NS41B-1136
3-D Velocity Imaging of the Shallow Subsurface Using Surface Seismic Reflection Data and Multi-offset VSP Data at a Site at Sandia National Laboratories
A 3-D surface seismic reflection study was conducted in the northeast portion of the Livermore, California facility of the Sandia National Laboratories. The surface seismic study covered an area of approximately 500 by 500 square meters, and consisted of 96 receivers, which recorded multiple events produced by an elastic wave generator at 120 shot locations. The reflection energy in the surface seismic data was low and continuous reflectors including the watertable surface could not be identified. To improve signal to noise ratio, a vertical seismic profiling study was conducted in a well adjacent to this site. The VSP method consisted of a 12-level 3- component receiver array deployed at 3.3 m intervals up to a maximum depth of 40 m down a well, which was grouted with a cement-bentonite mix up to a depth of 1 m. Multiple offset shot points were recorded at multiple azimuths around the well head, using an elastic wave generator source. First-break travel times were identified in the data and inverted to produce p-wave interval velocities. Existing borehole log information and other geologic information were combined with interval velocities from the 3-D surface seismic study and the VSP study to produce a 3-D model of the aquifer boundaries below the water table.
NS41B-1137
Compressional Wave Velocity and Attenuation in Carbonate Sediments of Kaneohe Bay, Oahu
Measurements of geoacoustic and physical properties were made of near-surface carbonate sediments at one coarse-grained site (Site 1) and one finer-grained site (Site 2) in Kaneohe Bay, Oahu. Velocity dispersion in the measured frequency range (20 - 100 kHz) was observed at both sites, with velocity increasing from 1691 to 1708 m/s at Site 1 and from 1579 to 1585 m/s at Site 2. At both sites, effective attenuation scaled linearly with frequency, increasing from 15 to 75 dB/m at Site 1 and from 22 to 62 dB/m at Site 2. Data were compared to predictions of two common sediment geoacoustic models, namely Biot-Stoll and the Grain Shearing model. In both models, two unknown parameters were varied to find best fits at each site to (1) both attenuation and velocity data, and (2) velocity data only. Both models gave similar fits. In the fits to attenuation and velocity data, both models predicted more velocity dispersion than was measured. In the fits to velocity only, both models predicted attenuation well below measured values. This can be partially attributed to scattering since the models predict intrinsic attenuation only and the abundant shell material at both sites suggests that scattering loss should be significant.
NS41B-1138
P and S Wave Measurements of Surface Geomaterials for the Geotechnical Map of Porto
During the past few years the Department of Geology, of the Faculty of Sciences of the University of Porto, has been actively engaged, along with the Local Town Hall, in Urban Geological mapping. This has culminated in two editions of the Porto Geotechnical Map. This map, composed by a number sheets with different parameters, has been put together by gathering information and data from many of the engineering projects conducted in Porto over more than two decades as well as by Geological mapping techniques applied to this urban environment. The data comprises of outcrops, lithological classification from boreholes, in situ tests, laboratory tests and was assembled together into a GIS database. Under the continuous tasks associated with the updating of this map we are currently undertaking measurements of P and S wave velocity on surface materials. The obtained values will be used to determine the dynamic elastic parameters and to correlate them with measurements made with the standard in situ geotechnical testing and, in some cases, with the latest direct measuring techniques made upon samples and other in situ geotechnical tests. The measured parameters can thus be used by the engineering community, for construction and project planning, as well as a starting point for the future Seismic Risk Map. The geophysical technique employed was, in a first stage, standard seismic refraction, using vertical and horizontal sensors, in very short arrays in order to obtain the elastic parameters of the surface materials (soil). We also conducted in all the tested sites a longer refraction profile in order to visualise the weathering structure although only in terms of compressional wave velocity interpreted using travel time tomography. With these initial results we aim to show how they correlate with some of the direct geotechnical measurements and the mapped Geology. We also aim to share the difficulties encountered in performing these tests in an urban setting. In order to overcome some of the difficulties in performing S wave measurements we are currently performing surface wave tests with both active and passive sources and some of these results will also be shown.
NS41B-1139
Characterization of Thin Beds by Spectral Decomposition of Ground Penetrating Radar Profiles
Ground Penetrating Radar profiles are extensively used for stratigraphic studies of depositional environments. Detection of internal stratification within layers and thin bed characterization in these environments are important aspect of stratigraphic interpretation. But GPR resolution is limited and identification of the presence of thin beds is difficult in the time domain. Thin bed detection from GPR data is better done in frequency domain. In order to better understand the frequency variations temporally/spatially as well as to relate these observed changes to bed geometry, time-frequency decomposition or spectral decomposition is required. Seismic resolution studies have shown that thin beds can be studied through frequency domain analysis by generating spectral decomposition maps. We have applied this technique to GPR profiles over depositional sequences of thin beds as well as to model simulations. Time-frequency representation methods such as the S-transform and the Adaptive Optimal Kernel representation have been used for obtaining spectral decomposition maps. Results indicate that this method can extract useful information from GPR data regarding depositional characteristics.
NS41B-1140
Mapping of heavy metal loadings in topsoils by means of rock magnetism in Mexico City.
In recent years studies of rock magnetism have become one of the most important methods in investigations of anthropogenic impact on topsoils from great urban regions. Mexico is one of the largest cities of the world, its extension is more than 2000 km2. Urban and industrial activities, like in many developed countries, are together, constituting a health problem. The determination of heavy metals by chemical methods is the most common chosen method used. The low cost and less time consuming of the magnetic methods had been droved to be one of the most useful. In this work we present the results of a preliminary campaign of sampling. The correlations of heavy metals contents determined by induced-coupled plasma mass spectrometry (ICP-MS) and magnetic parameters as magnetic susceptibility, NRM, IRM and ARM are presented. These correlations constitute the basis for an extensive magnetic sampling campaign to perform detailed maps of polluted areas.
