequency Rayleigh waves on topographic free surface, two main topographic models are designed in this study. One of the models contains a depressed topographic surface, and another contains an uplifted topographic surface. We numerically simulate the propagation of high-frequency Rayleigh waves on these two topographic surfaces by finite-difference method. Soon afterwards, we analyze the propagation character of high-frequency Rayleigh waves on such topographic models, and compare the variations on its energy and frequency before and after passing the topographic region. At last, we discuss the relationship between the variations and topographical steepness of each model. Our numerical results indicate that influence of depressed topography for high-frequency Rayleigh waves is more distinct than influence of uplifted topography. Rayleigh waves produce new scattering body waves during passing the depressed topography with reduction of amplitude and loss of high-frequency components. Moreover, the steeper the depressed topography is, the more energy of Rayleigh waves is lost. The uplifted topography with gentle slope produces similar influence as the depressed topography on propagation of high-frequency Rayleigh waves. When slopes are getting steep, however, the obstacle influence of the uplifted topography becomes weak. In addition, in the uplifted-topography case, part of Rayleigh-wave energy is absorbed by the uplifted terrain, which includes the trapped energy between the uplifted boundaries and the generation of scattering waves. Meanwhile, local topography strongly affects the character of Rayleigh-wave dispersion. Egregious error may be introduced, when pick Rayleigh-wave phase velocities on dispersion energy images for an MASW survey, if the local free-surface topography is ignored.

Theoretical and experimental examinations of the capacitively coupled resistivity (line antenna) method

The capacitively coupled (CC) resistivity method is an emerging geophysical technique for near-surface investigations. However, there are two major problems associated with using the CC resistivity (line antenna) system in ground investigations. These two problems are (1) the geometrical factor and (2) measurement biases. We first derive the geometric factor for the line-antenna array in a dipole-dipole configuration. The new expression of the geometric factor considers the separation between the two current (and the two potential) line antennas, thereby improving the accuracy of the measured apparent resistivity. Second, from the electroquasistatic point of view, a quadruline model that can describe the characteristics of the CC resistivity (line antenna) system is derived based on the previously published quadrupole model. The validity of the quadruline model is verified experimentally. Based on the quadruline model, it is found that the dielectric properties of the line-to-ground capacitor and the ground permittivity produce measurement bias at low and high current frequencies, respectively. If the operating current frequency is around the kHz range and the ground resistivity is relatively high, such as in permafrost areas, the ground permittivity can also create measurement bias to cause the actual value to be underestimated. The experimental results and the predictions by the quadruline model suggest that the measurement bias induced by the gap between the line antenna and the ground surface becomes significant and cannot be ignored as the gap height is large and the associated ground resistivity is low. In general, the CC resistivity (line antenna) measurements are not biased by the gap effect when the gap height is less than 0.01 m, which can easily be achieved for tests on flat ground.

Near-Surface Investigation of Groundwater Contamination in the Regolith Aquifer of Palladan, Zaria using Borehole log and Tomography Techniques

Two geophysical Tomography techniques- Electrical resistivity and Seismic refraction were used to investigate the subsurface of a potentially polluted dumpsite in Palladan a densely populated area of Zaria, with a view to examining the possible subsurface distribution of groundwater contamination plume. The presence of domestic wells in the residences of the people which are distances 10.0 to 30.0 m from the dump facilitated analysis of water chemistry to enhance the geophysical interpretation. The groundwater level in the dumpsite site was found to be higher than the surrounding area, thereby creating a local deviation from the regional groundwater flow. Due to this the contaminants from the waste site spread out in the nearby soil and groundwater. The resistivity models clearly show a top layer of about 10.0 m thickness with low resistivity, whereas the resistivity has an inverse correlation with distance from the waste disposal site. Bore hole log shows that the top upper 10.0 m of soil consists of loose permeable laterite with high water content followed by a layer of degraded sand before the weathered basement which suggests the possibility of the contamination penetrating deeper into the regolith aquifer. This agrees with the result of the water chemistry analysis which shows elevation in concentration of contaminants above the WHO guidelines. The borehole log also indicated the presence of fracture basement at a depth of 23.0 m this correlated well with the Seismic refraction result. The study therefore suggests that these fractures also facilitate the migration of the contaminants. Based on the combined results, the contamination plume seems to have migrated not less than 500.0 m in the southern direction which is also the direction of hydraulic gradient

Принципові положення нової технології дослідження природних і техногенних ґрунтів на основі комплексу радіоізотопного каротажу

Принципові положення нової технології дослідження природних і техногенних ґрунтів на основі комплексу радіоізотопного каротажу

Accurate 1D forward and inverse modeling of high-frequency helicopter-borne electromagnetic data

Frequency-domain helicopter-borne electromagnetic (HEM) data are commonly interpreted using quasistatic approximations in forward and inverse solutions. At high-frequencies ([Formula: see text]), the accuracy of this approach is often insufficient. Implementation of a full solution of the forward problem that includes displacement currents in both subsurface and air, however, may cause singularity problems during numerical evaluation of the secondary field integral using Hankel or Laplace transforms, particularly at high frequencies. These effects can be reduced by a wavenumber shift during numerical evaluation, a transformation of the integral by partial integration, or a combination of both. Based on these corrections, the resulting forward responses obtained with various fast Hankel transforms or numerical Laplace transforms are nearly identical. The inversion of HEM data also requires some modification. For a half-space inversion, the parameter sets, such as look-up tables, linear or polynomial approximations have to be modified for frequencies above some 10 kHz. The quasistatic formulas used in multi-layer inversion procedures have to be replaced by the accurate formulas. Fortunately, singularity problems are restricted to the forward calculation as the derivatives of the Jacobian matrix are not affected. Accurate high-frequency forward and inverse modeling enables a successful usage of helicopter-borne electromagnetics for near-surface investigations and expands the range of applications in environmental and geotechnical issues.

