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- Volume 9, Issue 5, 2011
Near Surface Geophysics - Volume 9, Issue 5, 2011
Volume 9, Issue 5, 2011
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Influences on the resolution of GPR velocity analyses and a Monte Carlo simulation for establishing velocity precision
Authors Adam D. Booth, Roger A. Clark and Tavi MurrayABSTRACTThe precision of ground‐penetrating radar velocity models is seldom reported, despite their common use for quantifying subsurface properties. We explore influences on the resolution of ground‐penetrating radar velocity analysis and demonstrate a Monte Carlo method for obtaining the implied precision in velocity estimates. A series of synthetic common‐midpoint gathers, which assume Ricker wavelets of 50 MHz, 100 MHz and 200 MHz frequencies as source pulses, simulate hyperbolic reflections from the bases of three horizontal layers, each with interval velocity and thickness of 0.1 m/ns and 2 m, respectively. With these, we use coherence analysis to show that the principal influence on stacking velocity resolution is the traveltime moveout, normalized by the wavelet period, exhibited across some offset range. Where moveout exceeds the period by factors of, e.g., 3 and 6, is resolved to [–6.5, +8.2]% to [–3.6, +4.0]%, respectively, at the 50% coherence threshold. The temporal duration of coherence responses is expressed as a one‐quarter wavelet period either side of the stacking traveltime. Coherence resolutions are used in Monte Carlo simulations to derive the implied precision in the interval stacking velocity and its derivative properties. These analyses are repeated for field common‐midpoint data, acquired using 50 MHz antennas, on Quaternary glacio‐fluvial media overlying a Cambrian basement (<15 m deep). For three reflections identified in these data, the velocity and temporal resolution of coherence responses vary in a quantitatively similar way to events in the synthetic data. For the corresponding layers, Monte Carlo simulations yield precision estimates for , layer thickness and fractional water content. The procedure predicts a reasonable model of water content decreasing with depth, in accordance with increasing pore compaction and provides a robust error analysis; the uncertainty in estimates of the fractional water content of two (assumed) water‐saturated layers is ±2.6% and ±2.1% (shallower and deeper, respectively). Monte Carlo simulations are recommended as an efficient means of establishing the precision in quantitative estimates of subsurface properties derived from ground‐penetrating radar velocities, particularly where ground‐truth data are unavailable.
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Integrated geophysical profiles and H/V microtremor measurements for subsoil characterization†
Authors Beatriz Benjumea, Albert Macau, Anna Gabàs, Fabian Bellmunt, Sara Figueras and Jordi CirésABSTRACTMapping bedrock structure beneath overburden is crucial for understanding geological and hydrogeological processes. Acquiring this information is generally done using well drilling or geophysical surveys; but these studies are expensive and require large periods of acquisition and processing time. In addition, geophysical data acquisition can be logistically challenging in urban zones with limited available areas for instrumentation deployment.
Under favourable conditions (1D structure and high acoustic impedance contrast) the H/V microtremor technique can provide estimates of bedrock depth. This technique is used to obtain the soil resonance frequency in seismic microzonation studies. It is based on the computation of the horizontal to vertical spectral ratio of microtremor recordings acquired at a single station. The soil resonance frequency is related to soil shear‐wave velocity and thickness.
Here we investigate the capability of combining microtremor and traditional exploration geophysical techniques (electrical resistivity and seismic tomography) to obtain an empirical relationship relating soil resonance frequency and overburden thickness. Subsequently we propose to extend microtremor measurements to adjacent areas that have not been covered by geophysical surveys.
This methodology has been applied at a test site located in a granitic environment where alluvial/colluvial sediments cover the granite weathering profile. This area is characterized by urban development and sectors having rugged topography. A priori, this area has suitable conditions to apply the H/V microtremor technique. Overburden thickness has been estimated to range between 20–50 m. The proposed methodology has been validated at the test site, encouraging us to apply the H/V method as an exploration tool in similar geological environments.
