Forensic Geophysics L2

Forensic Geophysics L2

1. Typical Targets

   • Clandestine Graves
     • Most cases solved by conventional police methods
       • Specialist dogs
       • Thermal imaging cameras
         • If recently buried
       • Site walkovers
       • Ecology/ entomology
       • Physical probing of lumps and bumps
     • Geophysical methods
       • Used in searches for mass graves
       • Complications
         • Body Decay
           • Temperature and depth dependant
             • 0.5 meter burial skeletonises in 1 year
             • 2 meter burial skeletonises in 5-10 years
               • Body intact ~1 year
         • High acidity
           • Peat
         • Ambient Temperature
           • Putrefraction process occurs
             • 40-50 degrees
             • Increases decay time
             • Reduces fluid discharge
       • Advantages
         • Does not disturb ground
         • Relatively rapid
   • Buried Weapons
   • Lost Vehicles
   • Disturbed Ground
   • Evidence of Occupation

2. Metal detectors

   • Electromagnetic field transmitted through coil
     • Secondary field produced from conducting objects
     • Smaller coils detect shallower items
   • Some discriminate between metals
   • Submersible versions available

3. GPR

   • Active field method
     • Sends induced signal into ground and measures return
       • Transimitter and reciever antennae take 1D recording
         • Sequentially moved to record 2D line
           • Successive recordings create 3d profile
   • Developed in 1970s
     • Mainly for landmine detection
   • Case study
     • Fred West Case
       • 1994
         • First public exposure of GPR
       • 25 CROMWELL STREET, GLOUCESTER
         • 9 bodies found beneath house and garden
           • 3 other bodies at second address and in a field
           • Victims include 2 of their daughters
     • IRA Case
       • Search for missing murder victim in N.Ireland
       • Observed collapse feature in ground
         • Surveyed with GPR showing no body present
           • Saved police time and money
       • Ruffel (2005)
   • Can be used indoor
     • Behind wall 1.2 ghz
       • Found pipe bombs
   • Advantages
     • Estimates depth/size of target
     • Data collected can be viewed in real time
     • Works well in sandy soils
     • Can work over concrete
     • Good resolution for near surface features
   • Disadvantages
     • Expensive
     • Can be difficult to collect data
       • Area ideally level and smooth
       • Trees, made ground, and saline ground water not ideal
     • Takes a lot of time over large areas

4. Conductivity

   • Transmits EM primary wave
     • Recieves secondary conductive fields
       • Near surface objects
   • Active method
   • Measures material ability to transmit electricity
   • Transmitter and reciever can be different distances apart
     • Larger distance has larger sampling area
       • Poorer resolution
   • Case study
     • New Zealand
     • Yvonne
       • Missing 12 years
     • Large, wooded survey area
       • Conductivity anomolies investigated
         • Body recovered
   • Advantages
     • Equipment can be carried
     • Surveys large areas
     • Detects change in disturbed soil
     • Can be used in woodland and over hard ground
     • Works over concrete
   • Disadvantages
     • Averages conductivity within range
     • Surface interference can occur
     • Doesnt work well in clay soils

5. Resistivity

   • Active method
     • Inverse of conductivity
       • Based on materials resistivity to current
   • Pigs in grave study
     • Human tissue act (2005)
       • Humans cant be used for research
     • Decay rates studied
       • Resistivity measure can be associated with human decay
         • Similar organ size, skin and hair covering
   • Case study
     • N. Wales
     • Keele university study
     • Victim buried in in rural area
       • Wrapped/in suitcase
       • Body probably buried for 1 year
         • Likely resistivity response of victim matched with simulated burials
           • Anomalies prioritised based on similarity to simulated data
             • Wrong field studied
       • Clay rich soils
         • Cant use GPR
     • Pringle and Jervis (2010)
   • Advantages
     • Flexible configurations can investigate at different depths
     • Repeatable
     • Less effected by surface noise
   • Disadvantages
     • Poor spatial resolution
     • Influenced by soil moisture
     • Slower coverage time than conductivity
     • Some surface destruction
       • Electrodes placed into ground

6. Water based search methods

   • 3x more people dumped in water
   • Side scan sonar
     • Towed 3-6 meters above sea floor
     • Sound waves reflect off bottom surface and surface objects
       • Converted into graphical image of reflected wave streangth
     • Used to find
       • Shipwrecks
       • Aircraft
       • Vehicles
       • Bodies