- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 19, Issue 3, 2021
Near Surface Geophysics - Volume 19, Issue 3, 2021
Volume 19, Issue 3, 2021
-
-
2D time‐domain full‐waveform inversion of SH‐ and Love‐waves for geotechnical site characterization
Authors Ruoyu Chen, Khiem T. Tran and Yao WangABSTRACTFull‐waveform inversion has proved to be an effective and robust approach for near‐surface site characterization. Past full‐waveform inversion studies have mostly focused on Rayleigh wave, with only a few studies focussing on SH‐ and Love‐waves. Compared with Rayleigh waves, the main advantages of using SH‐ and Love‐waves are that they are more sensitive to mass density of materials and require much less computing time for simulation. In this study, we present an efficient SH‐ and Love‐wave full‐waveform inversion method to extract both S‐wave velocity and mass density of soil and rock. The method is based on the solution of 2D elastic SH‐wave equations and the adjoint‐state gradient approach with an implementation of Tikhonov regularization. We use synthetic and field experiments to test the capability of the method. The synthetic experiment indicates that the presented method can accurately characterize a challenging soil profiel represented by the presence of a velocity reversal along with variable layers of high and low S‐wave velocity and density. Variable layer interfaces, true S‐wave velocity and density values are well recovered. The field experiment successfully characterizes the subsurface structure to a depth of 18 m, that includes a soil layer underlain by limestone bedrock. Two invasive standard penetration tests were conducted to verify the inverted seismic results. The trend in depth of the standard penetration test N‐values generally agrees with the inverted results, including identification of a soft soil layer.
-
-
-
Hidden layer imaging using joint inversion of P‐wave travel‐time and electrical resistivity data
Authors Mostafa Yari, Majid Nabi‐Bidhendi, Reza Ghanati and Zaher‐Hossein ShomaliABSTRACTThe combination of geophysical surface‐based imaging techniques, including seismic and electrical resistivity tomography (ERT), is now common practice to obtain a more accurate characterization of subsurface structures. Due to model non‐uniqueness and geological heterogeneity, conventional travel‐time tomography cannot solely reveal hidden layers (i.e., low‐velocity zones embedded between layers of higher velocities) in the subsurface. Hence, we present a joint inversion algorithm based on a normalized cross‐gradients function to detect hidden low‐velocity layers. The structural similarity between P‐wave velocity (Vp) and resistivity fields is enhanced by incorporating the normalized cross‐gradients constraint in the joint inversion algorithm. Improved structural similarity can mitigate the problem of the recovery of a hidden layer. We also take advantage of a priori information derived from borehole geological data to reduce the continuous range of possible solutions (i.e., exact‐data non‐uniqueness). In both joint and separate inversions, an auxiliary damping factor is used to ensure convergence, and also the smoothness constraints are applied to deal with instability stemming from error in the data. To verify the performance of the joint inversion procedure, the algorithm is tested on synthetic and real data examples with emphasis on hidden low‐velocity layer detection. Numerical experiments demonstrate that the joint inversion strategy can produce more reliable and better velocity models of the subsurface structures as compared with those obtained through individual inversions. We conclude that this simultaneous joint inversion of Vp and ERT integrates the best of both schemes and makes it possible to improve resolution, and, hence, reduces uncertainties in hidden low‐velocity layer problems.
-
-
-
Interpretation on water seepage and degree of weathering in a landslide based on pre‐ and post‐monsoon electrical resistivity tomography
Authors Philips Omowumi Falae, Rajesh Kumar Dash, D.P. Kanungo and P.K.S. ChauhanABSTRACTThe influence of rainfall on the Pakhi landslide has been examined using a pre‐ and post‐monsoon timelapse electrical resistivity imaging technique. The results obtained have been used to characterize the subsurface based on the depth and degree of water seepage and degree of weathering. The combined interpretation of pre‐ and post‐monsoon electrical resistivity tomography is not very common in landslide characterization. This study focuses on post‐monsoon electrical resistivity tomography investigation and its interpretation in conjunction with earlier research on pre‐monsoon investigations. The advancing water seepage across depth was reflected in the sharp decrease in the post‐monsoon resistivity value of the top (near‐surface) colluvial layer, mostly at the crown and toe portions of the investigated slope, as compared to that of weathered dolomites and fresh dolomites. Hence, the water‐saturated colluvium deposits could easily be delineated using a timelapse pre‐ and post‐monsoon electrical resistivity tomography technique. On the basis of degree of saturation, the materials were classified into four zones, namely: a highly saturated zone with resistivity values <600 Ωm; a fairly saturated zone with resistivity values 600–2500 Ωm; a low saturated zone with resistivity values 2500–4500 Ωm; and an unsaturated zone with resistivity values >4500 Ωm. Further, a twofold classification of dolomites based on the degree of weathering, namely highly weathered dolomites with resistivity values 1000–2500 Ωm and low weathered dolomites with resistivity values 2500–4500 Ωm, is presented. This type of information will add value to the design of landslide‐monitoring programmes and also for landslide stability analysis. Therefore, the method can be highly recommended as a cost‐ and time‐effective as well as an efficient way for characterization of potential landslide slopes.
