25th Symposium on the Application of Geophpysics to Engineering & Environmental Problems

Levee systems in Kanto Region, central Japan, were severely damaged at many places caused by the long-lasting strong ground motion of the magnitude (Mw) 9.0 East Japan Earthquake, which occurred at 05:46 UTC on Friday, 2011 March 11, even located more than 400 km far from the epicenter. Since 2005, we have conducted integrated geophysical surveying for the safety assessment of levee systems at 39 actual levee sites in Japan. Among them, severe damage took place in two sites by the East Japan Earthquake just at the anomaly part delineated by the survey. The anomaly part in one site was characterized as low S-wave velocity and low resistivity both for levee body and substrata. After the earthquake, we conducted comparative surveying on the same levee but the damaged part of which had been soon repaired. As a result, the characteristic low S-wave velocity and low resistivity zone was again identified just at the damaged or repaired part where substantial top subsidence had occurred. This suggests a physical model that nonlinear loosening of underlying clay layers had caused the ground failure and resulted in the damage of levee systems. The other site, where large sliding had taken place on a levee slope during the earthquake attack, was featured by the existence of high resistivity anomaly in the levee body. The anomaly was also identified by the comparative surveying at the same part in the line where the slope sliding had occurred. A different type of levee failure mechanism was interpreted as resulting from high contrast of physical properties in levee body, based on our integrated geophysical surveys. Thus the corresponding survey results lead us to the usefulness of the integrated geophysical surveying for understanding levee failure mechanism and for the assessment of present conditions of levee systems attacked by the earthquake.

Waterborne continuous resistivity profiling (WCRP) permits rapid collection of direct current (DC) resistivity data over water by means of towing a multi-electrode cable streamer behind a moving vessel. During WCRP electrical current is injected using a pair of electrodes, while multiple potential measurements are made using other pairs of electrodes. This acquisition results in a near continuous profile of measured resistances along the survey path. These data are then processed/inverted to produce a 2.5D resistivity model, which can be used to interpret physical subsurface properties (i.e. lithology, and pore fluid conductivity).

Typical methodologies for determining well spacing in groundwater air sparge systems, such as water table response, dissolved oxygen measurements, and conservative gaseous tracers have been known to give semi-quantitative estimates of the radius of influence. Placement of observation wells that intersect established air channels can lead to data that are very different than wells that do not intersect, while two such wells could be located only a short distance apart. Non-invasive geophysical methods such as electrical resistivity (ER) can provide more detailed data over a larger area, alleviating the data density issues associated with typical test methods. Data are presented from two sites with existing air sparge systems that were not performing to expectations, ER surveys were conducted to evaluate the air-filled pore space induced by the operating air sparge wells. The air sparge systems were turned off in advance for a baseline survey with no air-filled pore space. Multiple repeat surveys were then collected at progressively increasing air flow rates to evaluate differences in radius of influence or volume of air-filled pore space. An increasing area of resistivity values during air sparging at an increased flow rate is attributed to pore-water displacement by injected air, resulting in decreased relative electrical conductivity. This conclusion is supported by direct measurments at various observation wells of dissolved oxygen concentration and changes in depth to water.

The ACME Superfund site is one of many Superfund sites in Northern Illinois. This 20 acre (8.1 ha) site was contaminated by various volatile organic compounds (VOC’s) and heavy metals during the 1960-1980s. These contaminants have seeped into the Wise Lake dolomite (Galena Group) and contaminated local ground water supplies. The field site is located immediately south and hydrologically downgradient of the ACME site, where up to 10 m of unconsolidated sediments overlie the fractured and karstic Wise Lake Formation in a small valley adjacent to residential wells.

Flood control structures (FRS) or dams in the desert southwest are commonly located at the margins of deep alluvial basins that are subject to groundwater pumping-induced subsidence. Safety assessment of these facilities includes characterizing the existing or potential future hazard to these structures due to subsidence. Differential subsidence can lead to changes in surface hydrology and flood storage capacity, and possible earth fissuring that could cause cracking in an FRS or its’ foundation, resulting in possible piping erosion failure of the FRS during a flood event. Resistivity is an excellent reconnaissance tool to assess compressible basin alluvium that is prone to subsidence. However, with typical water table depths of 30 to over 100 meters, and potential bedrock depths underlying alluvium ranging from 0 to over 300 meters, resistivity soundings require large arrays. The authors use array electrode a spacing up to 300 meters for this work.

Remote detection techniques of accidental oil spills under ice are needed to support oil spill response in the Arctic. The various techniques tried so far have not been fully satisfactory. We present preliminary considerations of Earth’s field NMR (EFNMR) from the ice surface for this purpose.

Seismic refraction surveys and tomographic methods are commonly used to characterize the near-surface. The advantage of traveltime tomography is its robustness because it uses only traveltimes. Its disadvantage is that it provides relatively low spatial resolution velocity models. The advantage of waveform tomography is its ability to provide high-resolution models. The disadvantage of waveform tomography is its strong dependence on an accurate starting model since it is a very nonlinear inverse problem. The spatial resolution of traveltimes data is related to the Fresnel zone, which can be very wide, whereas for waveform data it is related to the seismic wavelength, typically a smaller length scale by comparison. A new integrated strategy for deriving velocity models from nearsurface seismic refraction data using two complementary tomographic methods is presented.

Since 2009, the environment ministry of Quebec (Canada) funded a huge characterization program aiming to improve the knowledge of the groundwater resources throughout the southern part of Quebec. Our research group was mandated to characterize the region of Montérégie Est that covers an area of approximately 9000 km2 at South-East of Montreal. The surveyed area has approximately 577 000 inhabitants, of which 28% use groundwater as a source of supply.

Surface waves have been extensively utilized to detect the properties of subsurface materials for geological inspections in many non-destructive testing methods, such as the Spectral Analysis of Surface Waves (SASW) and Multi-channel Analysis of Surface Waves (MASW). In recent years, these stationary measurement methods were also applied to concrete and pavement structures. Most of these implementations are still using contact sensors, such as accelerometers and geophones. The test efficiency is also significantly limited by the conventional inversion algorithms, which were established on the iterative forward analysis with trial solutions. These drawbacks don’t meet the challenges of fast inspection for highway pavements.

A method is proposed to localize preferential fluid flow pathways in earth dams and embankments based on time-lapse self-potential measurements associated with salt tracer injection upstream. This salt tracer is carried along the ground water flow paths by advection and hydrodynamic dispersion. A network of non-polarizing electrodes located at the ground surface is connected to a highly sensitive voltmeter and used to record the resulting electrical field fluctuations occurring over time at the ground surface. The transport of the salt through the conductive porous materials changes the localized streaming potential coupling coefficient associated with the advective drag of the excess of charge of the pore water and is also responsible for a diffusion current associated with the salinity gradient. Therefore, monitoring of the electrical field at the ground surface can be used to localize a pulse of saline water over time, and to determine its velocity. Possibly in real time This method can be used to track highly localized flow pathways characterized by high permeability that could not be monitored with DC resistivity tomography, which is limited in this regard because of temporal resolution.