NS41B-1141
Nitrate contamination in groundwater at farmlands in Nsawam, Ghana: The role of fractures from azimuthal resistivity surveys
Nitrate contamination of groundwater at farmlands in Nsawam, Ghana, has become a growing concern in recent times. Water samples were obtained from water groundwater wells in the study area and concentrations of nitrates, lead, arsenic, cadmium, copper, zinc and chromium were measured and analyzed. Three of the wells showed nitrate concentrations levels that reached 3-5 times the permissible limits for human consumption. The bed rock in the area is fractured and information about the orientations, apertures and lengths of fractures in the study area was obtained from geological mapping of few exposed outcrops. Azimuthal resistivity surveys (ARS) using the square array configuration were conducted in the vicinity of the wells with the aim of estimating the orientation and hydraulic properties of unexposed subsurface fractures which may serve as conduits for fluid flow and contaminant transport. It is thus shown that in the absence of rock exposure, ARS could be a viable complement in mapping subsurface fractures. Characteristic fracture parameters, fracture porosity, specific surface area which could give potential information on the hydraulic properties correlated with nitrate concentration in a given locality. The results serve to establish the role of fractures in groundwater contamination in the study area.
NS41B-1142
Electrical Resistivity Imaging Below Nuclear Waste Tank Farms at the Hanford Site
The Hanford Site, a Department of Energy nuclear processing facility in eastern Washington, contains a complex series of radiological liquid waste disposal and storage facilities. The primary method of interim storage is the use of large single-shelled steel tanks with capacities of up to 3790 m3 (1 million gallons). The tanks are organized below ground into tank farms, with about 12 tanks per farm. The liquid waste within the tanks is primarily comprised of inorganic salts with minor constituents of heavy metals and radiological metals. The electrical properties of the radiological waste are significantly different to that of the surrounding engineered fill and native geologic formations. Over the past 60 years since the earliest tanks have been in use, many have been known to leak. An electrical resistivity survey was conducted within a tank farm to map the extent of the plumes resulting from historic leaks. Traditional surface-based electrical resistivity surveys resulted in unusable data due to the significant subsurface infrastructure that included a network of delivery pipes, wells, fences, and electrical discharge sources . HGI adapted the resistivity technique to include the site infrastructure as transceivers to augment data density and geometry. The results show a distribution of low resistivity values within the farm in areas that match known historic leak sites. The addition of site infrastructure as sensors demonstrates that the electrical resistivity technique can be used in highly industrial sites.
http://www.hydrogeophysics.com
NS41B-1143
HydroImage-A User Friendly Hydrogeophysical Characterization Software Package
HydroImage is a user-friendly software package that integrated spatially extensive geophysical data, either high- resolution crosshole tomographic data or surface geophysical data, and borehole hydrogeological measurements, e.g., core data or conepenetration test (CPT) data to improve characterization and monitoring of the subsurface over a variety of resolutions and spatial scales. The ultimate product of HydroImage is estimates of hydrogeological parameters in 2 or 3-dimensional space. The software package is built as subsystem modules, so that additional capabilities can be added to the package as they are developed or refined. HydroImage links 1) a graphical user interface; 2) a geostatistical integration toolbox; 3) a geophysical inversion toolbox, including quality control steps; 4) a petrophysical and scale-matching toolbox; and 5) a Bayesian integration subsystem into a single package that will allow an investigator to efficiently and cost-effectively perform data analysis and make coherent interpretations. Because direct borehole measurements are typically limited in number and represent only single points within a large contaminated volume of the subsurface, estimation of hydrogeological parameters over large volumes using geophysical data represents a significant advance in characterization of contaminated sites and design of effective remediation systems.
NS41B-1144
Quality assurance and quality control for autonomously collected geoscience data
The growing interest in processes, coupled with the reduction in cost and complexity of sensors which allow for continuous data collection and transmission is giving rise to vast amounts of semi autonomously collected data. Such data is typically collected from a range of physical and chemical sensors and transmitted - either at the time of collection, or periodically as a collection of measurements - to a central server. Such setups can collect vast amounts of data. In cases where power is not an issue one datapoint can be collected every minute, resulting in tens of thousands of data points per month per sensor. Especially in cases in which multiple sensors are deployed it is infeasible to examine each individual datapoint for each individual sensor, and users typically will look at aggregates of such data on a periodic (once a week to once every few months) basis. Such aggregates (and the timelag between data collection and data evaluation) will impact the ability to rapidly identify and resolve data issues. Thus, there is a need to integrate data qa/qc rules and procedures in the data collection process. These should be implemented such that data is analyzed for compliance the moment it arrives at the server, and that any issues with this data result in notification of cognizant personnel. Typical issues (encountered in the field) include complete system failure (resulting in no data arriving at all), to complete sensor failure (data is collected, but is meaningless), to partial sensor failure (sensor gives erratic readings, or starts to exhibit a bias) to partial powerloss (system collects and transmits data only intermittently). We have implemented a suite of such rules and tests as part of the INL developed performance monitoring system. These rules are invoked as part of a data qa/qc workflow, and result in quality indicators for each datapoint as well as user alerts in case of issues. Tests which are applied to the data include tests on individual datapoints, tests on suites of datapoints, and tests applied over the whole dataset. Example of tests include: Did data arrive on time, is received data in a valid format, are all measurements present, is data within valid range, is data collected at appropriate time intervals, are the statistics of the data changing over time and is the data collected within an appropriate instrument calibration window? This approach, which is executed automatically on all data provides data end users with confidence and auditability regarding the quality and useability of autonomously collected data.
http://geophysics.inel.gov