Richard C. Benson

Evaluation of MASW Data Acquired with a Hydrophone Streamer in a Shallow Marine Environment

Seismic shear wave velocity is an important characteristic of sub-bottom materials for geotechnical and environmental characterizations in marine environments. Traditional marine shear wave velocity measurements utilize measurements within borings or data from geophones planted on the bottom. Multichannel Analysis of Surface Waves (MASW) provides a more efficient means to estimate shear wave velocity values of sub-bottom materials using a hydrophone streamer and an airgun seismic source. MASW data were acquired in a shallow marine environment and on adjacent land. Land data were acquired with a traditional spread of geophones, while the marine data were acquired with a 24-channel hydrophone streamer laid on the bottom. Phase velocity frequency spectra calculated from the data indicate that the measurements on land are repeatable with measurements underwater. Shear-wave velocity models estimated from the dispersion curves are consistent with each other and with expected geologic conditions. However, the sh...

CHARACTERIZATION OF KARST FEATURES USING ELECTROMAGNETICS AND MICROGRAVITY: A STRATEGIC APPROACH

The flat-lying limestones of western Texas are naturally jointed with preferential dissolution occurring along joints and bedding planes. This area has some of the largest air-filled open caves in West Texas and large paleokarst collapse features have been identified throughout a wide region. Aerial photos indicate the presence of extensive joints, lineaments and paleokarst. Site characterization to detect and map such subsurface features using borings alone is inadequate to produce a reasonable level of spatial sampling. This paper addresses the application of surface geophysical techniques, and an assessment of the spatial sampling and instrument sensitivities necessary to define the karst features of interest in this geologic setting. Electromagnetic measurements using a Geonics EM34 were selected because the measurements provide an excellent means of locating dissolution-enlarged joints. Microgravity was selected because it is the only surface geophysical method that will provide the location of karst feature regardless of their shape or fill material as long as there is a sufficient density contrast. While both methods have limitations, as do all methods of site characterization, the combination of gravity and EM measurements are complementary in this application. A known cave system was used to establish the spatial sampling criteria for the detection and characterization of such features. Field tests were then run over two known karst sites; a paleokarst collapse; and a localized doline to provide anomaly signatures in this geologic setting. It is clear that the EM and the gravity techniques are appropriate for detecting and characterizing karst features in this geologic setting. However, one of the key issues in planning and carrying out a geophysical survey, is developing a spatial sampling criteria. This criteria should be based upon an understanding of project objectives, a conceptual geologic model of site conditions and, if possible, existing data from the area of interest.

Site Characterization in Karst and Pseudokarst Terraines

There is a wide range of terminology encountered in the fi eld of karst. The following terms will set the stage for the topics to be covered. Additional terms will be presented throughout the book. The two key divisions are karst and pseudokarst. Karst conditions are formed due to a natural process of dissolution of soluble rock. Pseudokarst may look or act like karst, but it may be formed naturally in non-soluble rock or may be caused by man’s activities. In addition, the term paleokarst is used to indicate those karst features that are very old, in geologic terms, and are typically inactive. For a complete discussion of karst terminology see Monroe (A glossary of karst terminology. US Geologic Survey Water Supply Paper 1899K, 1970), Lowe and Waltham (Dictionary of karst and caves. In: Judson D (ed) British Cave Research Association cave studies series 10. British Cave Research Association, Malvern, 2002), and Fields (A lexicon of cave and karst terminology with special reference to environmental karst hydrogeology. US EPA/600/R-02/003, 2002).

Surface Geophysical Methods

We now have a work plan that is based upon regional and local existing data and possibly some site-specific data. A conceptual model has been developed and focuses our attention on what we know, what we need to know and what we might expect on site. As we begin to acquire on-site data using surface geophysical methods, we will begin to test the conceptual model. Their primary benefit is that of greatly increasing the data density over the site, which improves the definition of background and anomalous conditions. These measurements allow us to more accurately locate invasive measurements based upon data rather than guesses thereby improving the chances of accurately defining geologic conditions.

