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BLOOD SPATTER ANALYSIS

BLOOD SPATTER ANALYSIS. modified from: CP Forensics by Alvarado. A SOURCE OF FORENSIC EVIDENCE. Origin(s) of bloodstain Distance of bloodstain from target Direction from which blood impacted. Speed with which blood left source Position of victim and assailant

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BLOOD SPATTER ANALYSIS

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  1. BLOOD SPATTER ANALYSIS modified from: CP Forensics by Alvarado

  2. A SOURCE OF FORENSIC EVIDENCE • Origin(s) of bloodstain • Distance of bloodstain from • target • Direction from which blood • impacted

  3. Speed with which blood left source • Position of victim and assailant • Movement of victim and assailant • Number of blows/shots

  4. PROPERTIES OF BLOOD BLOOD VOLUME • On average, blood accounts for 8 % of total body weight • 5 to 6 liters of blood for males 4 to 5 liters of blood for females

  5. BLOOD VOLUME (con't) • 40 percent blood volume, or about 2 L, lost internally or/and externally is required to produce irreversible shock (death) • A blood loss of 1.5 L, internally or externally, is required to cause incapacitation.

  6. SURFACE TENSION • Surface tension is slightly less than that of water • A drop of blood tends to form into a sphere in flight (not the artistic teardrop shape). • Formation of the sphere is a result of surface tension that binds the molecules together. • This elastic like property of the surface of the liquid makes it tend to contract.

  7. More rapid bleeding may result in slightly larger drops. • BUT, slower bleeding does not always result in smaller drops.

  8. Blood cast from a moving source will tend to consist of smaller droplets. • Blood behaves as a projectile in motion and obeys the laws of physics and mathematics.

  9. CATEGORIES OF BLOOD STAINS • PASSIVE • TRANSFER • PROJECTED

  10. PASSIVE DEFINITION: drops created or formed by the force of gravity acting alone.

  11. EXAMPLES: Drops Drip Patterns Pools Clots

  12. PICTURES:

  13. TRANSFER DEFINITION: created when a wet, bloody surface comes in contact with a secondary surface.

  14. EXAMPLES: Contact bleeding Swipe or Smear Wipe Smudge

  15. PICTURES: A recognizable image of all or a portion of the original surface may be observed in the pattern.

  16. PROJECTED DEFINITION: created when an exposed blood source is subjected to an action or force, greater than the force of gravity.

  17. EXAMPLES: Arterial Spurt / Gush Cast-Off Impact Spatter

  18. PICTURES:

  19. Categories of Impact Spatter Low Velocity (usually >4 mm) Medium Velocity High Velocity (usually < 1mm)

  20. LOW VELOCITY Relatively large stains4mm in size and greater. Gravitational pull up to5 feet/sec.

  21. MEDIUM VELOCITY • Most stains 1 to 4mm in size. • Force of 5 to 25 feet/sec.

  22. HIGH VELOCITY • Most stains 1mm or less (much variablity) • Force of 100 feet/sec or greater

  23. DIRECTIONALITY OF BLOOD The spherical shape of blood in flight is important for the calculation of the angle of impact (AOI θi) of blood spatter when it hits a surface.

  24. When a droplet of blood strikes a surface perpendicularly (90 degrees) the resulting bloodstain will be circular. The length and width of the stain will be equal. 90○ ANGLE

  25. Blood that strikes a surface at an angle less than 90 degrees will be elongated or have a tear drop shape. 70○ ANGLE

  26. 45○ ANGLE 10○ ANGLE 30○ ANGLE 5○ ANGLE

  27. “Fitting” of an ellipse in blood drop Tail or spine Parent Drop

  28. ANGLE OF IMPACT ANGLE of IMPACTis the acute angle formed between the direction of the blood drop and the plane of the surface it strikes.

  29. By utilizing trigonometric functions, it’s possible to determine the impact angle for any given blood droplet.

  30. By accurately measuring the length and width of a bloodstain, the impact angle can be calculated using the SIN formula below: θi = sin-1 (W / L)

  31. EXAMPLE Finding AOI LENGTH = 5.9cm WIDTH = 2.6cm

  32. SOLUTION: θi = sin-1 (W / L) θi = sin-1 (2.6/5.9) θi = sin-1 (.44) θi = 26.2°

  33. POINT OF CONVERGENCE DEFINITION: The common point, on a 2 dimensional surface, over which the directionality of several bloodstains can be retraced.

  34. Once the directionality of a group of stains has been determined, it's possible to determine a 2D point (or area) for the group of stains.

  35. By drawing a line through the long axis of a group of bloodstains, the point of convergence can be determined. LONG AXIS / LENGTH

  36. Point of Convergence (2D)

  37. POINT OF ORIGIN DEFINITION: lies at a point in space above the point of convergence. Measurement of the impact angle allows for translation of the 2-D image (convergence) into a 3-D one (origin).

  38. TO DETERMINE WHERE THAT POINT IS LOCATED: • Measure the distance • from each blood stain • along its central axis to the • POC (distance = y) • 2) Find the tangent of impact angle ( tan θi )

  39. 3) Multiply the TAN of the AOI by the distance. 4) Measure that distance from floor up the perpendicular axis and you will arrive at the Point of Origin (PO) FORMULA: PO = y · tan θi

  40. EXAMPLE Finding PO GIVEN: DISTANCE FROM BLOODSTAIN (to POC): 90cm AOI (calculated from AOI formula): 30° SOLUTION: PO = TAN (30°) x 90cm PO = .577 x 90cm = 52cm

  41. Point of Origin (3D --- use Z axis)

  42. In practice (at a crime scene), strings, tapes and protractors are commonly used.

  43. Computer programs can also be used.

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