Soil moisture content estimation using ground-penetrating radar reflection data

Ground-penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100- and 200-MHz GPR survey was conducted on aeolian deposits in a quarry. Time-domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (er) and relative magnetic permeability (lr), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and erlr. Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in erlr, which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in erlr coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0AE055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference.

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Identifying causes of ground‐penetrating radar reflections using time‐domain reflectometry and sedimentological analyses

We present two regional, lithospheric cross sections that illustrate eastward‐ and southeastward‐dipping, subducted slabs to depths of 315 km beneath the surface of Colombia in northwestern South America. These cross‐sectional interpretations are based on relocated earthquake hypocentral solutions, models supported on gravity and magnetic regional data, and coda‐Q ( Q c ) tomography. The method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system. Mapping of earthquake‐defined Benioff zones combined with tomographic imaging reveals the presence of an ∼240  km long east–west‐striking slab tear, named here the Caldas tear. The proposed Caldas tear separates a zone of shallow, 20°–30°‐dipping, southeastward subduction in the area of Colombia adjacent to Panama and the Caribbean Sea, which is not associated with subduction‐related volcanism, from an area of steeper, 30°–40°‐dipping, slab adjacent to the eastern Pacific Ocean that is associated with an active north–south chain of active arc volcanoes. We propose that the Caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the Panama indenter, an extinct island arc that began subducting beneath northwestern South America about 12 Ma. The area south of the Panama indenter is Miocene oceanic crust of the Nazca plate, which subducts eastward beneath northwestern South America at normal angles and melts to form a north–south‐trending active volcanic arc. In addition to the formation of the Caldas tear, we propose that impedance of the thicker crustal area of the Panama arc‐indenter over the past 12 Ma may have led to down‐dip break‐off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate‐depth earthquakes beneath east‐central Colombia.

Tearing and Breaking Off of Subducted Slabs as the Result of Collision of the Panama Arc‐Indenter with Northwestern South America

Earthquake nests as natural laboratories for the study of intermediate-depth earthquake mechanics

http://www.lucabaradello.it/carcione/CSG03.pdf

Acoustic and electromagnetic properties of soils saturated with salt water and NAPL

In 1995 and 1996, researchers associated with the US Air Force’s Phillips and Armstrong Laboratories took part in an extensive geophysical site characterization of the Groundwater Remediation Field Laboratory located at Dover Air Force Base, Dover, Delaware. This field experiment offered an opportunity to compare shallow‐reflection profiling using seismic compressional sources and low‐frequency ground‐penetrating radar to image a shallow, unconfined aquifer. The main target within the aquifer was the sand‐clay interface defining the top of the underlying aquitard at 10 to 14 m depth. Although the water table in a well near the site was 8 m deep, cone penetration geotechnical data taken across the field do not reveal a distinct water table. Instead, cone penetration tests show a gradual change in electrical properties that we interpret as a thick zone of partial saturation. Comparing the seismic and radar data and using the geotechnical data as ground truth, we have associated the deepest coherent event in...

/pdf/seismic-reflection-and-ground-penetrating-radar-imaging-of-a-26ockhb30k.pdf

Seismic reflection and ground‐penetrating radar imaging of a shallow aquifer

The Bucaramanga earthquake nest is situated at a depth of about 160 km beneath Colombia at 6.8° N, 73° W, and it produces about eight earthquakes of mb 4.7 or more each year from a source region having dimensions of about 10 km or less. In this study, we evaluate nest properties using reported locations, focal mechanisms, and moment tensors, as well as relocations determined from teleseismic P, pP , and PKP arrivals. We also introduce a novel method for determining the “volume” of a group of hypocenters and use this volume as a statistic for evaluating the accuracy of nest hypocenters. We have relocated 26 nest earthquakes occurring between 1964 and 1986 using the JHD method and carefully reread teleseismic P, pP , and PKPdf arrivals. The relocated earthquakes cluster within a region having a volume of 776 km3 and east-west, north-south, and vertical dimensions of 13 × 18 × 12 km, a smaller region than any found previously from analysis of teleseismic data. In practical terms, this represents an approximate upper bound on the accuracy of relative earthquake locations possible using teleseismic data and standard relocation methods. This compares with a volume of 11 km3 and dimensions of 5.1 × 3.4 × 3.8 km as determined by Schneider et al. (1987) from observations of microearthquakes collected over 16 days with a temporary local network. These dimensions are comparable to those of the rupture dimensions of the largest observed nest earthquakes, as determined from an analysis of their seismic moments (∼5 × 1024 dyne-cm). Furthermore, an evaluation of cumulative moment rate since 1964 indicates that strain rates within the nest volume must be between 10−13/sec and 10−11/sec. The b values (∼2.0) and β values (∼0.9) determined from available magnitudes and moments are remarkably high, suggesting that earthquakes of all observed sizes (3 × 1020 to 5 × 1024 dyne-cm) contribute significantly to the moment release. Focal mechanisms determined from teleseismic observations exhibit a consistent pattern, most having westward-dipping P axes and eastward-dipping T axes; this differs from the more varied pattern reported by Schneider et al. (1987) for microseismic earthquakes.

A reexamination of the Bucaramanga, Colombia, earthquake nest

Environmental site assessment usually includes determining the geologic character of the subsurface after contamination has occurred. However, at the Groundwater Remediation Field Laboratory (GRFL), we acquired extensive geophysical and geotechnical data before planned contaminant releases happened. These prespill geophysical images provide background data for comparison with data acquired in the future. Additionally, a large amount of cone penetrometer (CPT) data provide in situ geotechnical measurements to compare with surface geophysics.

/pdf/site-characterization-at-the-groundwater-remediation-field-1jkypmeuc9.pdf

Site characterization at the groundwater remediation field laboratory

In June of 1995, the Earth Sciences Division of the Air Force Phillips Lab, with survey equipment from the University of Delaware and assisted by the Kansas Geological Survey and Elohi Geophysics, conducted a geophysical site characterization of the SERDP-funded Groundwater Remediation Field Lab (GRFL) located at Dover AFB, Delaware and administered by Applied Research Associates for USAF Armstrong Lab. Seismic data were collected in order to 1) compare the results using three different compressional sources and 2) cover the field site well enough to characterize the seismic response of the shallow subsurface. This paper will focus primarily on the first of these two goals.

Katharine Kadinsky-Cade

Papers

Seismic reflection and ground‐penetrating radar imaging of a shallow aquifer

A reexamination of the Bucaramanga, Colombia, earthquake nest

Site characterization at the groundwater remediation field laboratory

High-resolution seismic reflection survey at dover afb: a comparison of three seismic sources

Coincident ground penetrating radar and seismic imaging of an aquitard boundary