Near Surface Geophysics - Volume 18, Issue 2, 2020

In this work, we explore the use of biochar as a remediation agent, and the sensitivity of the spectral‐induced polarization method as a remediation monitoring aid. Biochar amended columns were fully saturated with industrial wastewater (olive oil mill waste) with very high concentration of phenols (∼2485 mg/L) and other substances. The biochar‐amended columns achieved very high removal rates of phenols compared to the control (sand only). Geophysical monitoring over the duration of the experiment (10 days) showed changes in the spectral‐induced polarization signal (imaginary conductivity) consistent with phenol removal as confirmed by geochemical monitoring. This experiment confirmed the utility of biochar as a remediation agent. Furthermore, spectral‐induced polarization can serve as long‐term, high resolution, monitoring aid in organic contaminant degradation processes.

In this research, we have studied the capabilities and limitations of the transient electromagnetic sounding applied to investigate the structure and composition of the moraine deposits covering a crystalline basement. It has been experimentally demonstrated that using the ungrounded antennas one can not only determine the electrical resistivity of different layers in the alluvium deposits and map the surface topography of the crystalline basement down to a depth of 70–100 m, but can also minimize the costs and the damage done to the environment. Additionally, transient electromagnetics helps to reveal the layers within the moraine strata that often have different resistivities and depths. The use of single antennas of size 12 m × 12 m–25 m × 25 m for field excitation and registration of transient responses allows for maximum localization of the targets and provides a high horizontal resolution in strongly heterogeneous media. The limitations of the method due to the induced polarization effect have also been investigated. Overall, despite some limitations, the transient electromagnetic method appears to be effective in the mapping, assessment and analysis of hazards in landslide‐prone areas, prior to geotechnical soundings in wells.

Obtaining high‐quality ground penetrating radar (GPR) images in karst is difficult because materials resulting from the weathering of carbonate rocks might be electrically conductive. As a consequence, penetration depth and signal resolution might be greatly reduced due to attenuation. In addition, fractures and faults might cause a significant amount of electromagnetic wave scattering. We present a 2D data processing flow which allows improving the quality of GPR images in carbonate karst. The processing flow is composed of the following steps: obtaining a zero‐offset section by removing the direct wave, low‐frequency noise removal, geometrical spreading and exponential gain compensation, spectral balancing, Kirchhoff migration, bandpass filtering, amplitude‐volume enhancement, and topographic correction. For a 200‐MHz dataset, we present in detail each step of the processing flow, exemplifying how to parameterize every step. Spectral balancing is of key importance because it can approximately replenish the high‐frequency content lost due to propagation effects. In this step, we recommend to shift the centroid frequency as much as possible to high‐frequency values, even exceeding the nominal value of the antenna center frequency, but still looking for a band‐limited spectrum as the goal. Despite the difficulty of migrating GPR data, we show that migration (even assuming a constant velocity) might enhance the lateral continuity of the reflection events and allows identification of discontinuities such as faults and fractures. If imaged in a better way, these structures can have special importance as they are often the boundaries of dissolution features. Obtaining images based on amplitude‐volume enhancement techniques allows to better visualize karst voids and deep‐rooted discontinuities because these features are often associated with low‐amplitude zones, which are highlighted in such images. Due to this processing flow, stratigraphic, structural and dissolution features can be enhanced, allowing the interpreter to establish spatial and genetic associations among these elements to obtain a better understanding of the karst formation process.

An interesting geological objective of quantitative interpretation of magnetic data is to find inverse models which can determine sharp geological interfaces below the surface. The stabilizing function in the Tikhonov parametric functional governs sparseness constraint in the recovered model. This paper introduces a novel stabilizer based on a sigmoid function which can provide non‐smooth models in the inversion of magnetic data efficiently. An inversion algorithm is developed based on the reweighted regularized conjugate gradient to get the solution of the inverse problem using this stabilizing function. The performance of the proposed algorithm is checked on two synthetic data sets and real aeromagnetic data from McFaulds Lake in Ontario, Canada, in comparison with the results of the minimum support stabilizing function. The inverse problem converges to the solution faster when the sigmoid stabilizing function is used instead of the minimum support stabilizing function.

