Presentation on theme: "Introduction to Bloodstain Pattern Analysis (BPA): (Continued)"— Presentation transcript:
Introduction to Bloodstain Pattern Analysis (BPA): (Continued)
Mechanism of how blood forms elongated stains is critical for understanding how to determine the angle at which the blood impacted with the surface. When a blood droplet hits a surface it forms a right triangle, whose angle can be calculated using trigonometric functions. The step-by-step procedure Approximate the outline of the stain as shown in the slide on the right below - white oval. Do not include the stain’s tail (E) in the approximation. Measure the length (D) and the width (d) from the stain. Calculate the fraction by dividing the width by the length (d/D). Calculate the impact angle by taking the arcsine (sin -1 ) of the fraction (d/D). A fraction of 0.5 represents a 30 o angle. Calculating the Angle of Impact
Length (L) Width (W) Angle of Impact Arc sin W/L
2D Area of Impact
Point of convergence or Back Projection in 2-Dimensions Represents the 2D location of an impact, Blunt force trauma when someone has been beaten with an object or a fist. For impact stains, approach is helpful in determining a possible minimum number of blows Spatter cannot occur until blood present to be splashed). Multiple points of convergence suggest multiple blows. Mechanism for selecting the proper bloodstains from within a bloodstain impact spatter pattern to use in determining the 3D area of impact. Repeat for 20 stains Determining the Origin of an Impact
2D Points of Convergence
3D - Area of Origin
Area of Origin in 3-Dimensions The spatial location of an impact that caused impact spatter pattern Third dimension of the 2D determination. If impact location determined in 2D, converting that into a 3D image is simple Accomplished using 3D imaging techniques, such as 3D scene imaging systems, computer aided design (CAD) programs or software specifically designed for the application. Also scene using string method o Manually or by using lasers. Alternatively, the spatial area (point) can be calculated using a combined manual/trigonometric method..
1. Back Project 1. Back Project 2. Determine Impact Angle Using sin -1 (w/l) function Z (height) 3D Point of Impact 2D Pt. of Impact Tangent Method z= tan (impact angle) x Length ? The Tangent Method – Step-by-Step Use 2D back project method discussed above and back project stains to point of convergence Step 1 in the diagram below and illustrated in previous slide. Measure the width (d) and the length (D) of two droplet stains. Calculate the impact angle by taking the arcsine of the width divided by the length of the bloodstain. Measure the distance from the point where the blood droplet hit the surface to the point of convergence. The height of the area of impact (Z) is calculated using the tangent function as shown in the slide below.
String Method 3D - Spatial Determination of Impact Area
Manual procedure for approximating the area of origin of an the scene Select several stains (10-20) from bloodstain pattern Most easily from a previous 2D back projection determination. These stains have specific characteristics: Characteristics of Droplets for String Method Droplets must be traveling linearly - means they still posses most of the energy imparted by impact. The easiest way of determining which stains are appropriate is to identify them by performing 2D analysis using back projection. Stains losing energy begin to arc and fall - no use for determining the spatial origin of an impact. Stains should be as elongated as possible, given the complexities of the pattern analyzed. Measuring these stains is less error prone than more circular stains. The String Method
Choose as many as 20 elongated stains traveling linearly from impact point before striking the surface. Number each stain sequentially and tape number to the wall (surface) next to stain. Record the information in a log. Calculate the impact angle for each stain (arcsine width/length). Record angle in log and onto tape next to the stain on the impact surface. Tape end of a long piece of string where the blood droplet hit the surface. The String Method: Step-by-Step
Ensure tape holding the string is secure. Use zero edge protractor to track string through the center of the stain’s axis along the protractor at the calculated impact angle. Pull the string taut and attach it to the surface, e.g., floor or other object. Re-check the measurements and the stain trajectory (the string) to ensure the angle of the droplet impact is correct. Repeat the process for as many as 20 stains in the pattern. String Method – Continued
