Nigel J. Cassidy

Time-Lapse Geophysical Investigations over a Simulated Urban Clandestine Grave

Abstract:  A simulated clandestine shallow grave was created within a heterogeneous, made‐ground, urban environment where a clothed, plastic resin, human skeleton, animal products, and physiological saline were placed in anatomically correct positions and re‐covered to ground level. A series of repeat (time‐lapse), near‐surface geophysical surveys were undertaken: (1) prior to burial (to act as control), (2) 1 month, and (3) 3 months post‐burial. A range of different geophysical techniques was employed including: bulk ground resistivity and conductivity, fluxgate gradiometry and high‐frequency ground penetrating radar (GPR), soil magnetic susceptibility, electrical resistivity tomography (ERT), and self potential (SP). Bulk ground resistivity and SP proved optimal for initial grave location whilst ERT profiles and GPR horizontal “time‐slices” showed the best spatial resolutions. Research suggests that in complex urban made‐ground environments, initial resistivity surveys be collected before GPR and ERT follow‐up surveys are collected over the identified geophysical anomalies.

BISTATIC TOMOGRAPHIC GPR IMAGING FOR INCIPIENT PIPELINE LEAKAGE EVALUATION

In this work, we present an inverse scattering approach to address the timely detection of damage and leakage from pipelines via multi-bistatic ground penetrating radar (GPR) surveys. The approach belongs to the class of linearized distorted wave models and explicitly accounts for the available knowledge on the investigated scenario in terms of pipe position and size. The inversion is regularized by studying the properties of the relevant linear operator in such a way to guarantee an early warning capability. The approach has been tested by means of synthetic data generated via a flnite-difierence time- domain forward solver capable of accurately and realistically modeling GPR experiments. The achieved results show that it is possible to detect the presence of leakage even in its flrst stages of development.

Geophysical Monitoring of Simulated Clandestine Graves Using Electrical and Ground-Penetrating Radar Methods: 0-3 Years After Burial

Abstract:  This study provides forensic search teams with systematic geophysical monitoring data over simulated clandestine graves for comparison to active cases. Simulated “wrapped” and “naked” burials were created. Multigeophysical surveys were collected over a 3‐year monitoring period. Bulk ground resistivity, electrical resistivity imaging, multifrequency ground‐penetrating radar (GPR), and grave and background “soil‐water” conductivity data were collected. Resistivity surveys revealed the naked burial had consistently low‐resistivity anomalies, whereas the wrapped burial had small, varying high‐resistivity anomalies. GPR 110‐ to 900‐MHz frequency surveys showed the wrapped burial could be detected throughout, with the “naked” burial mostly resolved. Two hundred and twenty‐five megahertz frequency GPR data were optimal. “Soil‐water” analyses showed rapidly increasing (year 1), slowly increasing (year 2), and decreasing (year 3) conductivity values. Results suggest resistivity and GPR surveys should be collected if target “wrapping” is unknown, with winter to spring surveys optimal. Resistivity surveys should be collected in clay‐rich soils.

The use of microgravity for cavity characterization in karstic terrains

Microgravity is the interpretation of changes in the subsurface density distribution from the measurement of minute variations in the gravitational attraction of the Earth. As a technique, it is particularly suited to the investigation of subsurface structures, mapping of geological boundaries and, most importantly in this case, the location and characterization of voids or cavities. Gravity variations due to the geological/petrophysical changes associated with fracturing and changes in pore composition are superimposed upon much larger variations due to elevation, latitude, topography, Earth tides and regional geological variations. However, these external changes can be modelled or monitored with sufficient accuracy to be removed from the data. With the recent development of high-resolution instruments, careful field acquisition techniques and sophisticated reduction, processing and analysis routines, anomalies as small as 10 microgal can be detected and interpreted effectively. This paper describes the ‘state-of-the-art’ application of the microgravity technique for the detection and characterization of karstic cavities in a variety of limestone terrains, including the Carboniferous Limestone of the United Kingdom and Eire and the coral limestones of the Bahamas. The case study examples show how the recorded gravity anomalies have revealed the location of density variations associated with underground cave systems and, ultimately, provided information on their depths, shapes and morphology from a combined analysis of their spectral content, characteristic gradient signatures and modelling responses. In addition, mass deficiencies have been estimated, directly from the anomaly map, by the use of Gausss theorem without any prior knowledge of the exact location, or nature, of the causative bodies.

