19th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems

Migrating, contaminated groundwater plumes continue to be a problem around the world. Over the last decade geophysical investigations have been performed to determine the depth to bedrock and fault structures at the Tooele Army Depot (TEAD) in support of development of a groundwater model. Borehole, seismic, time-domain electromagnetic, gravity, and audio-magnetotelluric surveys have been performed. As the investigations progressed, inconsistencies became apparent between the interpretations. Portions of some of the surveys became essentially useless to the Department of Defense client because they couldn’t be “trusted”. The geophysical results, processing, and interpretation for the different systems have been revisited and the results integrated into one interpretation. In the end, site survey coverage, relative to the complex local geology was determined to be the main issue and not geophysical data acquisition and interpretation. The complicated and rapidly varying geology at the site prevents full continuity of the geophysical interpretation of faulted blocks between the different lines and stations. A case is made for “wide-area” airborne electromagnetic and magnetic surveys of the Depot that would result in a more comprehensive data set and interpretation of the subsurface geology and groundwater flow in the TEAD area. This study reiterates that, in complicated areas, no single technique can stand on its own nor be interpreted in isolation. A suite of geophysical techniques must be applied and integrated into one interpretation.

Near-surface riverine, land, and borehole geophysical surveys have been used to refine the hydrostratigraphic model of a glaciofluvial river valley aquifer at Fredericton, New Brunswick, Canada. Waterborne electromagnetic apparent conductivity profiling has revealed where the aquifer penetrates through an overlying clay/silt aquitard and is in direct hydraulic communication with the Saint John River. High-resolution acoustic sub-bottom profiles have imaged the clay/silt aquitard’s structure and stratigraphy, including its termination against the esker-like sand and gravel ridge that makes up the main part of the aquifer. A land-based CMP seismic reflection survey has penetrated through the aquifer system to confirm major variations in the thickness of the underlying impermeable glacial till layer that overlies fractured bedrock at 50 – 60 m depth. Finally, integration of geological boreholes logs with the geophysical information has yielded a contour map showing the structure and paleogeography of the aquifer sand body and major variations in the thickness of its clay/silt cap. This map shows that the aquifer ridge exhibits both topographic and lateral irregularities and crosses beneath the Saint John River near two well fields on opposite sides of the downtown area. The hydrostatigraphic model has been used on a local scale to help design and constrain hydrogeological and hydrogeochemical studies of aquifer recharge and riverbank filtration. Hydraulic head data and temperature time series, collected at discrete points beneath the riverbed, have been used to confirm recharge and determine spatial and temporal patterns in infiltration velocities. The geophysical results are also relevant, on a larger scale to aquifer protection and future well field development.

A land streamer together with MASW-technique has been used for surface wave seismics in a number of different projects, ranging between environmental and soil mechanical purposes. A few cases will be presented. In the first case a new regional road was built with 2 km passing over an area of peat. In order to get the peat stiff enough it was stabilized with lime and merit to a depth of approximately 3 m. In order to avoid heaving this stabilization process demands a layer of gravel or fragmented rock to be placed on top of the surface immediately after the stabilization. Among the drawbacks with this technique is the problem of testing the success of the stabilization – both in testing/measuring the reached stiffness or strength, and finding out the rate of homogeneity that has been reached. First, traditional geotechnical/soil mechanical sounding methods are time consuming and thereby costly since they need casing to get through the top layer. Secondly, those methods as usual can be looked upon as needle sticks and hence don’t give a representative picture of how much of the volume that actually has been treated by the stabilization. A test of land streamer and MASW-technique was carried out aiming to fill the gap, and the results will be presented. The second case treats an area previously used for storage of oil and other chemicals in large containers of different kinds, with sometimes environmentally doubtful handling, is to be converted into an area for storing containers (for ships). The containers are carried from the ships to the container terminal with very heavy trucks requiring very stiff road constructions. The Swedish standard road design procedures do not include such high loads, and hence a more modern analytical road design methodology was proposed. In order to perform such a concept among the parameters needed was the shear modulus of the soil. Soil sampling and environmental tests showed a highly contaminated soil. Those two facts made the non-penetrating land streamer useful. The methodology and the results will be presented.
In the final case presented leakage of DNAPL (TCE) to the groundwater from an old underground container was detected in a resident area, (next to a kindergarten). A crucial point in the
following project was to get a detailed picture of the topography of the rock surface, in order to get as good data as possible for he groundwater modelling to be carried out. In the area of paved roads, houses with gardens, people living and working, a land streamer and MASW-technique and a sledgehammer as a source was used. The project and the results will be presented.