If pattern is from a single event AND if the appropriate stains were selected, AND if measurements determined correctly AND if the string was run at the correct angle Strings should converge in an area in space that approximates the area of the impact. If the area of convergence seems too large, the process might not have been performed correctly. o Or indicates multiple blows String Method
ReasonCorrection Method Poor stain selection Re-examine the impact pattern and the stains selected. Incorrect stain measurement Repeat measurements to ensure measurement accuracy Incorrect angle using the zero edge protractor If angles were calculated correctly (above), check the protractor positioning. If placed correctly, re-check string positioning. Multiple impacts occurring in the same general spatial location Use 2D back projections to determine if multiple impacts occurred. If so, use 2D method to choose stains from each impact and make overlapping 3D area of impact determinations. Troubleshooting The String Method
Cast off Blood Leaving a Moving Object An Example of Centripetal Force
Centripetal Force Only one force is at work, centripetal force. Directed toward the center of the path of the moving object. When the adhesive forces holding the blood onto the object are greater than the centripetal force, the blood will fly off the object in a tangentially straight line. Straight line … impact site and hence its angle … direct link to location of object at the exact time blood left it. Movement of Object Left-to-Right Blood Castoff – Tangentially to Movement of Object Centripetal Force Toward Center of Object
When blood leaves a blood-covered object it can Drip passively or be propelled. If propelled blood leaves the blood-covered object when the centripetal force acting on it overcomes the adhesive forces holding the blood on the object. True whether blood cast from object occurs while the bloody object is being swung or if it comes to an abrupt halt (cessation castoff). Example Blood on a bloody knife Knife is swung in an arc, Blood will be propelled (castoff) the knife in a path that is tangentially straight from that point and travel until it hits impacts a surface. If blood hits the surface while still traveling in the straight line, the shape it takes will represent the angle of the impact. Trigonometric functions can be used to calculate the impact angle Backtrack to the spatial origin of the castoff when blood left the object. Castoff
Bevel & Gardner Spatter Family - Linear - Cast- off James, Kish & Sutton Spatter - Projection Mechanism - Cast-off Wonder Spatter Groups - Cast-offs - Swing Old IABPACast-off Pattern Created from bloody knife blade and swinging the blade in a downward motion in front of the target. Volume of drops and impact angles change as the knife continues its downward motion. Alternate Terminology Castoff
Direction of Travel Cessation Castoff Bloody knife hitting floor
Breadth of Castoff Pattern More than one droplet usually leaves bloody object as it moves through its path Preponderance form a pattern characteristic of the surface of the bloody facing the impact surface. Example Bloody hand is held sideways to the impact surface, say a wall, the castoff pattern formed represent that surface of the hand facing the impact. Pattern will be broad line of individual droplets. As hand moves through its arc, it will change position causing individual fingers to face wall.
Castoff pattern broadens and might show castoff from individual fingers. Breadth of pattern reflects the blood-covered surface area of the surface facing the impact site. Thus, a knife blade will often give a single line of castoff staining, while a baseball bat can give a broader castoff pattern. If the knife’s flat side of blade faces impact area, the breadth of the castoff pattern will reflect the width of the knife blade, but not its actual dimensions.
Cast Off Sequence First blow causes bleeding Subsequent blows contaminate weapon with blood Blood is cast-off tangentially to arc of upswing or backswing Pattern & intensity depends on: type of weapon amount of blood adhering to weapon length of arc
Bubble Stains Bubble stains form when air is present in the droplet. An example or expectorated blood is shown below. The arrows point to the air bubbles in the stain, which are expected in blood expectorated from the lungs for from the mouth.
