Chapter 10

How are blood stain patterns useful?

The crime scene investigator must remember that the location, distribution, and appearance of bloodstains and spatters may be useful for interpreting and reconstructing the events that produced the bleeding.

Define crime-scene reconstruction

The method used to support a likely sequence of events at a crime scene by the observation and evaluation of physical evidence and statements made by individuals involved with the incident.

How to find the origin of blood stain patterns

The origin of a blood spatter in a two-dimensional configuration can be established by drawing straight lines through the long axis of several individual bloodstains. The intersection or point of convergence of the lines represents the origin point. The origin of a blood spatter in three-dimensions can be established from the point of convergence plus calculating the angle of impact from the length and width of blood droplets. The intersection of the three lines represents the point of origin.

Common ways to document blood stain patterns

Two common methods of documenting bloodstain patterns are the grid method and the perimeter ruler method.

Discuss the information that can be gained from bloodstain pattern analysis about the events involved in a violent crime

-The direction from which blood originated - The angle at which a blood droplet struck a surface - The location or position of a victim at the time of a bloody wound was inflicted - The movement of a bleeding individual at the crime scene. -The minimum number of blows that struck a bleeding victim -The approximate location of an individual delivering blows that produced a bloodstain pattern.

Important Blood Spatter Patterns

A cast-off pattern is created when a blood-covered object flings blood in an arc onto a nearby surface. Arterial spray spatter is created when a victim suffers an injury to a main artery or the heart. Commonly, the pattern shows large spurted stains for each time the heart pumps. A pattern created by blood that is expelled from the mouth or nose from an internal injury is called an expirated blood pattern. A void is created when an object blocks the deposition of blood spatter onto a target surface or object. When an object with blood on it touches one that does not have blood on it, this produces a contact or transfer pattern. The edges of a stain will dry to the surface, producing a phenomenon called skeletonization. Patterns made by drops or large amounts of blood flowing by the pull of gravity are called flows. A trail pattern is a series of drops that are separate from other patterns, formed by blood dripping off an object or injury.

Understand how various bloodstain pattern types are created and which features of each pattern can be used to aid in reconstructing events of a crime scene

A shooting may leave a distinct gunshot spatter pattern. This may be characterised by both forward spatter from an exit wound and back spatter from an entrance wound. A cast-off pattern is created when a blood-covered object flings blood in an arc onto a nearby surface. This kind of pattern commonly occurs when a person pulls a bloody fist or weapon back between delivering blows to a victim. The bloodstain tails will point in the direction in which the object was moving. Arterial spray is created when a victim suffers an injury to a main artery or the heart. The pressure of continuing pumping of blood causes blood to spurt out of the injured area. Expirated blood pattern is a pattern created by blood that is expelled out of the nose, mouth, or respiratory system as a result of air pressure and/or airflow. Avoid patterns- An area within a deposited spatter pattern that is clear of spatter, caused by an object or person blocking the area at the time of the spatter's deposition. Contact/Transfer patterns- A bloodstain pattern created when a surface that carries wet blood comes in contact with a second surface; recognisable imprints of all or a portion of the original surface or the direction of movement may be observed. Flow Pattern- A bloodstain pattern formed by the movement of small or large amounts of blood as a result of gravity's pull. Pool Patterns- A pool of blood occurs when blood collects in a level (not sloped) and undisturbed place. Skeletonization- The process by which the edges of a stain dry to the surface in a specific period of time ( dependent on environmental and surface conditions.) Skeletonization will remain apparent even after the rest of the bloodstain has been disturbed from its original position. Drip Trail Patterns- A pattern of blood stuns formed by the dripping of blood off a moving surface or person in a recognisable pathway separate from other patterns.

Gunshot Blood Spatter Patterns

A shooting may leave a distinct gunshot spatter pattern. This may be characterized by both forward spatter from an exit wound and back spatter from an entrance wound. The location of injury, the size of the wound created, and the distance between the victim and the muzzle of the weapon all affect the amount of back spatter that occurs.

Discuss the methods to determine the area of convergence and area of origin for impact spatter patterns.

Area of convergence- The area on a two-dimensional plan where lines traced through the long axis of several individual bloodstains meet; this approximates the two-dimensional place from which the bloodstains were projected. Area of origin- The location in three- dimensional space from which blood that pro ducted a blood-stain originated. The location of the area of convergence and the angle of impact for each bloodstain is used to approximate this area. A common method for determining the area of origin is the string method. String Method: 1) Find the area of convergence for the stain pattern. 2) Place a pole or stand as an axis coming from the area of convergence. 3) Attach one end of a string next to each droplet. Place a protractor next to each droplet and lift the string until it lines up with the determined angle of impact of the drop. Keeping the string in line with the angle, attach the other end of the string to the axis pole. 4) View the area of origin of the drops where the strings appear to meet. Secure the strings at this area.

Describe the classifications of low, medium and high-velocity impact spatter and appreciate how these classifications should be used.

Impact Spatter- A bloodstain pattern produced when an object makes forceful contact with a source of blood, projecting drops of blood outward from the source. Forward Spatter- Blood that travels away from the source in the same direction as the force that caused the spatter. Back Spatter- Blood directed back toward the source of the force that caused the spatter. Low-Velocity Spatter- An impact Spatter pattern created by a force traveling at 5 feet per second or less and producing drops with diameters greater tun 3 millimetres. Medium-velocity spatter- An impact spatter pattern created by a force traveling at 5 to 25 feet per second and producing drops with diameters between 1 and 3 millimetres. High-velocity spatter- An impact spatter pattern created by a force traveling at 100 feet per second or faster and producing drops with diameters less than 1 millimetre.

Describe the methods for documenting bloodstain patterns at a crime scene

Investigators must accurately record the location of specific patterns and to distinguish the stains from which laboratory samples were taken. The photographs and sketches can also point out specific stains used in determining the direction of force, angle of impact and area of origin. The grid method involves setting up a grid of squares of known dimensions over the entire pattern using string and stakes. Perimeter ruler method, involves setting up a rectangular border of rulers around the pattern and then placing a small ruler next to each stain.

Calculate the angle of impact of a bloodstain using its dimensions

It is possible to determine the impact angle of blood on a flat surface by measuring the degree of circular distortion of the stain. Angle of impact- the angle formed between the path of a blood drop and the surface that it contacts. Buildup of blood will show up in angles of larger than 90 whereas longer and longer trails will appear as the angle of impact becomes smaller.

Explain how surface texture, directionality, and the angle of impact affect the shape of individual bloodstains

Surface Texture- In general harder and non porous surfaces result in less spatter. Rough surfaces, such as a concrete floor or wood, usually result in irregularly shaped stains with serrated edges, possibly with satellite spatter. Satellite spatter- small drops of blood that are distributed around the perimeter of a drop or pool of blood and were produced as a result of the blood impacting the target surface. Directionality- An investigator may discern the direction of travel of blood striking an object by studying the stain's shape. As the stain becomes more elliptical in shape, its direction becomes more discernible because of the pointed end of a bloodstain faces its direction of travel. Satellite spatter coming off of parent stains will have the pointed end facing against the direction of travel.

How blood stain patterns are affected

Surface texture and the stain's shape, size, and location must be considered when determining the direction, dropping distance, and angle of impact of a bloodstain. Surface texture is of paramount importance. In general, the harder and less porous the surface, the less spatter results. The direction of travel of blood striking an object may be discerned because the pointed end of a bloodstain always faces its direction of travel. The impact angle of blood on a flat surface can be determined by measuring the degree of circular distortion. At right angles the blood drop is circular, as the angle decreases, the stain becomes elongated.