A shallow seismic survey was conducted on a flat asphalt covered storage area. It was designed to use the Multi Channel Acquisition of Surface Waves (MASW) method; however, the data also displayed a clear pattern of a refraction response. The refraction pattern was unexpected, since soil wave velocities were much slower than those of the asphalt surface. The data were subsequently analyzed using both MASW and refraction techniques. The survey procedures were typical for shallow seismic exploration. The equipment consisted of a Geometrics Geode Seismograph and a Geostuff land-streamer with 24 4.5Hz Mark vertical geophones. The geophones were spaced at 2.15 feet, and the shot point was offset 15 feet from the end of the line. The spread and shot point were moved forward in 5-foot increments. The MASW model shows S-wave velocities of 500 to 1500 feet per second that varied with depth. The seismic refraction analysis shows Pwave velocities for a direct wave of about 700 feet per second, and refracted waves of about 1500 to 2500 feet per second. Both models were examined to depth of 30 feet.
The refraction results were unexpected, but provide a beneficial addition to the exploration of the site, and may be applicable in similar environments.

The feasibility of a novel pseudo-3D technique for seismic profiling in shallow marine environments has been investigated by application to sub-bottom imaging of a transgressive beach in the
Bay of Fundy at Waterside, New Brunswick. The survey site is characterized by a 10 m tidal range, occasionally choppy waters and water depths of 2 to 30 m overlying highly variable sea bottom sediments. Two IKB SeistecTM sub-bottom profilers were towed side by side 12 m apart, fired alternately, and positioned by dual frequency DGPS capable of giving centimetre accuracy in three dimensions. The intention was to achieve sufficient data density and positioning precision to allow heave and tidal effects to be removed and geological cross-line dips to be resolved, thereby producing high resolution subsurface maps without the cost of acquiring a full 3D dataset. Results from the trials indicate that near perfect line levelling is possible based on the +/- 5 cm misties observed between orthogonal seismic lines. Heave can be tracked using GPS heights but heave removal by conventional water bottom smoothing proved more robust for most of our data. The dataset exhibits four seismically and sedimentologically distinct, post-glacial deposits with a vertical resolution on the order of 20 cm and depths of penetration of up to 12 m sub-bottom. These units include mixed sand and gravel beach deposits, salt marsh, laminated offshore mud sediments, and steeply dipping reflectors that likely represent glacio-marine sediments. Additionally, gravel bars that extend 6 m above the surrounding beach and paleo-tidal channels were imaged as components of the four main deposits.