-
-
-
Coupling hydrogeophysics with hydrodynamic modelling to infer subsurface hydraulic architecture of an alluvial floodplain
Authors J. Michael Martin, Mark E. Everett and Peter S.K. KnappettABSTRACTThis 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.
-
-
-
Mapping hazardous cavities over collapsed coal mines: Case study experiences using the microgravity method
Authors Pavol Zahorec, Roman Pašteka, Juraj Papčo, René Putiška, Andrej Mojzeš, David Kušnirák and Marian PlakingerABSTRACTExamples of the application of microgravity mehtod for the detection of potentially hazardous (empty) underground cavities caused by the collapse of coal mines are presented. Within these areas some alteration by previous remediation activity had occurred. This was not documented earlier and, therefore, such alteration was often unknown prior to the current investigations. We show a successful application of microgravity, leading to the detection of an empty cavity, in close proximity to where concrete injection was earlier performed. This was subsequently verified thorugh measurements in a borehole. In contrast, two other examples delivered results where the microgravity data, along with signals from other geophysical methods, led to the interpretation that subsidence resulting in the formation of sinkholes had been in‐filled, and in these situations the detection of more recent cavities was considered to be unreliable. These results point to the conclusion that microgravity interpretation of cavities is an effective approach, but the success of this approach may be compromised at sites where remediation activity has already occurred; in such situations the approach should be supplemented by other geophysical methods.
-
-
-
Gravity survey and modelling of the Nemocón salt mine, Colombia
ABSTRACTA gravity survey was conducted at a salt mine in Nemocón, Cundinamarca, Colombia, located in the axial part of the Eastern Cordillera of the northern Andes. This part of the Eastern Cordillera is characterized by successions of synclines and anticlines sometimes containing salt bodies at their core, and the mine is located at the top of one of these anticlines called the Nemocón anticline. As is typical for salt and other mines in Colombia, very little data on the internal structure and geology are publicly available. The purpose of our study is to show how gravity data and modelling can be used to infer near‐surface properties above the mine in the absence of other sources of information. Relative gravity measurements were made over an irregular grid of 107 stations and tied to the Instituto Geográfico Augustin Codazi national absolute gravity network. The total and residual Bouguer anomalies as well as the analytic signal were calculated over the area of the salt body. Using grid data, we first determine the main inversion parameters (e.g., depth weighting) and model resolution using a checkerboard model with the addition of random noise. Then, applying 3D inversion modelling to the residual Bouguer anomaly, we find that the positive and negative anomalies correspond to few zones of positive density contrasts and two main zones of negative density contrasts. Some of the positive anomalies are located above the fault trace putting in contact the Arenisca Dura formation with the Conejo formation containing the salt body. The negative anomalies are not well correlated with the underground position of the salt mine, but some influence of the low‐density ‘rute’ material, consisting of black clays with shaly claystone parts covering the salt body might be important. This study showcases how 3D inverse modelling can provide important information on the near‐surface structure of a salt mine.
-
-
-
Airborne geophysical data levelling based on variational mode decomposition
Authors Qiong Zhang, Fei Yan and Yunqing LiuABSTRACTA new technique is proposed to level airborne geophysical data based on the assumption that the level errors along the flight‐line direction have a distinguishable centre frequency. In the levelling method, airborne geophysical data from the entire survey area are corrected automatically after sorting the data into a group according to the measurement order. Variational mode decomposition is applied to the profile data to adaptively extract the level‐error component with distinct spectral bands. In airborne geophysical surveys, data in the anomalous region show visible peaks with strong gradients along the flight‐line direction, which would impact the decomposition. To minimize their effect on variational mode decomposition, the higher amplitude anomalies are grouped and discarded by K‐means clustering to obtain the real data level. The levelling method extracts the level errors of the entire survey area simultaneously, thus avoiding the regional error caused by strong noise, missing data or error transfer in the common levelling process. Moreover, the approach is automatic and applicable to both irregular and regular line patterns without the participation of staff members or tie‐line control. The levelling method may require much time when decomposition is performed on the extended survey area. We have applied the method to the airborne electromagnetic and magnetic data acquired by Geotech Limited to confirm its validity and have compared the obtained results with those based on the flight‐line correlation levelling algorithm.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)