Blood spatter analysis
Pattern Evidence: Reconstruction Patterns and Individualization Patterns
Many patterns that appear at crime scenes are useful for reconstruction. In some of the cases, these patterns can be “collected” only by documentation; that is, they cannot be “packaged” and taken back to the crime lab. If these patterns are interpreted properly, they can provide information about the events and can often times be more reliable than witnesses. Blood spatter is probably the most common type of reconstruction pattern. Blood spatter pattern interpretation is something of a subspecialty in forensic science. There are laboratory examiners as well as investigators who specialize in blood pattern interpretation as a result of training and experience. |
Basics of Blood Pattern Interpretation
Droplets of blood falling or projected through space follow standard physical laws. Those laws provide the scientific basis for understanding blood patterns. By virtue of its makeup, viscosity, density, and other physical properties, blood forms predictable patterns when it falls or is projected through the air and impacts a surface.
Most of the time blood forming a pattern of interest at a scene has already dried. But occasionally it can still be in a liquid or semicongealed state. Blood deposited outside the body onto a surface will clot within a matter of minutes. Then it will dry. The larger the quantity of blood the longer it will take to dry. Investigators sometimes use the state of the partially dried blood to draw inferences about how much time has elapsed since the blood was shed. Blood drying time will also be influenced by heat, humidity, air circulation, and the target surface.
Droplets of blood falling or projected through space follow standard physical laws. Those laws provide the scientific basis for understanding blood patterns. By virtue of its makeup, viscosity, density, and other physical properties, blood forms predictable patterns when it falls or is projected through the air and impacts a surface.
Most of the time blood forming a pattern of interest at a scene has already dried. But occasionally it can still be in a liquid or semicongealed state. Blood deposited outside the body onto a surface will clot within a matter of minutes. Then it will dry. The larger the quantity of blood the longer it will take to dry. Investigators sometimes use the state of the partially dried blood to draw inferences about how much time has elapsed since the blood was shed. Blood drying time will also be influenced by heat, humidity, air circulation, and the target surface.
Velocity and Impact Angle
Blood spatter is often classified as low, medium, or high velocity. A low-velocity blood spatter pattern is any pattern formed where gravity is the only force acting on the blood. These are typically drip patterns. A medium-velocity blood spatter pattern is formed when moderate force from some object causes pooled blood to “scatter” in all directions. The force causes the droplets produced to be smaller so the resulting pattern consists of more and smaller droplets than are seen in typical low-velocity patterns. These patterns are typically produced by an external force such as the use of blunt force or someone stomping his foot or shoe into pooled blood. It could also be produced by an arterial spurt. A high-velocity blood spatter pattern is the result of extreme force acting on a blood source. As a rule, such patterns are seen only in connection with gun shots, explosions, or the high impact forces of a vehicle crash. The shape of a blood spatter droplet indicates the angle from which it impacted the surface. A blood droplet hitting a nonabsorbent surface at a 90-degree angle (perpendicular to the surface) results in a circular stain. As the angle changes, the resulting stain becomes more elliptical. In fact, using measurements and some elementary trigonometry can estimate the angle of incidence of a blood stain. |
Various Blood Spatter Patterns
Falling droplets from a bleeding source that is stationary will generally result in blood pooling below the source. It may be possible to discern separate“droplet” patterns in the pool. If a dripping blood source is moving fast enough, the droplets may show which direction the source was moving
Contact deposit patterns result from an object coming into direct contact with a blood pool or blood source. These patterns can appear on clothing or on objects at scenes. Sometimes it is important to determine what object caused the pattern. Wipe and swipe patterns result when an object contacts or transfers wet blood and smears on a surface. A wipe pattern is created by an object contacting an existing bloody surface with motion. A swipe pattern is created by a bloody object contacting another surface with motion. Running patterns are just what the name says. Blood hits a vertical surface but the volume is sufficiently high that gravity causes the droplet to run down. |
An arterial spurt pattern results when an artery is cut or severed and blood is literally
pumped out of the body by the beating hear and onto a nearby surface. These patterns generally contain quite a bit of blood. In addition, an individual with a seriously severed artery is losing blood at such a rate that he or she will not be able to move too far and without immediate and extreme medical intervention will die fairly quickly Cast-off patterns result when a bloody object is swung and throws off droplets onto a nearby surface. These patterns may be seen on ceilings or walls and occasionally on floors. Secondary spatter patterns result when blood drops fall into a preexisting pool of blood. As each drop hits the liquid surface, it can cause small droplets to splash upward and some of these may hit a nearby vertical surface. |