School of Geography, Archaeology & Palaeoecology, Queen's University, Belfast, Northern

Lacustrine Flow (Divers, Side-scan Sonar, Hydrogeology, Water Penetrating Radar) Used to Understand the Location of a Drowned Person

Alastair Ruffella*

School of Geography, Archaeology & Palaeoecology, Queen's University, Belfast, Northern Ireland, BT7 1NN, UK

SUMMARY

An unusual application of hydrological understanding to a police search is described. The lacustrine search for a missing person provided reports of bottom-water currents in the lake and contradictory indications from cadaver dogs. A hydrological model of the area was developed using pre-existing information from side scan sonar, a desktop hydrogeological study and deployment of water penetrating radar (WPR). These provided a hydrological theory for the initial search involving subaqueous groundwater flow, focused on an area of bedrock surrounded by sediment, on the lake floor. The work shows the value a hydrological explanation has to a police search operation (equally to search and rescue). With hindsight, the desktop study should have preceded the search, allowing better understanding of water conditions. The ultimate reason for lacustrine flow in this location is still not proven, but the hydrological model explained the problems encountered in the initial search.

*Corresponding author.

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Keywords: police search; side scan sonar; water penetrating radar; hydrogeology

1. Introduction

This work aims to demonstrate how hydrologists (hydrogeologists, limnologists included) can assist in understanding the results of a police search that could be useful in other searches (by the police or search and rescue personnel) where understanding water conditions is required, be they like this case, or generally. The location of objects submerged in water is commonly undertaken by search and rescue, law enforcement, environmental, and engineering personnel [McGrane et al., 2013; Schultz et al., 2013). Visual inspection by divers is often augmented by use of search dogs, side-scan sonar and geophysics (high resolution seismic [CHIRP], ground penetrating or water penetrating radar [WPR] and magnetometers, [Schultz et al., 2013; Parker et al., 2010]). The conjunctive use of two or more search assets is mentioned in textbooks on canine behaviour (Rebmann & Sorg, 2000; Snovack, 2004) and drowning victims (Armstrong & Erskine, 200X). This work amplifies such recommendations by demonstrating the combined use of information from divers and local people; cadaver dogs; a desktop geological/hydrological study and WPR. More importantly, the study shows that an understanding of karstic groundwater in a lacustrine system can assist the search and explain results. The forensic search methods also demonstrate how limnologists may contribute to search and rescue and how search methods may help the study of lake (and other)aquatic systems.

2. The Importance of the Lacustrine Environment to the Case

The sequence of events leading up to and including this case is important to understanding the search scenario. The injured party (IP) was a 50 year old male with known illegal drug, alcohol and depression problems who lived with his wife, his children having left home. He was unemployed and had involvement with local drug-dealers whom (it was rumoured) he owed money: she worked part-time in a rural town in the west of Ireland (location anonymised). At 1800hrs in September (autumn) 2006 his wife walked home from work to find their vehicle (a saloon car) absent with no specifics of where her husband may have gone. She alerted the police some 12 hours later, who could not find him or the vehicle following an All Points Bulletin alert.24 hours later local volunteers began a search of the town. The vehicle was located that evening (some 50 hours from the last known activity from the missing person – a sighting in the local shop) in a car park adjacent to a boating slipway at a nearby lake (Figure 1). People visiting the car park, especially those fishing, knew the IP and his vehicle and presumed he was arriving early each morning to go fishing or sit by the lake.

