Mapping Sediment Thickness Using Airborne Electromagnetics
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Summary An airborne electromagnetic (AEM) survey was conducted to support a feasibility study for the placement of a riverbank filtration system. The goal of the survey was to map sediment thickness along a narrow corridor centered on the Missouri River in North Dakota. This case history presents the results of that survey. A total of 1,184 line km of data were acquired along a 116 km stretch of the Missouri River using the SkyTEM 301 time domain electromagnetic system. This system, using a combination of early time (approximately 5 microseconds for the first time gate) and spatially dense data, successfully mapped the alluvial sediment - bedrock contact. Using a spatially constrained inversion in conjunction with borehole data, the AEM data reveal a paleochannel where sediment thicknesses exceed 75 meters. Six locations were identified for future investigation based on interpreted alluvial sediment thickness, sediment resistivities, and location.Keywords:
Bedrock
Palaeochannel
Electromagnetics
Alluvion
abstract An attenuation formula was derived for the Arias instrumental intensity on bedrock based, in part, on the source spectrum function obtained by Aki. The constants in the formula were calibrated for the San Fernando earthquake by using eight bedrock spectra derived from surface accelerograms. The calibrated formula was then used to compute the Arias bedrock intensities at most of the sites of the ground-level accelerographs and seismoscopes that recorded the earthquake. Maxima accelerations from accelerograms and spectral accelerations from seismoscope records were then used to compute the Arias intensities at the surface by using an empirical relation obtained by Arias. After the instrument sites were classified into four groups, (1) crystalline rock, (2) sedimentary rock, (3) shallow alluvium, and (4) deep alluvium, surface-to-bedrock intensity ratios were correlated with these site classifications which leads to four surface attenuation curves constrained to have the same slope. From the constant differences between these curves, it is possible to define site factors to be applied to bedrock intensity in order to estimate surface intensity for zoning purposes. Conversely, these factors can be applied to surface intensities for deriving bedrock attenuation curves for other earthquakes in which the geology of the instrument sites is only generally known. The site factors relative to unweathered, unfractured crystalline rock outcrops are 1.80, 3.63, 3.74, and 5.12 for classifications 1 through 4, respectively.
Bedrock
Outcrop
Intensity
Alluvion
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Abstract Recent studies reveal that low‐slope bedrock reaches (bedrock surface slope milder than ~5 m/km) are more common than previously thought and can be found in engineered rivers and densely populated deltas. Here we present a novel formulation of alluvial morphodynamics of low‐slope bedrock rivers transporting nonuniform bed material that accounts for the nonuniformity of the sediment size and the presence of small scale bedforms such as dunes and can thus be of aid to solve management/restoration problems in low‐slope bedrock rivers. The formulation is implemented in a one‐dimensional morphodynamic model. Numerical results are compared with laboratory experiments on equilibrium bedrock reaches downstream of stable alluvial‐bedrock transitions. The differences between experimental and numerical results are comparable with those obtained in the alluvial case. Model applications simulate (1) bedrock reaches with a stable bedrock‐alluvial transitions, (2) an alluvial‐bedrock transition subject to sea level rise, and (3) steep bedrock reaches. Upstream of a stable bedrock‐alluvial transition the flow decelerates in the streamwise direction with the formation of a stable pattern of downstream coarsening of bed surface sediment. In response to sea level rise, alluvial‐bedrock transitions migrate downstream and bedrock‐alluvial transitions migrate upstream. Opposite migration directions are expected in the case of sea level fall. When applied to steep channels, the model predicts gradual alluviation, but it fails to reproduce runaway alluviation.
Bedrock
Beach morphodynamics
Bedform
Alluvion
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Summary The importance of determining the shape of the soil-bedrock interface in the alluvial Argive Bain, Peloponnese, Greece, stems on the fact that it is a fundamental in a current seismic ground response analysis in areas around the archaeological sites of Midea and Tiryns, the later listed by the UNSECO as world heritage. Although newly collected Transient Electromagnetic (TEM) soundings were modeled and inverted in one-dimension (1D) and used to create two-dimensional (2D) pseudo-sections and maps of resistivity distribution at several depth intervals. This pseudo-2D and -3D imaging procedure has been successful in other alluvial basins allowing inferences of their structure. The 1D inverted models were stitched together to form thirteen 2D geoelectrical resistivity pseudo-sections, some traversing the whole basin. Additionally, the 1D models were interpolated to form maps of resistivity distribution at various depth intervals from 0 to 130 m depth. The previously uncertain soil-bedrock contact outside populated zones within the Argive Bain leading to soil thickness varying from 20 to at least 130 m thick in some areas. The bedrock is uplifted in some areas and highly fractured in others.
Bedrock
Alluvion
Electrical Resistivity Tomography
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The thickness of the alluvial and tuffaceous deposits that overlie bedrock in Yucca Valley has been inferred from gravity and seismic measurements. Preliminary interpretations indicate that these deposits are thickest in a narrow north-trending trough in the eastern part of the valley. The gravity data delineate a buried north-trending ridge of bedrock that extends from Mine Mountain almost to Quartzite Ridge. Seismic refraction measurements confirm the existence of the bedrock ridge and indicate that the bedrock is as close as 100 feet to the surface. The buried bedrock high is important because it may alter concepts of the movement of groundwater within the valley. A single seismic-refraction profile was located near the area of thickest alluvium and tuff to determine the feasibility of using refraction techniques for determining the depth to bedrock where it is covered with several thousand feet of alluvium and tuff. The results are encouraging but not enough data were acquired to give a reliable depth estimate. Seismic-refraction measurements were used successfully to determine the thickness of alluvium in narrow valleys partly filled with alluvium. This work was in the northwestern part of Yucca Valley and was done to choose drilling sites for studies of ground-water movement.
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Seismic refraction
Trough (economics)
Alluvion
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Shallow seismic‐reflection techniques were used to image the bedrock‐alluvial interface, near a chemical evaporation pond in the Texas Panhandle, allowing optimum placement of water‐quality monitor wells. The seismic data showed bedrock valleys as shallow as 4 m and accurate to within 1 m horizontally and vertically. The normal‐moveout velocity within the near‐surface alluvium varies from 225 m/s to 400 m/s. All monitor‐well borings near the evaporation pond penetrated unsaturated alluvial material. On most of the data, the wavelet reflected from the bedrock‐alluvium interface has a dominant frequency of around 170 Hz. Low‐cut filtering at 24 dB/octave below 220 Hz prior to analog‐to‐digital conversion enhanced the amplitude of the desired bedrock reflection relative to the amplitude of the unwanted ground roll. The final bedrock contour map derived from drilling and seismic‐reflection data possesses improved resolution and shows a bedrock valley not interpretable from drill data alone.
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Reflection
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Modeling of the annual heat flow within a thin alluvium veneer on a granitic bedrock substrate in desert environments, such as found in the southwestern United States, predicts that at certain times of the year the depth to bedrock has a measurable effect on the surface temperature if the alluvium cover is less than 2 m thick. Changes in the thickness of the alluvial cover caused by bedrock topography will produce contrasts in the surface temperature. If temperature contrasts as small as 0.1 C can be resolved, a linear topographic feature having several metres of relief buried by 1.5 m of alluvium may be visible in thermal imagery acquired during January or August in the southwestern U.S. under optimal conditions. Thermal remote sensing may provide a means for delineating some buried faults, fluvial channels, and other features of interest on buried, granitic pediment surfaces.
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Alluvion
Alluvial fan
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