GPR derived architecture of November 1996 jökulhlaup deposits, Skeiðarársandur, Iceland

Abstract Skeiðarársandur in southeastern Iceland, with an area of > 1000 km2, is the world’s largest active proglacial outwash plain. In July–August 2000, a total of over 10 km of ground penetrating radar (GPR) profile data (at 50 MHz and 100 MHz) was collected from a variety of proglacial outwash sediments across the Gígjukvísl channel region of the Skeiðarársandur plain. GPR-profile results and their corresponding facies interpretations are presented for the flood deposits of a single supraglacial outwash fan and its associated source-proximal ice-walled canyon created entirely by the November 1996 jökulhlaup event. By combining the GPR data with ground surveying, photogrammetry and detailed sedimentary outcrop evidence, this study adds a new perspective to the large-scale analysis of single, high-magnitude flood events and the sedimentary record of former, ice-proximal outwash plains. The GPR derived architectures point to a higher degree of sediment reworking than predicted by previous sedimentary models and may provide a useful analogue for the study of sedimentation within similar bedrock fluvial and alluvial fan feeder systems.

Gpr-derived Sedimentary Architecture and Stratigraphy of Outburst Flood Sedimentation Within a Bedrock Valley System, Hraundalur, Iceland

Jokulhlaups and lahars are both types of outburst flood that commonly comprise a glacial meltwater and volcaniclastic sediment mix, and have discharges that are typically several orders of magnitude greater than perennial flows. Both types thus constitute a serious threat to life, property and infrastructure but are too powerful and too short-lived for direct measurements of flow characteristics to be made. Consequently a variety of indirect methods have been used to reconstruct flow properties, processes and mechanisms. Unfortunately, limited observations of sedimentary architecture and stratigraphic relationships are hampering our ability to discriminate fluvial magnitude-frequency regimes and fluvial styles of deposition, particularly those produced by rapidly-varied flows. This paper therefore uses Ground Penetrating Radar (GPR) to obtain quantitative data on subsurface sedimentary character of high-magnitude outburst flood sediments, including architecture and stratigraphy, from a bedrock-valley system in north-central Iceland. Basement pillow lava and subaerial lava flows are characterised by chaotic and hummocky GPR reflectors with a lack of coherent structure. They also feature an upper rough surface as evidenced by concentration of hyperbolae point sources. Unconsolidated sedimentary units are interpreted due to occur where laterally-persistent horizontal and sub horizontal reflectors occur. Deposition produced spatially diverse sediments due to rapidly- varied flow conditions. Observations include prograding and backfilling architecture, intercalated slope material and fluvial sediments, and multiphase sedimentary deposition. We suggest that these outburst flood sediments were initially deposited by traction load of coarse- grained material on prograding bedforms, and subsequently by drop-out from suspension of finer-grained material. The latter phase produced laterally extensive tabular sedimentary architectures that in-filled pre-existing topography and masked the complexity of bedrock forms beneath. Existing qualitative concepts of high-magnitude fluvial deposition within a topograph- ically confined bedrock channel are therefore now refined with quantitative data on sediment architecture and thus on flow regimes.