3. The Initial Search of the Lake

Upon discovery of the vehicle, it was searched and impounded by police, and the surrounding area searched. His home was re-visited and searched. No sign of the IP could be found: search dogs traced trails along the lake banks that led to no further discoveries. Theories explaining the disappearance included: suicide; a faked suicide and absconding (to avoid debt); homicide and accidental death. The location of the vehicle could have been a false lead and the IP was elsewhere (dead, injured or alive) but lead to the possibility that his body was in the water adjacent to the vehicle and car park/slipway (Fig. 1). Whilst terrestrial investigations were carried out, a search around the slipway car park was organised. The landward area around the car park comprises an access road to boating jetties (Fig. 1), fields with stone walls with isolated trees and a tree-lined lake bank. A search of these locations revealed nothing, so a water-search was required. Available search assets included three inflatable boats, fire brigade divers, cadaver dogs (trained for work on boats) and sonar. A dive team arrived first at the scene, followed by dog team and lastly, sonar. A standard dive-search protocol was deployed, with two divers working alternate 15 minute shifts from one support boat in a zigzag pattern in a series of grids radiating north and west from the slipway. A second boat deployed the cadaver dogs, also in standard deployment as one dog was taken independently of the second. Any dog indication was located by GPS and a unique buoy as other buoys occur throughout the area. The sonar (StarFish 100KHz) was deployed last (due to availability) in a series of ENE – WSW strips along the length of the lake west of the slipway, and N – S transects east of the slipway (Fig. 2a), as following the dog zigzag pattern proved difficult with short runs and possible snagging of the sonar fish close to shore. As the diver’s search was completed the second boat was used to deploy the cadaver dogs, both of which reacted at a location south west of the slipway. Redeployment of the divers found no human remains (or anything else of significance) at this location.

3. 1. Initial Search Results

The dive team reported that water conditions were still and clear to 2m depth but had a moderate current with sediment movement from 2m depth to base, which was surprising given the lacustrine environment. The team noted two types of substrate. First, a rock outcrop north of the slipway was observed with either hollows filled with silt and mud with vegetation, or a veneer of sand and silt on rock. Second, the remainder of the lake floor comprised silt and sand with mud in broad (10-20m wide) depressions. The sonar results were excellent in this second area, although rock protuberances above the lake floor caused problems with the sonar signal: this shows up as a ‘stripy’ appearance to the data (Fig. 2b). Air bubbles emanating from the rock also caused some loss of sonar signals. A simple inverse-distance weighting (12 nearest neighbours) was applied to the data in ARC GIS, which removed the stripes (Figure 2c) and revealed an anomaly that was 250-300m northeast of where the dogs indicated. Other methods of interpolating the data could have been used (Lloyd, 2011), had time allowed. The cadaver dog location was re-surveyed with the sonar and searched by divers: no object was found at this location, suggesting the dogs had picked up a remnant lake-side scent from the IP or from some other submerged or nearby material. The dogs did not react to the sonar anomaly in the rocky area, yet when investigated, the IP’s body was found wedged in amongst the rocks with arms folded in to the torso (Fig. 2c). A postmortem found death by drowning and the cause: misadventure (high levels of alcohol and veterinary drugs were found in the IP’s blood). The police are still investigating whether he tried to wade out across the shallower, rocky area, or was taken to this location as a place people could stand and push someone under water.

3.2 Initial Search Exit Strategy

The negative results from the initial lake search caused refocus on the IPs home and other possible known locations, as well as a review of results so far. Two main questions remained from the lake: the false-positive reaction from the cadaver dogs and the existence of water-bottom currents and gas release in a non-organic lake environment. As part of the exit strategy, an integrated geological and hydrological study of the location and surrounding environs was initiated in order to explain the various observations from the search.

4. Desktop Geology and Hydrology of the Search Area

The solid geology of the area surrounding the car park, slipway and searched lake comprises early Carboniferous (Pennsylvanian in some countries) Dartry Limestone Formation (Long & McConnell, 1997), deformed into gentle anticlines and synclines and cut by ENE- WSW and NW-SE oriented normal and strike-slip faults as well as Palaeogene dykes. The drift geology and geomorphology of the area comprises a 1-2m thick veneer of glacial till (thicker in valleys, up to 10m thick), with glaciofluvial sands and gravels. The limestone is karstic, with extensive cave systems recorded by outcrop mapping in surrounding upland areas and by local cavers at lower altitudes. Cave systems are commonly open above 150m elevation, becoming phreatic below this height. Springs are common at the base of the Darty Limestone, where artesian flow is common, provided by the considerable hydraulic head from surrounding uplands (up to 300m above sea level). The area to 3-4km around the search site comprises flat fields and water, with little solid or drift geology exposed. This was also noted in comparison to the reports from divers of rock at the base of the lake: these reports were not detailed enough to allow the desktop geology authors to decide whether this was loose rock (e.g. boulders) or outcrop. Results from both the desktop study and search indicated that geological mapping of the lake floor could explain the lake floor topography and existence of gas bubbles and flow, and thus possibly the cadaver dog reaction. Interviews with local rivers agency staff and those who use the lake for fishing and water sports confirmed the observation of the dive team, that at the narrow area of the lake adjacent to the slipway, a current occurs below 1-2m depth. This was ascribed to flow between the two areas of the lake: the desktop study suggested that flow could be due (wholly or partly) to phreatic groundwater flow from a karstic system below the lake floor.

