Journal of Environmental & Engineering Geophysics (JEEG)

Identification of Double-layered Water-filled Zones Using TEM: A Case Study in China
Guoqiang Xue, Dongyang Hou, and Weizhong Qiu

ABSTRACT: At present in China, the use of the transient electromagnetic method (TEM) is emerging as a leading geophysical technique for exploration of water-filled zones in coal mines. These zones are more conductive than the host coal and are easy targets to map. However, there is a growing interest for the investigations of double-layered or multi-layered mined-out zones. Therefore, it is necessary to study the feasibility of TEM's abilities to detect double-layered, water-filled voids. In this study, the basic hydrogeological conditions of a survey area, located in the northern China, are described. The corresponding geophysical models of the single- and double-layered water-filled zones are developed from borehole logging data. Then, forward calculations of different models are carried out with 1D numerical simulations. The modeling results show that it is feasible for TEM to identify these types of targets under certain conditions, including instrument sensitivity, low resistivity for the water-filled zones, and shallower depths. Moreover, the field survey for locating double-layered water-filled zones in coal mines in the Datong region of Shanxi Province is verified by well drilling.

Design of Analog Front-end Circuit for Audio-frequency Magnetotelluric Instrument
Yinglu Zhang, Zhenzhu Xi, Xingpeng Chen, Honglan Wei, Long Huang, and Wei Xiao

ABSTRACT: High-performance audio-frequency magnetotelluric (AMT) instrument is one means of obtaining high-quality electromagnetic (EM) data. To improve the ability of AMT system to obtain high-quality data, this paper presents a design for a high-performance analog front-end circuit for AMT instrument. It mainly consists of the input protection, preamplifier, passive high pass filter, power frequency notch filter, programmable amplifier, and active low pass filter. In addition, this paper proposes a design of low-noise, high-performance preamplifier, which improves the common-mode rejection ratio (CMRR) of analog front-end circuit and effectively enhances the signal-to-noise ratio (SNR) of the circuit. The front-end circuit utilized two-stage twin-T notch filter to effectively suppress the strong interference of fundamental component of power frequency. Also, it used signal relays to control circuit gain and selection of cutoff frequency of anti-aliasing filter, resulting in the improvement of the capability of the analog-to-digital Converter (ADC) to distinguish weak EM signal. The measured results of the electric field and magnetic field channel showed that: 1) The circuit works in frequency range of 1 Hz∼100 kHz; 2) The CMRR values of the preamplifier of electric field channel at low frequencies (1 Hz∼1 kHz) are 111 dB and 97 dB when the gains are 20 dB and 6 dB respectively; 3) The maximum attenuation fundamental power frequency can reach −39.46 dB and −39.04 dB respectively; 4) The total harmonic distortion rate at 1 kHz is 0.022% and 0.029% respectively; 5) The input noise levels of electric field channel are 12.67nV / @10Hz and 8.15V / @1kHz, while the input noise levels of magnetic field channel are 8.97nV / @10Hz and 6.16V / @1kHz; and 6) In conclusion, the analog front-end circuit is superior to meet the requirements of the AMT methods, and provides a useful reference for the development of AMT instrument.

Geophysical Monitoring of Blast-induced Liquefaction at the Mirabello (NE Italy) Test Site
Federico Passeri, Cesare Comina, Valeria Marangoni, Sebastiano Foti, and Sara Amoroso

ABSTRACT: A blast-induced liquefaction test was conducted in the surroundings of Mirabello (NE Italy), where extensive liquefaction phenomena were observed after the 2012 Emilia earthquake. This experiment is the first blast-induced liquefaction test carried out in Italy. Several geophysical investigations were performed at the site to define initial soil condition and to evaluate the variations of the geophysical parameters over time. Specifically compressional (VP) and shear (VS) wave velocities were measured using both invasive (down-hole) and non-invasive (surface wave) tests. Electric Resistivity Tomography (ERT) tests were also carried out. Tests results before and after the blast-induced liquefaction are here presented and discussed with respect to the observed liquefaction effects. The evolution of measured geophysical parameters suggests that the soil modifications due to blasting (i.e., changes in porosity and soil structure) can be imaged with the adopted approaches.

CO₂ Injection Monitoring Using Transient Electromagnetic in Ground-Borehole Configuration
Kai Li, Huaifeng Sun, Ming Cheng, and Jianlei Guo

ABSTRACT: Using geophysical methods, specifically transient electromagnetic (TEM), for CO₂ monitoring is an effective way to detect CO₂ diffusion. In this work, a multi-scale finite-difference time-domain (FDTD) algorithm was established to monitor CO₂ by defining new iterative relations and approximating boundary conditions, which achieves unification in the time and space domain. The response curve characteristics of different forms of CO₂ were acquired by changing the receiver's depth and position, CO2 resistivity, scale, and injection stage. Different models considering a planar, tilted, and large-scale CO₂ bodies, which were established to test the capacity of TEM monitoring for CO₂. The TEM response of injected CO₂ bodies had obvious characteristics and the response curve had distinguishable differences from background. This phenomenon could provide reference models for real TEM CO₂ monitoring.