Do peatland microforms move through time? Examining the developmental history of a patterned peatland using ground‐penetrating radar

Using ground-penetrating radar (GPR) to map subsurface patterns in peat physical properties, we investigated the developmental history of meso-scale surface patterning of microforms within a raised bog. Common offset GPR measurements were obtained along a 45-m transect, at frequencies ranging from 100 to 900 MHz. We found that low-frequency (central frequency = 240 MHz) showed a striking pattern of subsurface reflections that dip consistently in a northerly direction. The angle of these dipping reflectors is calculated using a semblance algorithm and was shown to average 3.9 degrees between a depth of 1.0 and 2.5 m. These dipping reflectors may indicate downslope migration of surface microforms during the development of the peatland. Based on the estimated angle and the rate of peat accumulation, the average rate of downslope propagation of these surface microforms is calculated at 9.8 mm per year. Further survey work is required to establish whether the downslope migration is common across the peatland.

Interpreting complex, three-dimensional, near-surface GPR surveys: an integrated modelling and inversion approach

With the increasing computational power of modern personal computers, sophisticated modelling and inversion techniques are becoming popular tools for the interpretation of high-resolution, fully three-dimensional GPR surveys. In this paper, we present the latest results of ongoing practical research into the development of novel, integrated, finite-difference time-domain (FDTD) numerical modelling and linear tomographic inversion methods for the interpretation and analysis of near–surface, 3D GPR data. The proposed approach utilizes the Born approximation solution to the inverse-scattering problem and a truncated singular value decomposition (TSVD) to create the final, inverted reconstructions. A three-dimensional, full-field, O(2,4) accurate FDTD modelling scheme is used to generate the numerical-based Green’s functions and incident fields for the inversion. As such, accurate antenna sources (including the influence of shields) and near-field air/ground interface effects are inherently included in the inversion formulation. The performance of this integrated method is evaluated via a simulated, 3D, forensic-based, test-case example (a 900 MHz survey over a clandestine human burial target) including coherent noise from near-surface clutter. Although the example is simplistic, the results show that the scheme works well, despite some assumptions in the inversion methodology. As such, useful information can be gained on the true form, depth, location and spatial interrelationships of the buried features and, therefore, improved interpretations can be obtained in a three-dimensional context.

Long‐term Geophysical Monitoring of Simulated Clandestine Graves using Electrical and Ground Penetrating Radar Methods: 4–6 Years After Burial

This ongoing monitoring study provides forensic search teams with systematic geophysical data over simulated clandestine graves for comparison to active cases. Simulated “wrapped,” “naked,” and “control” burials were created. Multiple geophysical surveys were collected over 6 years, here showing data from 4 to 6 years after burial. Electrical resistivity (twin electrode and ERI), multifrequency GPR, grave and background soil water were collected. Resistivity surveys revealed that the naked burial had low‐resistivity anomalies up to year four but then difficult to image, whereas the wrapped burial had consistent large high‐resistivity anomalies. GPR 110‐ to 900‐MHz frequency surveys showed that the wrapped burial could be detected throughout, but the naked burial was either not detectable or poorly resolved. 225‐MHz frequency GPR data were optimal. Soil water analyses showed decreasing (years 4 to 5) to background (year 6) conductivity values. Results suggest both resistivity and GPR surveying if burial style unknown, with winter to spring surveys optimal and increasingly important as time increases.

Soilwater conductivity analysis to date and locate clandestine graves of homicide victims

In homicide investigations, it is critically important that postmortem interval and postburial interval (PBI) of buried victims are determined accurately. However, clandestine graves can be difficult to locate; and the detection rates for a variety of search methods (ranging from simple ground probing through to remote imaging and near‐surface geophysics) can be very low. In this study, simulated graves of homicide victims were emplaced in three sites with contrasting soil types, bedrock, and depositional environments. The long‐term monthly in situ monitoring of grave soil water revealed rapid increases in conductivity up to 2 years after burial, with the longest study evidencing declining values to background levels after 4.25 years. Results were corrected for site temperatures and rainfall to produce generic models of fluid conductivity as a function of time. The research suggests soilwater conductivity can give reliable PBI estimates for clandestine burials and therefore be used as a grave detection method.