Sung-Ho Song

Resistivity Imaging Using Borehole Electrical Resistivity Tomography: A Case of Land Subsidence in Karst Area Due to the Excessive Groundwater Withdrawal

Electrical resistivity tomography surveys using boreholes were applied to reveal the cause of a catastrophic land subsidence accompanied by the excessive groundwater withdrawal in urban karst area and to map the connectivity of disseminated cavities over the study area. In order to understand the hydrogeological characteristics, resistivity using exsitu core samples, groundwater level for five boreholes, and hydraulic conductivity using slug test were measured. The hydraulic conductivity variation ranging from 0.8 to for five boreholes and a gentle slope of groundwater level indicated that there is no significant characteristics of hydraulic heterogeneity. Core samples of the lime-silicated rock were classified as three groups including cracked, weathered, and fresh and measured the resistivity values ranged from 103 to 161, 218 to 277, and 597 to 662 ohm-m, respectively. Drilling results that showed the cavity filled with clay materials and tomogram for this region indicated resistivity value lower than 50 ohm-m. From the inverted resistivity results for each section with five boreholes, cavity and fractured layer were distributed along the depth between 10 and 20 m overall area and cavities ranging from 4 to 6 m filled with clay materials.

Application of a streamer resistivity survey in a shallow brackish-water reservoir

To delineate the resistivity structure of sub-bottom sediments in a shallow brackish-water reservoir in the western coastal area of Korea, we carried out a streamer resistivity survey using a dipole–dipole array. First, through numerical testing, we confirmed that the resistivity method with a dipole–dipole array could be applied in a shallow marine environment, when the resistivity contrast between water and the underlying sediments ranges from a factor of 3 to 5. Also, inversion with a water layer explicitly included is more effective than the conventional inversion method in resolving power, which we confirmed by observing that the inversion results for synthetic datasets matched better when a water layer was included in the inversion procedure. After constructing a data acquisition system composed of a resistivity meter, GPS, and echo sounder, and developing data processing software, we conducted a streamer resistivity survey and inverted the data obtained to identify the hydrogeological sequences and sediment characteristics at the bottom of the shallow brackish-water reservoir. Drill logs identified three sediment layers, including silty sand, fine sand, and mixed sand. The resistivity distributions from inversion matched the resistivity ranges measured on materials obtained by sampling near the drilling points. We constructed a contour map of the top of the mixed-sand layer, using semivariogram analysis. Comparing these results with the drilling results, the depth to each layer, and the measured and estimated resistivity range of the materials, also corresponded to resistivity profile. From this study, we are assured that the streamer resistivity method would be a useful tool for surveying shallow brackish-water reservoirs.

SP Monitoring at a Sea Dike

The self-potential (SP) method is widely used in seepage evaluation hydrological studies to monitor the integrity of infrastructure such as dams, sea dikes, and other types of flood control devices because the electric signals that are measured are directly related to seepage rate. At leaking areas along sea dikes, large SP anomalies can be generated by the rising and falling of tides. Unfortunately, SP data are often contaminated with several types of noise, such as that from drifting electrodes, telluric dis- turbances, and external electrical noise. Furthermore, SP signals can have high levels of spatial vari- ability due to heterogeneity in lateral resistivity at the locations where the electrodes are installed. Because of these issues, it is very difficult to correlate the measured SP voltages with the streaming potentials associated with groundwater flows at particular points in time. To alleviate these problems, we developed a simple but effective interpretation method for SP monitoring data that involves sub- tracting consecutive SP voltages collected at different time points from a particular monitoring station. This subtracting procedure is able to effectively reduce spurious SP anomalies caused by electrode drift, change in resistivity, and other types of interference. Therefore, any changes observed in SP measurements over certain time frames were interpreted as resulting primarily from temporal changes in seepage flow. To demonstrate the performance of this method, we analysed SP monitoring data measured at a sea dike located on the southern coast of Korea. Our results confirmed that the SP interpretation method is able to explain changes in streaming potentials depending on the tide change over time and to detect the horizontal location of anomalous seepage zones along the sea dike.

Time-lapse Inversion of 3D Resistivity Monitoring Data

We developed a time-lapse inversion using new cross-model constraints based on change ratio and resolution of model parameters. The cross-model constraint based on change ratio imposes the same penalty on the model parameters with equal change ratio. This constraint can emphasize the model parameters with significant change regardless of their increase or decrease. The resolution cross-model constraint imposes a small penalty on the model parameters with poor resolution, but a large penalty on the model parameters with good resolution. Thus, the model parameter with poor resolution can be effectively identified in the inversion result if they are significantly changed with time. Through the numerical tests for 3D resistivity monitoring data sets, the performance of these two cross-model constraints was confirmed. Finally, for the safety estimation of a sea dyke, we applied the developed time-lapse inversion to the 3D resistivity monitoring data that were acquired at a sea dike located in western coastal area of Korea. The result of timelapse inversion suggested that there were no significant changes at the sea dike during the monitoring period.