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Deterministic deconvolution for GPR data in the t‐f domain †
Authors Nikos Economou and Antonis VafidisABSTRACTDeconvolution methods encounter difficulties in increasing the temporal resolution of GPR data due mainly to non‐stationarity of the records. GPR wavelets are typically mixed phase, which is additionally a major failure of standard deconvolution methods. Here, we propose a deterministic deconvolution method for GPR data, implemented in the t‐f domain, which utilizes narrow time windows and sets spectral balancing as a precondition. A reference wavelet is estimated experimentally for the calculation of a time varying deconvolution operator. Its phase variation is extracted from the spectrally balanced deconvolved GPR trace. The algorithm, tested on synthetic and real data, produces very promising results. In particular the deconvolved GPR section acquired over sands exhibits better temporal resolution and reveals reflected waves travelling through high loss media.
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Focusing frequency and significance of multi‐frequency very low‐frequency electromagnetic measurement in delineating near‐surface conducting structures
Authors S.P. Sharma and V.C. BaranwalABSTRACTThe very low‐frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near‐surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement.
A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi‐frequency VLF data yields better results. Inversion of multi‐frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi‐frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.
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2D‐3D resistivity and microgravity measurements for the detection of an ancient tunnel in the Lavrion area, Greece
Authors C. Orfanos and G. ApostolopoulosABSTRACTAt the archaeological site of Bertseko, Lavrion, Greece, the entrance of an ancient tunnel was found during archaeological excavations. Approximately 15 m from the entrance the tunnel remains inaccessible – blocked with filled material. The main aim of the geophysical survey was to verify if the tunnel is unblocked after that point and to delineate its direction. Another objective was to understand the purpose of its construction. As a first detection approach, eight 2D resistivity profiles were carried out with a pole‐pole array in a non rectangular grid, in order to image a wider area, a greater depth and to avoid several surface obstacles. Then the most promising area was outlined and selected for analysis with 3D resistivity tomography using parallel survey lines with a pole‐dipole array. Moreover, the microgravity method was used, for better resolution and verification of resistivity results. This study has shown that the influence of 3D effects in resistivity arrays, the choice of the 3D or 2D (pseudo‐3D) inversion approach, the position and the direction of the target in respect to the 2D profiles or 3D survey grid, are crucial factors that significantly affect the accuracy of the resistivity method in tunnel detection. Moreover, the design of the microgravity measurements, based on results of other geophysical methods, permits a flexible survey with a moderate acquisition time. If the geological environment is not complex, inversion of microgravity data is feasible and can be very useful, as it offers depth information about the target and can be directly comparative with resistivity models. The reconnaissance 2D resistivity survey was an important step for the optimized application of 3D resistivity and microgravity methods. The integrated results provided answers to archaeologist’s questions, delineating the direction of the tunnel with minimum ambiguity and showing that the ancient tunnel was not constructed to connect the two ancient tanks but is part of an ancient underground mine.
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Statistical stacking and adaptive notch filter to remove high‐level electromagnetic noise from MRS measurements
Authors Chuandong Jiang, Jun Lin, Qingming Duan, Shuqin Sun and Baofeng TianABSTRACTTraditionally, one of the major limitations for magnetic resonance sounding (MRS) measurement is that the weak signal generated by subsurface water molecules is prone to be disturbed by high‐level electromagnetic noise. In China, the power grid coverage is 94.6% and spiky noise and powerline harmonic noise are always present when utilizing MRS measurement in suburban areas or towns. In order to improve the performance of the MRS method, two new techniques, statistical stacking and adaptive notch filter, are introduced to remove spiky noise and power‐line harmonic noise. Firstly, four stacking procedures are analysed to suppress the natural noise and spiky noise. It could be found that statistical stacking can be utilized in the areas with serious spiky noise and can improve the signal‐to‐noise ratio by a factor of 4 to 7. Moreover, the stacking number is less than other stacking procedures and the measurement time may decrease by nearly 50% in some suburban areas or towns. Secondly, there are a variety of filtering procedures available to suppress power‐line harmonic noise, which are all based on analogue or digital notch filtering. But nearly all of them may cause distortion. An adaptive notch filter is applied here to remove power‐line harmonic noise because harmonic frequencies are away from and (or) close to the Larmor frequency, even when the frequency offset between them is zero. From simulation results, it could be noted that the signal can be recovered after adaptive notch filtering because it is not irretrievably distorted but proportionally attenuated. Thus, the amplitude attenuation can accurately be compensated. The effectiveness of the two techniques applied to MRS measurements is demonstrated by field testing with the prototype of the MRS system developed by Jilin University, China. The results show that the statistical stacking and adaptive notch filter are effective methods to remove high‐level electromagnetic noise from MRS measurements.