To improve the resolution of seismic data, it is important to accurately estimate the near‐surface quality factor, Q, which provides a measure of seismic wave attenuation. In view of the unique advantages provided by uphole surveys when investigating near‐surface structures, they are widely employed to estimate the near‐surface Q factor. However, the Q factor estimated using the traditional spectral ratio method is not always precise and provides larger oscillations in the estimated Q factors due to errors associated with first‐break picking and velocity estimation. Following on the traditional logarithmic spectral ratio method, a new method called the logarithmic spectral ratio integral method was proposed to estimate the layer Q factor using uphole survey data. It calculates first the weighted integral of the logarithmic spectral ratio in an effective frequency interval between non‐adjacent traces, then makes a linear regression between the inter‐trace travel moveout and the weighted integral of logarithmic spectral ratio under the constraint of velocity stratification. The result of model analysis shows that under an ideal condition (without first‐break picking errors), the layer Q values estimated by the logarithmic spectral ratio integral method are fairly consistent with the true layer‐specific Q values in the model. In addition, the Q values estimated from field‐measured data and data from forward modelling with 10% random noise added, both have smaller mean relative errors than the results using traditional spectral ratio method and the double‐linear regression method. A case study is employed and the results show that the layer Q factor estimated using the new method correlates well with the velocity stratification and is thus applicable for use with various uphole survey observation systems. Furthermore, all results indicate that the logarithmic spectral ratio integral method delivers a more precise and stable estimation of the layered Q than the other methods, and the anti‐noise characteristics are stronger.

Guadalajara, Jalisco, is the second largest city in Mexico with around 4.5 million inhabitants. A high seismic hazard exists in the city due to forces produced by the interaction between the Rivera, Cocos and North American plates and the smaller Jalisco Block. Guadalajara is one of the largest cities built over pumice soil deposits. Furthermore, the near‐surface phreatic level causes a high susceptibility to liquefaction. All these features can cause extreme earthquake site effects. Due to the fragile inner structure of pumice sands, traditional geotechnical tests are inappropriate to characterize the seismic response. Therefore, we propose the use of surface wave analysis methods (multichannel analysis of surface waves and refraction microtremor), which we applied in 33 sites to define the soil classification in terms of VS30 (the average shear wave velocity between the surface and 30 m depth), the bedrock depth and the fundamental period. From the soil classification, we construct a microzonation map consisting of four geotechnical zones, which we superimpose on the known construction systems within the city. The comparison between the construction period of the buildings and the fundamental frequencies of the soil indicates a high vulnerability to resonance in 1‐ to 4‐storied old buildings constructed of adobe and unreinforced masonry within zones II and III, followed by a medium vulnerability to seismic resonance in compact buildings of 1–4 stories within zone I and 1–12 stories within zones II and IV.

A 3D geological model of the Torre‐Bembibre Carboniferous sub‐basin, a potential CO2 storage reservoir, is proposed here. The Torre‐Bembibre sub‐basin is part of the El Bierzo coal‐bearing Carboniferous basin located in the westernmost part of León province (NW Spain). It covers an area of about 150 km². Carboniferous deposits (up to 1500 m of continental siliciclastics) are divided into nine intervals, defined by earlier coal exploration work. They were deposited as intramontane basin infill within a transpressional regime during the last stages of the Variscan Orogeny and are partially covered by Neogene and Quaternary continental siliciclastic sediments, formed by alluvial plains, fans and terraces. To resolve a 3D geological model of the Torre‐Bembibre sub‐basin, including outcropping layers and their prolongation at depth, a 3D dip‐domain method was applied, where data from all stratigraphic positions were used to compose a coherent geological reconstruction of the stratigraphic surfaces. Subsurface information was available from previous and newly acquired seismic reflection profiles and boreholes. The 3D model helped resolving some uncertainties, specifically: (i) the western termination of the Carboniferous basin under the Tertiary sediments, corresponding to an NNE–SSW normal fault, (ii) the depth and morphology of the base of the Carboniferous strata and (iii) the morphology of the intra‐Carboniferous layers. The syncline/anticline shape of the Torre‐Bembibre sub‐basin suggests a reservoir which may be capable of storing CO2 under the Tertiary cover. The total estimated volume is 6.4 km³, while the total capacity for CO2 storage could be as much as 7 Mt. The reservoir is most likely located within the major massive sandstone bodies in the Chuchu interval, in the syncline–anticline structure north of the town of Bembibre. The potential capacity of the reservoir calls for undertaking further studies using the proposed 3D geological model in order to determine more precisely the petrophysics and geometry near Bembibre.