Richard D. Markiewicz

Improvements in shallow high-resolution seismic reflection through PC-based systems

Abstract Increased dynamic range of recording equipment and decreased cost of processing hardware and software have made seismic reflection a cost-effective way of imaging geologic targets significant to environmental site classification. Recent application of shallow seismic reflection to environmental, geotechnical, and groundwater problems using a hand-portable PC-based seismograph for data acquisition and a stock 80486 PC for data processing demonstrates the feasibility of the technique to resolve efficiently key geoscience questions. Some applications with demonstrated success include fault mapping with less than 4 m of vertical resolution at 300 m, detecting structure and stratigraphy associated with a hydrologic basin divide, and groundwater modeling of thick sequences of alluvial, colluvial, and glacio-fluvial material. Shallow high-resolution seismic reflection has the potential to enhance significantly the economics and accuracy of many groundwater and geotechnical studies.

Advantages of wet work for near-surface seismic reflection

Benefits of shallow water settings (0.1 to 0.5 m) are pronounced on shallow, high-resolution seismic reflection images and, for examples discussed here, range from an order of magnitude increased signal-to-noise ratio to resolution potential elevated by more than 8 times. Overall data quality of highresolution seismic reflection data at three sites notorious for poor near-surface reflection returns was improved by coupling the source and/or receivers to a well sorted and fully saturated surface. Half-period trace-to-trace static offsets evident in reflections from receivers planted into a creek bank were eliminated by moving the geophones to the base of a shallow creek at the toe of the bank. Reflections from a dipping bedrock were recorded with a dominant frequency approaching 1 KHz from hydrophones in 0.5 m of water at the toe of a dam using a hammer impact source. A tamper impacted by a dead blow hammer in a shallow (10-20 cm) deep creek produced reflections with a dominant frequency over 400 Hz at depths as shallow as 6 ms.

Seismic Reflection: Upstream, Downstream, And On Earthen Dams And Dikes

High-resolution seismic reflection has been used successfully to characterize material and investigate a variety of problems associated with earthen dams and abutments. Limitations and challenges of seismic reflection when interrogating these structures and lithologies are nontrivial and require very critical thinking. Seismic reflection has proved an effective tool for mapping confining units for integration into the cutoff wall at the Keechelus Dam in Cle Elum, Washington; mapping lithologies and bedrock structures for earthquake retrofitting at Bend, Oregon; delineating karst in bedrock beneath the dam core responsible for subsidence on the upstream side of a major flood control structure at Clearwater Dam, Missouri; and detecting high permeability zones within a glacial outwash embankment of a water retention dam near Enumclaw, Washington. Extreme geometries and material variability associated with any man-made structure are the most formidable challenge to seismically imaging. Inconsistent source wavelets, out-of-the-plane energy, extreme statics (topography and velocity based), and source noise (disproportionately high percentage of surface waves) are all problems that are not unique to earthen dams, dikes, and levees, but they are certainly more prevalent with those types of structures. Success of the technique in these settings is source characteristics and spatial oversampling.

Shallow Seismic Reflection Survey at Wickiup Dam in Central Oregon

Concerns about potential liquefaction and resulting flowslide type failures at Wickiup Dam in Central Oregon prompted geophysical studies of the left wing dike, studies which included a high resolution seismic reflection survey. High resolution seismic reflection data effectively delineated structures and correlated lithologies to shallow borehole data above the surface of the dipping, basalt bedrock on the downstream side of the dam. Unconsolidated, Quaternary lacusnine, fluvial, and volcanic-derived sediments overlay the bedrock surface, which is encountered within 50 feet of the ground surface near the dam outlet works and plunges to depths of at least 1000 fi below ground surface 5000 ft west of the outlet works. Sediments that filled the seismically defined bedrock surface possess a wide range of bedding geometries likely indicative of multiple episodes of erosion and deposition, driven by the modern and ancestral Deschutes River system. Dominant frequencies in excess of 250 Hz evident on stacked sections provide vertical bed resolution on the order of 10 ft at depths of over 900 ft. Layering interpreted on stacked seismic data within the basaltic bedrock has been inferred to represent contacts between different basalt flow episodes and volcanic sediments (ash, cinder, etc.) originating from Wickiup Butte. Frequency inversions, which are common indicators of inappropriately stackedinterpreted seismic sections, evident on these stacked data are actual inversions that can be verified by correlating the stacked sections with shot gathers, a process rarely possible on high resolution stacked seismic sections. If geologic interpretations, instrumental in engineering designs for resistance to seismic loading, would have been based on existing drill data from the upper 300 ft alone, a relatively flat laying, cyclic sequence of sands, gravels, volcanic ash, silt, and clay overlying the mudflow and basalt materials would have been inferred to an estimated depth of several hundred to more than one thousand feet. Seismic data provided an extremely detailed image of the subsurface, delineating channel features within the sediments and a dipping basalt bedrock surface. Amplification potential of these kinds of geometries may impact the levels of expected ground motion for a given size seismic event.

IMPLICATIONS OF VP/VS RATIO ON SHALLOW P AND S REFLECTION CORRELATION AND LITHOLOGY DISCRIMINATION

Knowledge of subsurface material response to seismic energy can be important in developing an accurate siteresponse model. Seismic velocities can give insight into ground response and therefore construction requirements. An important aspect of this study is the accurate calculation of Vp/Vs ratio using P-wave and S-wave reflections from the same reflector. Accurate Vp/Vs ratios can give insight into lithology, pore fluid pressure and porosity, as well as engineering characteristics. However, correlating different reflection modes can be difficult due to changes in the bulk modulus not consistent with changes in the shear modulus for the same reflector. This can lead to inaccurate reflection correlation and incorrect Vp/Vs ratios. By utilizing multiple data types in the very near-surface, this study attempts to accurately correlate P-wave and S-wave reflections with reflectors, giving reflector specific rather than interval based calculations for Vp/Vs ratios, which can be used for lithologic interpretation, thus improving site response models.

Refraction Tomography Mapping of Near-surface Dipping Layers Using Landstreamer Data At East Canyon Dam, Utah

We apply the P-wave refraction-tomography method to seismic data collected with a landstreamer. Refractiontomography inversion solutions were determined using regularization parameters that provided the most realistic near-surface solutions that best matched the dipping layer structure of nearby outcrops. A reasonably well matched solution was obtained using an unusual set of optimal regularization parameters. In comparison, the use of conventional regularization parameters did not provide as realistic results. Thus, we consider that even if there is only qualitative a-priori information about a site (i.e., visual)—as in the case of the East Canyon Dam, Utah—it might be possible to minimize the refraction nonuniqueness by estimating the most appropriate regularization parameters.

Impact of four geophysical surveys on engineering design decisions

The borrow investigations provide estimates of quantity and quality (particle aiae gradations) for alluvial materials at two dam sites: Jackson Lake Dam modification, and Jordanells. At Jackson Lake, results of reaiativity surveys indicated significant quantities of sands and gravela within economic hauling distance of the embankment site. At Jordanelle, baaed on geophysical survey results, embankment designs were changed to accommodate an indicated shortage of suitable borrow.