Inverse Methods to Improve Accuracy of Water Content Estimates from Multi-offset GPR
Andrew D. Parsekian

ABSTRACT: Ground penetrating radar (GPR) is a powerful hydrogeophysical tool for estimating porosity and water content of geologic materials using radar wave velocities and appropriate petrophysical relations. In substrates with more than one layer of interest, surface-based multi-offset measurements require careful analysis to accurately retrieve physical properties for each layer. Frequently, Dix inversion is used to calculate interval velocities, however the assumptions and limitations of this approach are widely known. In particular for survey geometries and targets encountered with GPR, the assumptions inherent to Dix inversion are readily violated, and therefore inverse modeling is required to avoid velocity error. While the impact on velocity incurred by violating the assumptions of Dix inversion is well understood, the effects on water content estimates have not been widely reported and are therefore the subject of this work. In a subsurface representative of an unsaturated zone overlying an aquifer, error in excess of 50% in water content due to violating the assumptions of Dix inversion is demonstrated. Examples are shown using raytracing inversion to solve for subsurface water content structure that avoids the errors inherent to Dix inversion. These results are intended to underscore the importance of minimizing assumptions and using more correct physics when analyzing multi-offset GPR data, particularly due to the large potential errors that may be encountered if water content estimation is the main objective.

Study in the Qale-Alimoradkhan Area
Seyyed Reza Mashhadi and Hamidreza Ramazi

ABSTRACT: This paper investigates the capability of the resistivity and induced polarization (IP) methods to delineate skarn alteration haloes within the Qale-Alimoradkhan skarn copper deposit. This deposit is located in the Sanandaj-Sirjan geological and structural zone, Hamedan province, Iran. It is understood that fresh limestone and metamorphosed limy units including skarns represent high resistivity anomalies whereas granodiorite intrusions represent medium to low resistivity responses. Four profiles were selected to explore the sulfide-rich zones within the deposit using the Combined Resistivity Sounding and Profiling (CRSP) array. After the appropriate selection of inversion parameters, the inverted models were in good agreement with the known geological features. The resistivity response of the intrusive rocks were found to be alteration dependent. Furthermore, IP targets likely represent sulfide-rich zones. Our study suggests that if preexisting knowledge regarding the geological setting is available, then resistivity and IP can be helpful in the exploration of skarn mineralization.

Application of 3-D FKK Filtering in 3-D High-density Onshore Seismic Field Data
Jianping Liao, Songyuan Fu, Yungui Xu, Weibo Li, Jianxiong Chen, Huazhong Wang, Suping Peng, and Andrew Hursthouse

ABSTRACT: For linear noise such as seismic ground roll, 3-D frequency-wavenumber-wavenumber (3-D FKK) domain filtering suppression is better than 2-D frequency-wavenumber (F-K) domain filtering. In recent years, with the continuous development of computer processing speed and memory capacity, high-density data acquisition in seismic exploration has been widely applied in the hydrocarbon industry, opening up the application of 3-D FKK filtering methods. We applied the 3-D FKK filtering software to a 3-D high-density onshore seismic field dataset from a coal mine in western China. The case study demonstrates that the linearity of the noise in the field data is better represented by constructing the single shot records as a minimum dataset. Both theoretical synthetic models and the 3-D high-density onshore seismic field data numerical filtering experiments demonstrate that the feasibility of 3-D FKK filtering.

Exploring the Japan Cave in Taman Hutan Raya Djuanda, Bandung Using GPR
Widodo Widodo, Azizatun Azimmah, and Djoko Santoso

ABSTRACT: Investigating underground cavities is vital due to their potential for subsidence and total collapse. One of the proven geophysical methods for locating underground cavities at a shallow depth is ground penetrating radar (GPR). GPR uses contrasting dielectric permittivity, resistivity, and magnetic permeability to map the subsurface. The aim of this research is to prove that GPR can be applied to detect underground cavities in the Japan Cave of Taman Hutan Raya Djuanda, in Bandung, Indonesia. Forward modeling was performed first using three representative synthetic models before field data were acquired. The data acquisition was then conducted using a 100 MHz GPR shielded antenna with three lines of 80 m and one additional line 10 m long. The result showed a region of different reflection amplitude, which was proven to be the air-filled cavities.

Influence of GNSS Receivers on GPR Results
Dariusz Tanajewski, Dariusz Popielarczyk, and Adam Ciecko

ABSTRACT: Even though satellite positioning has been used in ground penetrating radar (GPR) measurements for years, there are no studies ruling out the influence of modern satellite positioning receivers on the operation of GPR antennas. In order to rule out mutual influence between devices, a field study was carried out to determine the possible influence of a Global Navigation Satellite Systems (GNSS) receiver on the results obtained from GPR. To this end, several equipment combinations based on two receivers were compared. This was followed by a numerical analysis of selected samples from the recorded data sets. The following were calculated: average values of signal amplitudes, their standard deviations and the signal-to-noise ratio, coefficient of variation, and signal stability parameters. We also suggested using a modified standard deviation based on the properties of the reference signal. Based on the results, we concluded that there were rather significant changes between the data sets for various equipment combinations, which may indicate that a GNSS receiver affects GPR data in some way. However, the influence was not significant enough to result in the qualitative misinterpretation of data.

Author Biographies

EEGS is grateful for the following companies and organizations which have chosen to advertise in the September, 2018 issue of the JEEG and support EEGS. 

   

Upcoming Issue - Volume 23 (4): 2018 IWAGPR2017: International
Workshop on Advanced Ground Penetrating Radar

Guest Editors: Nectaria Diamanti, Craig Warren, and Antonis Giannopoulos

This issue is scheduled for publication in December 2018, with a focus on papers from the International Workshop on Advanced Ground Penetrating Radar, organized by the School of Engineering at The University of Edinburgh. The conference was held in June 2017 with 84 oral and poster presentations. This special issues features 8 of those papers.