Impact Force Droplet Size
Drip Pattern: Secondary Spatter Free-falling drops dripping into wet blood Large irregular central stain Small round & oval satellite stains
Arterial Spurt Blood exiting body under arterial pressure Large stains with downward flow on vertical surfaces wave-form of pulsed flow may be apparent Heartbeat
Wipe Patterns Object moves through a wet bloodstain Feathered edge suggests direction Bevel & Gardner Non-spatter Family - Irregular Margin - Smear - Wipe James, Kish & SuttonAltered - Sequenced - Wipe Wonder Spatters Not a Criteria - Transfers - Moving - Wipe Old IABPAWipe Pattern Alternate Terminology
Swipe Patterns Wet, bloodied object contacts a secondary surface Transfer from: hand, fingers shoes, weapon hair Transfer to: walls, ceilings clothing, bedding Can produce mirror-image of bloodied object Bevel & Gardner Non-spatter Family - Irregular Margin - Smear - Swipe James, Kish & Sutton Passive - Contact/Transfer - Swipe Wonder Spatters Not a Criteria - Transfers - Moving - Swipe Old IABPASwipe Pattern Alternate Terminology
Flow Patterns Blood flows horizontally & vertically seeking the path of least resistance Altered by contours, obstacles Often ends in pool at the lowest or most blocked point
Forward and Back Spatter Forward SpatterBackspatter (AKA Blow Back)
Bloodstains on Clothing Whose blood on the clothing? How was blood deposited? Passive staining Transfers Fabric Impressions Flow patterns Saturation stains Dripping blood Dynamic staining Impact spatters Arterial spurts/sprays Expirated stains Castoff
Bloodstains on Clothing How was garment collected? Photos of person wearing garment Examine stains BEFORE DNA analysis Non-destructive examination History of garment How handled Emergency room floor o After suspect’s injuries treated » Additional bloodstains added » Existing ones altered
Sources of Error Differential expansion of blood droplet upon impact Where width of stain is NOT equivalent to droplet diameter! Fabric dependent Measurement error – significant figures Droplet volume and velocity unknown Surface properties Texture Porosity Resiliency Droplets not originating from a single source
Spattered Blood Random distribution of bloodstains that vary in size Produced by variety of mechanisms Size range varies considerably by any one mechanism Quantity varies depending on quantity of blood available Force applied Gunshot Beating stabbing
Identify Spatter Patterns
Archiving Bloodstain Patterns
All archiving principles apply. However scene investigators are not BPA analysts … need to understand and recognize them and then follow guidelines to properly preserve them. Acquire knowledge: Study bloodstain patterns and the underlying scientific principles relevant to BPA and Take an approved BPA workshop, Gain on-scene experience, Experiment First: Examine bloodstain pattern carefully to ascertain its overall characteristics. Answer the following questions. How much area does the pattern encompass? Is the pattern a composite of multiple patterns? How might this pattern have occurred? Begin archiving. Capturing these properly requires several photographs: … Establishing Midrange of each staining area. Close up photographs provide detail, o Misting, etc. o Close-ups of the relevant details present in various areas of the overall pattern. o Iinitial photographs taken without scales & then repeated with scales. Preserve the overall size of the pattern as well as sub-patterns within the whole.
Photograph before scales in place … at a scene as midrange shot of the bed and wall behind the bed. The stains on the bed, those on the wall behind the bed and those on the floor next to the bed were important. Not shown in the photograph are the castoff stains: on the ceiling above the bed, on the wall to the left of the bed and wall opposite the bed. Multiple superimposed patterns to capture for meaningful archive. Multiple impact spatters Large swipe/wipe pattern on the wall behind the bed leading from the just above the bed and flowing downward toward the floor Smaller swipe patterns on the wall to the right of the main impact spatter. Scene Example
Archiving Bloodstain Patterns
StepReason Determine the total area the pattern covers by examining adjoining walls, floors and ceiling. Ensures that subsequent archiving will not miss important areas of the pattern Measure perimeter of each area of pattern staining. Record measurements in appropriate log. Bloodstain patterns should be photographed first in the raw, without scales, so that subsequent analysis is not hindered by their presence. Photograph the area(s) without scales. Archives entire pattern without scales which can cover droplets from adjoining or overlapping patterns. Cordon off each pattern using adhesive measuring tape ruled in inches. Include the entire pattern - width and length. Photograph the pattern with the scales in place using forensically appropriate photos: establishing, mid range and close-ups, if appropriate. If not possible to capture its entirety, grid the area and photograph grid each separately Scales ensure that the size (with and length) of a pattern is captured. Gridding is a least preferred method, but is useful to capture detail in extremely large patterns. Video the pattern using a digital camera having video capability (or a dedicated video camera). Capture each area of the pattern. Show relationship of each to the whole. Ensure to capture areas adjacent to the pattern - walls, floor and ceiling. Gives the BPA analyst the overall perspective of the pattern. Photograph each area of the pattern having special characteristics. Radial impact spatter might have value in determining the spatial area of the impact. If analysis not be done at scene, capture individual droplets (numbered sequentially and taped to the surface) so that subsequent length/width measurements for stain pattern analysis software. Impact pattern with imbedded mist pattern - capture the latter, measuring overall size and that of representative droplets. Complex pattern with multiple, overlapping characteristics, (multiple impact patterns and/or multiple castoff patterns or is an impact pattern containing cessation cast off). Capture sub-patterns separately showing relation to overall pattern. Alerts the BPA analyst to pattern complexity or that information within the pattern might have additional interpretative data.