5. Consequences of the Desktop Study: Deployment of Water-borne Ground Penetrating Radar (WPR)

WPR is a useful compliment to side scan sonar in that it provides sub-bottom imagery in enclosed fresh water bodies where use of the seismic towfish is problematic (Sambuelli et al, 2009; Parker et al, 2010). Deployment of the radar antennas is not easy, requiring either mounting alongside a boat (Arcone, 1996; Sellmann et al., 1992) or placed in the base of a thin-skinned inflatable (Ruffell, 2006): both have attendant problems of snagging and equipment loss or damage. Nonetheless, if shallow geological investigations are required, the method can yield excellent results. In this case, a 250MHz shielded antenna (Mala Geoscience) was placed in a rubber dinghy with hull support slats removed, and the area surveyed on the same grid as the sonar survey. Data was analysed in the proprietary software ReflexW with no processing as field results were good enough for geological interpretation (Fig. 3).

6. Development of a Conceptual Hydrogeological Model of the Lake System

A preliminary model of how lake floor topography, sub-bottom imagery and reports of current flow from divers was integrated onto the 2D WPR data (Fig. 3). With this, the observations from the desktop geological study were used to provide a plan of where the cadaver dogs indicated, the likely position of entry to the water by the IP and the location of the body. Onto this model, the possible movement of groundwater was suggested, given the surrounding outcrop geology (Fig. 4). Groundwater has a more extensive catchment to the south of the lake and survey area and may thus provide a greater hydraulic gradient and input to the lake: flow tests would confirm or deny this. Other untested aspects of the model remain. First is whether the rocky peninsula on which the slipway is built is also a positive geological feature and thus a focus for upwelling groundwaters under artesian pressure from the hinterlands to the south. Second is whether one of the prominent horizontal reflectors in the water column may not be a surface multiple (‘ringing’ in the sense of Sellmann et al., 1992) but a real feature from mixing of lake and ground water, such as a thermocline. This could also be tested with temperature experiments.

7. Conclusions

Individual elements of this study do not provide many surprises. The dive team were deployed in a logical, if incorrect location, yet importantly, commented on water flow in the lake, confirming reports from local fishing enthusiasts. The cadaver dogs indicated on the presence of human remains. Sonar displayed good results from areas of silt, sand and mud on the lake floor, yet suffered from poor data quality where rocky substrates with or without air bubbles occurred. This was overcome by simple data processing and an anomaly that was the body of the IP identified and recovered. WPR provided information on the bathymetry and shallow geology of the lake, yet was deployed following the recovery of the body so imaging of the cadaver did not occur. A desktop study of geology and topography was conducted in order to understand the contradictory dog reactions, when if deployed earlier could have provided an explanation for the observations of bottom water flow. The order of asset deployment was thus not ideal, but dictated by availability and need, but together shows the usefulness of integrating data sources in order to explain both the results of the search, as well as the hydrogeology of this part of a lacustrine system. The latter is focused upon here, as non-forensic readers may be unaware of some of the methods used in these environments, how they may assist the integrated scientific study of lake hydrogeology, and how they may contribute to criminalistic or humanitarian ventures such as search.

Acknowledgements

Land and Property Services (N.Ireland) kindly provided aerial images for use in this work. The assistance in data manipulation of Conor Graham (Queen’s University, Belfast) is gratefully acknowledged. The work was initiated by detectives and officers from the Garda an Siochana (John Shanaghan) and Police Service of Northern Ireland (Nigel Johnson and Gary Arbuthnot, dog handlers; John Gilmore, detective). The National Trust for Northern Ireland and Waterways Irelandand Cavan County Council assisted with access to waterways.

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