A Field Study of Surfactant Enhanced In-Situ Remediation using Injection Wells and Recovery Trench at a Jet Oil Contaminated Site

This study reports a surfactant-enhanced in-situ remediation treatment at a test site which is located in a hilly terrain. The leakage oils from a storage tank situated on the top of the hill contaminated soils and groundwater in the lower elevation. Sixteen vertical injection wells (11 m deep) were installed at the top of the hill to introduce 0.1-0.5 vol.% of non-ionic Tween-80 surfactant. The contaminated area that required remediation treatment was about . Two cycles of injecting surfactant solution followed by water were repeated over approximately 7.5 months: first cycle with 0.5 month of surfactant injection followed by 3 months of water injection, and second cycle with 1 month of surfactant followed by 3 months of water injection. The seasonal fluctuation in groundwater table was also considered in the selection of periods for surfactant and water injection. The results showed that the initial Total Petroleum Hydrocarbon (TPH) concentration of 1,041 mg/kg (maximum 3,605 mg/kg) was reduced significantly down to 76.6 mg/kg in average. After 2nd surfactant injection process finished, average TPH concentration of soils was reduced to 7.5% compared to initial concentration. Also, average BTEX concentration of soils was reduced to 10.8%. This resultes show that the surfactant enhanced in-situ remediation processes can be applicable to LNAPL contaminated site in field scale.

Pilot Scale Feasibility Test of In-situ Soil Flushing by using `Tween 80` Solution at Low Concentration for the Xylene Contaminated Site

This study was performed to identify the optimal operating conditions and to evaluate the xylene removal efficiency, applying in-situ soil flushing with the low concentrated solution of `Tween 80` at the xylene contaminated site. The pilot scale test site (), was mainly composed of `sandy loam`, with the average hydraulic conductivity of . The average xylene concentration of the site was 42.1 mg , which was more than 2.5 times higher than Korea soil pollution warning limit (15 mg ). For the soil flushing, 7,800 L of 0.1~0.2% surfactant solution was injected into three injection wells at the average injection time of 9 hr for 10 days, followed by the additional only groundwater injection of 6,000 L. The same amount of the effluent solution was extracted from three extraction wells. From the analysis for xylene concentration of all effluent at 3 extraction wells, total 166 g of xylene was removed by in-situ surfactant flushing. Even though the residual xylene concentrations of 7 soil sampling locations in the test site were different due to the soil heterogeneity, from the comparison of xylene concentration at 7 locations before/after the feasibility test, 53.9% of the initial xylene in the site was removed from three extraction wells (mainly Ext-N and Ext-M well). The results showed that the in-situ soil flushing by using low concentrated `Tween 80` solution had a great potential to remediate the xylene contaminated site.

Suggestion for the Maintenance Program of the Sea Dike Using Geophysical Methods

The sea dike is the most important facility of reclamation projects, and plays an important role in securing freshwater in the reservoir. Systematic research on practical approaches and data analysis techniques are lacking even though some geophysical methods such as electrical resistivity and self-potential surveys are included within the inspection processes. Hence, geophysical methods were considered for improvement of precision safety diagnosis methods after problems in the maintenance system have been identified, such as safety checks and precision safety diagnoses. In addition, geophysical methods customized according to variations in ambient environmental limiting factors such as pore pressure changes by tidal fluctuation, compaction characteristics of the fill materials, and the surface condition of the embankment were suggested.

Identification of leachate from livestock mortality burial using electrical resistivity and small-loop EM survey: case history

Leachate from livestock mortality burial is harmful to the soil and groundwater environment and adequate assessment approaches are necessary to manage burial sites. Among the methods used to detect leachate, geophysical surveys, including electrical resistivity and electromagnetic (EM) techniques, are used in many engineering approaches to environmental problems, such as identifying contaminant plumes and evaluating hydrogeological conditions. Electrical resistivity, with a small-loop EM survey, was used in this study as a reconnaissance technique to identify the burial shape and distribution of leachate from livestock mortality burial in five small separate zones. We conducted a multi-frequency small-loop EM survey using lattice nets and acquired apparent conductivity values along several parallel and perpendicular lines over a burial site. We also compared geophysical results to the geochemical analysis of samples from both a leachate collection well and a downstream observation well within the study area. Depth slices of apparent conductivities at each frequency (obtained from the small-loop EM survey data) clearly identified the subsurface structure of the burial shape and the extent of leachate transport. Low-resistivity zones, identified from two-dimensional (2D) electrical resistivity imaging results, were matched to the five burial zones (within a depth of 5 m), as well as high electrical conductivity of the leachate obtained from leachate collection wells, and depth slices of the apparent conductivity distribution obtained from the small-loop EM survey. A three-dimensional (3D) inversion of resistivity data provided a detailed 3D structure of the overall burial site and leachate pathways. Moreover, these zones were widely spread over the burial site, indicating that leachate potentially extended through damaged regions of the composite liner to a depth of 10 m along the downstream groundwater flow. Both the small-loop EM method and the electrical resistivity method were considered suitable for identifying the shape of the livestock mortality burial and the extent of leachate. A small-loop EM survey and electrical resistivity survey were applied to a carcass disposal site to delineate the burial shape and extent of leachate arising from the burial. The small-loop EM survey was adequate for a reconnaissance survey of the burial zone and the resistivity survey was useful for quantitatively interpreting the pathway of leachate flows in the site.