Deblurring of frequency-wavenumber images from small-scale seismic arrays

Temporary arrays installed in urban areas for investigating the upper-most geological structure typically comprised of a limited number of stations and are arranged in geometries constrained by environmental boundaries. Therefore, it is expected that the frequency–wavenumber images are significantly blurred by the array transfer function and are corrupted by noise. In this paper, the effect of the Richardson–Lucy regularization method applied to the problem of deblurring frequency–wavenumber images is investigated. The images are computed by analysing data from two small-aperture 2-D arrays, installed with different configurations in a test-site within the town of Potenza (Southern Italy) for near-surface investigations. We show that removing the effects of the array response from the frequency–wavenumber images improve the phase-velocity estimation, reducing the relevant level of uncertainty. Furthermore, the Richardson–Lucy regularization method is effective in reducing the level of noise related to spatial aliasing by eliminating spurious peaks, allowing the maxima related to different seismic sources to be better discriminated.

EM-I sensor NEMFIS: method, equipment and case stories of archaeological prospection

Sounding with alternating electromagnetic fields, including Frequency Domain Electromagnetic Induction (FD-EMI) sounding methods, have received increasing attention and broad application during the last three decade. A portable electromagnetic sensor (NEMFIS, aka EMS), developed at IPGG (Siberian Branch of the Russian Academy of Sciences), and based on alternating electromagnetic fields, is described in this work. The paper presents a specific application to the near-surface investigation of ...

Accelerators and x-rays in cultural heritage investigations

Abstract In the following article a review is given on the use of accelerators in studies connected to our cultural heritage. It focuses on making use of the production and detection of x-rays as a general tool. At “small accelerators”, the proton induced x-ray emission (PIXE), especially when combined with Rutherford backscattering spectroscopy (RBS), has been developed to a very versatile and powerful technique for near-surface investigations. It is well complemented by larger facilities, synchrotron radiation sources as well as medium energy ion accelerators for high energy PIXE. With the development of small compact electron accelerators, a new generation of mono-energetic high-energy high-intensity x-ray sources will add a very comfortable complement in cultural heritage studies. To cite this article: H.-E. Mahnke et al., C. R. Physique 10 (2009).

Field investigation of Love waves in near-surface seismology

We examine subsurface conditions and survey parameters suitable for successful exploitation of Love waves in near-surface investigations. Love-wave generation requires the existence of a low shear-velocity surface layer. We examined the minimum thickness of the near-surface layer necessary to generate and record usable Love-wave data sets in the frequency range of [Formula: see text]. We acquired field data on a hillside with flat-lying limestone and shale layers that allowed for the direct testing of varying overburden thicknesses as well as varying acquisition geometry. The resulting seismic records and dispersion images were analyzed, and the Love-wave dispersion relation for two layers was examined analytically. We concluded through theoretical and field data analysis that a minimum thickness of [Formula: see text] of low-velocity material is needed to record usable data in the frequency range of interest in near-surface Love-wave surveys. The results of this study indicate that existing guidelines for Rayleigh-wave data acquisition, such as receiver interval and line length, are also applicable to Love-wave data acquisition.

O arranjo pólo-dipolo como alternativa ao dipolo-dipolo em levantamentos 2D de eletrorresistividade

This study shows the results of a near-surface investigation conducted to compare pole-dipole with dipole-dipole array. The survey was carried out in a single profile, 40 meters long. In the dipole-dipole array, the inter-electrode spacings used were 2 m and 1 m. In the pole-dipole array, spacing was 1 m. The data collected from the dipole-dipole array were combined into a single section and then compared with the results of the pole-dipole array. The investigation levels used in each array were selected in order to achieve, a priori , the same depth in the subsurface. The corresponding apparent resistivity pseudosection (field data) and the model (obtained by an inversion procedure) were very similar, proving that the pole-dipole array is a possible alternative to the dipole-dipole array, since it provided similar results for resolution and depth of investigation, as shown in the final models.

Multichannel analysis of surface waves (MASW)—active and passive methods

The conventional seismic approaches for near-surface investigation have usually been either high-resolution reflection or refraction surveys that deal with a depth range of a few tens to hundreds meters. Seismic signals from these surveys consist of wavelets with frequencies higher than 50 Hz. The multichannel analysis of surface waves (MASW) method deals with surface waves in the lower frequencies (e.g., 1–30 Hz) and uses a much shallower depth range of investigation (e.g., a few to a few tens of meters).