This paper will discuss a number of airborne electromagnetic salt-mapping projects undertaken by the Australian Government in the last five years. These surveys were flown primarily to provide information to inform land management decisions in areas of high value agricultural land. These techniques are also being applied in the management of water quality and to inform decision making in high conservation value wetlands. In 2001 an airborne electromagnetic (AEM) survey was flown across an area of 870,000 hectares in the Lower Balonne river catchment of eastern Australia. This area is a major centre of cotton production, and the survey was flown as part of a project to identify where salt was stored in the landscape and to test how land management options influenced potential salt mobilization. Also in 2001 a combined AEM and airborne magnetics survey was flown over approximately 120,000 hectares in an area of the Honeysuckle Creek catchment of southern Australia. The airborne magnetics survey mapped magnetic gravels infilling a series of major subterranean paleochannels, and showed that these channels intersected buried landscape salt stores of approximately 16 million tonnes. These paleochannels were identified by the study as the primary mechanism of salt export from the Honeysuckle Creek catchment. In 2002 an AEM survey was flown over the Jamestown area of South Australia, a major agricultural district. The AEM data was acquired to provide information for the development of salinity management plans for the district, which are now being used to protect high value cropping land. The study identified bottlenecks in groundwater flow caused by the subsurface geology, and showed that these groundwater bottlenecks are areas of rapid groundwater rise during high rainfallyears, causing salinity and waterlogging at the surface. This work has contributed to a significant change in the understanding of salt in the Australian landscape. The traditional view that salt is relatively widespread has been confirmed, but it is now understood that much of this salt is stable and locked up in the landscape, and therefore unlikely to become a salinity problem.

The Land Streamer developed by the author uses non-stretch woven belt as a towing member but also as a component to mount geophone units in which geophone elements are incorporated. Because the belt is made of textile fabrics, the guided waves through the belt are negligibly small under the nontensioned condition. A metallic baseplate bolted under the belt ensures the coupling of the geophone unit with the ground. This firm fixation of geophone elements in the unit and plane contact of the unit to the surface with the aid of the belt weight brings high quality data comparable to those acquired by planted geophones. The surface condition of the ground is another important factor which strongly affects the data quality in high-resolution seismic survey. Comparative tests revealed the effect of pavement as a high-pass filter, and proved the advantages of the Land Streamer for the high-quality data acquisition at paved areas. One disadvantage of the tool is its prefixed spacing of geophone units or the baseplate. We therefore manufactured various types of belts and sensor units and combined them to meet with the targeting depths and structures of a survey. The tools have been successfully utilized to high-resolution seismic reflection surveys especially in urban areas, where the surface is mostly paved and cultural noises had perturbed the collection of high quality geophysical data.

With increasing urbanization, the need for detecting unknown hazards, like hidden cavities or gas venues in populated areas, is of significant importance. The acoustic geophysical methods present a great potential to map such underground anomalies. At the Illinois State Geological Survey (ISGS), we have developed SH-wave and P-wave landstreamers to map large areas. More than one-hundred kilometers of data acquired in 2004-2005 have shown the extreme diversification of the shallow geology in the Midwest (< 100 m). Features such as buried channels, shallow thrusts and high-angle faults, and karst commonly are detected using landstreamers. The detection of gas venue from natural gas storage facilities using seismic reflection is the object of intense research at the ISGS. The P-wave is well known for its strong response to the presence of gas within pore spaces characterized by a high reflectivity and absorption of the acoustic energy. Our new results detect the presence of gas in the upper 100 meters of the Paleozoic bedrock. Whereas gas is mainly mapped using P-wave reflection, S-wave sections show the lithological layering and the location of bedrock fractures. The integrated P-wave and SH-wave reflection analysis of the sections demonstrates that the gas migrates in the most fractured areas.

For the most accurate results, a 2D receiver array such as a cross or circular type should be used in a passive surface wave survey. It is often not possible to secure such a spacious area, however, especially if the survey has to take place in an urban area. A passive version of the multichannel analysis of surface waves (MASW) method is described that can be implemented with the conventional linear receiver array deployed alongside a road. Offline, instead of inline, nature of source points on the road is accounted for by a dispersion analysis scheme that tries to resolve azimuths of the responsible points by scanning through all possible incoming angles of 180 degrees. On the other hand, it is possible to account for cylindrical, instead of planar, nature of surface wave propagation that often occurs due to the proximity of source points by considering the distance between a receiver and a possible source point. Performance of the processing schemes is compared to that of the scheme that accounts for inline propagation only. Comparisons made with field data sets showed that the latter scheme can result in overestimation of phase velocities up to 30 percent, whereas the overestimation can be reduced to less than 10 percent if these natures are accounted for according to the proposed schemes.