Hydrologic Characterization and Measurements

This chapter focuses attention upon the unique hydrologic characteristics of fractured and karst aquifers and the wide range of measurements that can be utilized to characterize them. At this stage of the characterization we should have a well-developed geologic conceptual model of conditions with many of the questions about the site already answered (Tables 13.1, 13.2 and 13.3). We should already know the type of surface hydrologic features at the site; the surface water and groundwater boundaries, in general if not in detail; the depth, thickness and level of complexity of the epikarst; whether the aquifer is confined or unconfined, and whether there is more than one. In addition, we will have preliminary data from existing monitoring wells or initial borings on water levels, potentiometric surfaces and flow directions. The objectives for this phase of work will focus upon the details to quantify and support our conceptual model. This is obviously important for projects focused on groundwater resources or contaminant remediation. But surface and groundwater in a fractured or karst setting will also impact most engineered structures, if not managed well they can result in unstable conditions.

What Is Site Characterization

Site characterization is the process of developing an understanding of the geologic, hydrologic and engineering properties at the site including the soil, rock, along with groundwater and in many cases, man-modified conditions in the subsurface (e.g. utilities, structures, mines and tunnels) that can impact site conditions. It also includes the spatial and temporal assessment of contaminants when they are present. Various terms such as site investigation, site assessment and site characterization have been used to describe this process and are often used interchangeably. Many case histories have shown that most failures are caused by not properly understanding the site conditions that can impact the project. These failures could have been avoided by focusing attention on the geologic and hydrologic conditions.

Engineering Measurements and Monitoring

At this stage of the site characterization some geotechnical data has already been obtain which may include soil and rock samples, STP blow counts, and RQD values. Geophysical logging data has provided additional geotechnical details from borings. Some laboratory analysis may have been made on selected samples from borings. With a refined conceptual model of conditions in hand, the engineering measurements and monitoring is now focused upon obtaining very site-specific measurements and data to address specific engineering questions and quantify additional engineering parameters. This is a broad range of measurement types and data. Within this chapter, we will focus specifically on the engineering measurements and monitoring of subsidence and collapse. We summarize some of the common methods for measuring and monitoring land subsidence and potential collapse, cover the general applications, some considerations and provide a few examples. As always, each technique or measurement has an associated scale of measurement, advantages and limitations.While the many details for making these geotechnical measurements are beyond the scope of this book, further details can be found in Dunicliff (1988), Hunt (2005) and by specialty conferences (DiMaggio and Osborn 2007) who all provide a solid foundation for these types of measurements. Due to the advances that continue to occur in geotechnical instrumentation this information should be supplemented with recent manufacturers literature and recommendations.

Aerial Photography and Remote Sensing Data

With the desk study completed, the objectives of the site characterization should be well defined, the area of investigation known and the regional geologic setting established. While the overview aerial photo or photo mosaic of the area of interest has provided an initial familiarization of the site, we are now concerned with additional details that can be obtained from a detailed analysis of the aerial photos.

EPA Superfund Site

This third case history was a 52 ha EPA Superfund site located in Tarpon Springs north of Tampa, Florida that had processed phosphate ore to produce elemental phosphorus. While the site had been investigated and a Record Of Decision (ROD) had been completed from EPA regarding the remediation, there remained concerns with the site. These concerns included the development of sinkholes on the site, as well as the presence of buried drums of elemental phosphorus waste material, and the potential for groundwater contamination. The project was carried out between 2001 and 2003 over a period of 30 months following our standard site characterization strategy. Because this was a superfund site, there were a number of stakeholders involved from beginning to end. The project included extensive meetings with local interests, county, state and federal agencies with EPA oversight. The client wanted this project completed right the first time with no questions left unanswered. Therefore, a large number of methods were utilized with heavy emphasis on surface, borehole and marine geophysical methods. Some of these methods were used to provide 100 % coverage of the property. The work at this site is probably one of the most complete karst site characterizations carried out.

Cave and Cavern Collapse

This chapter presents the concept of large caverns and domes found in many caves. It discusses the process of their development and modes of breakdown. This includes discussion of factors necessary to prevent or minimize the risk of subsidence or collapse at the surface including bulking of rock, which can sometimes restrict further collapse and the thickness of rock needed over a cave or mine to be considered safe. Regardless of the many rules of thumb and safety factors there are cases where collapse has propagated to the surface from great depths.