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Informed experimental design for electrical resistivity imaging
Authors Vanessa Nenna, Adam Pidlisecky and Rosemary KnightABSTRACTElectrical resistivity imaging has been successfully used to monitor near‐surface hydrologic processes but use of standard measurement arrays may not provide the greatest data sensitivity to the imaged region. We present a method of experimental design based on the concept of informed imaging for creating an electrical resistivity imaging experiment to monitor flow beneath a recharge pond. Informed imaging is the integration of all available data about a site into the acquisition, inversion and interpretation of electrical resistivity data. Informed experimental design uses all available information to develop an a priori model of the subsurface conductivity structure that guides the selection of measurement arrays for an electrical resistivity imaging experiment given spatial and temporal constraints on the acquisition. Selection of arrays focuses on maximizing the amount of unique information acquired with each source pair. We apply the method to the selection of arrays for imaging the top 5 m of the subsurface beneath a recharge pond in Northern California, which is part of an aquifer storage and recovery project. Decreasing infiltration rates over time reduce the effectiveness of the recharge pond. We seek to monitor infiltration processes at the contact between a fines‐rich sand layer and coarser sand layer in an effort to understand the hydrologic controls on infiltration. The performance of the arrays selected using informed experimental design relative to two standard arrays (Wenner and dipole‐dipole) is validated on two synthetic subsurface conductivity models, which are representative of conductivity structures that may arise during an infiltration event. Performance is evaluated in terms of a singular value decomposition of the sensitivity matrix produced by the three types of arrays, as well as a measure of the region of investigation. Results demonstrate that arrays selected using informed experimental design provide independent information about the imaged region and are robust in the presence of noise, improving the ability to image changes in a conductivity structure that result from infiltration processes.
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Geophysical and intrusive site investigations to detect an abandoned coal‐mine access shaft, Apedale, Staffordshire, UK
Authors S.G. Banham and J.K. PringleABSTRACTDerelict coal mine workings at Apedale in Staffordshire, United Kingdom were the focus for a multi‐disciplinary geophysical and intrusive site investigation. Objectives were to: 1) locate the surface entrance to a coal mine access shaft, 2) determine the inclined shaft’s changing depth below present ground level, 3) determine if it was open, partly or fully filled, 4) locate it beneath a known shaft if (1) was unsuccessful and finally 5) compare geophysical mineshaft detection techniques in difficult ground conditions.
After initial site reconnaissance, desktop study and modelling, field work collected surface micro‐gravity and electrical resistivity imaging (ERI) 2D profiles to locate the shaft and entrance area. The made‐ground nature of the site made identification of clear geophysical anomalies challenging. Subsequent intrusive investigations to locate the entrance were unsuccessful. A second phase of fieldwork down a known mineshaft imaged three geophysical anomalies beneath this shaft floor; after comparison with modelled data, subsequent intrusive investigations of the ERI anomaly successfully located the target shaft. Collapsed material was progressively cleared to the surface and a new shaft entrance stabilized.
Surface micro‐gravity 2D profiles surprisingly did not produce clear target anomalies, likely to be due to the target depth below ground level and the variety of above‐ground, relict mine structures present. Surface ERI 2D profiles were less affected by above‐ground structures but investigated anomalies were found to be heterogeneous ground materials. Comparisons of 2D micro‐gravity, ERI and ground‐penetrating radar profiles collected within a mineshaft showed ERI data were optimal. 2D micro‐gravity and ERI modelling were shown to aid geophysical interpretations.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)