Bloodstain Artifacts – Selected Examples No Scene is virginal … must be considered a scene that is not as it was when the crime took place. Blood evidence is not exempt from evidence dynamics. Examples of things that happen to blood evidence range from cleanup activity, officials responding to the scene, emergency medical personnel trying to save lives are common to weather complications. Artifacts, a common concern Can occur from people who work on the scene Insects who dine on blood or walk through the scene.
Insect Activity It is well known by experienced BPA analysts that insects can leave artifactual marks in blood at the scene. Forensic Entomologist Jason Byrd wrote “ Insects can also affect the interpretation of blood spatter pattern analysis. Roaches simply walking through pooled and splattered blood will produce tracking that may not be readily recognizable to the untrained observer. Specks of blood in unique and unusual areas (such as on ceilings) may mislead crime scene technicians unless they are aware of the appearance of blood contaminated roach tracks. Similarly, flies and fleas may also track through pooled and spattered blood. However, flies will also feed on the blood and then pass the partially digested blood in its feces, which are known as "flyspecks". Flies will also regurgitate and possibly drop a blood droplet on a remote surface, which may serve to confuse bloodstain analysis. Fleas feeding on the living pass a large amount of undigested blood (used as the larval food source) on many household surfaces. If a crime occurs in a heavily infected apartment, fecal drops already present would serve to confuse analysts as those droplets would test positive for human blood. Therefore it is important to recognize and properly document the natural artifacts that may occur from the presence, feeding, and defecation of roaches, flies, and fleas.”
Flies Contamination and artifacts from insects dining on dried or wet blood at the scene should be anticipated by the BPA analyst, Investigators should understand that such activity can be common. Artifactual staining has confused BPA analyists but that from flies should not have. Shapes and sizes of the individual stains are characteristic of the patterns that form, and these have been described TypeCharacteristics Size FecalSymmetrical and asymmetrical round spots having 3 levels of pigmentation: Creamy, brownish and dark mm Vomit regurgitation Craters from sucking activity are surrounded by raised edges having a dark perimeter. The surface is irregular and reflective. 1-2 mm Trailing (swiping due to defecation) Distinguished by two segments, a body and a tail appearing sperm-like, tear shaped or snake-like mm
Cockroaches Cockroaches feast on blood. Like flies, they leave telltale marks of their activity that can be misinterpreted by inexperienced scene scientists/investigators.
The following illustrate how crime scene unit activity can affect bloodstains. Note the wall next to the deceased. The photographs illustrate the importance of on-scene archiving before actively working the scene or removing evidence. The bloodstain artifacts on wall in the left photograph were created by the crime scene unit removing the evidence from the corner. The perspective in each photograph is slightly different, which is why some of the detail in the left photograph is missing in the right photograph. On-Scene Investigative Activity
Before Processing Began After Processing Photograph: original before the crime scene unit arrived at scene.. Photograph taken after the unit completed processing but before the body removed
Fabrics – Differential Absorption Interpreting bloodstain patterns from blood deposited on garments can pose problems because modern fabrics are often blends of different types of fibers, each of which has different absorptive properties.
Packaging/Preserving Bloodstain Evidence the scene Clothing of homicide victims/witnesses Dry all wet evidence if possible If not possible – do not fold stains on top of stains Procedure Place clean paper under & over garment Fold garment over the paper so that no stains come into contact with other stains Place folded garment into paper bag & seal