Week 3B - 4 Blood Spatter Analysis

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Del curso dictado por Nanyang Technological University, Singapore

Introduction to Forensic Science

467 calificaciones

Nanyang Technological University, Singapore

Introduction to Forensic Science

467 calificaciones

We have all seen forensic scientists in TV shows, but how do they really work? What is the science behind their work? The course aims to explain the scientific principles and techniques behind the work of forensic scientists and will be illustrated with numerous case studies from Singapore and around the world. Some questions which we will attempt to address include: How did forensics come about? What is the role of forensics in police work? Can these methods be used in non-criminal areas? Blood. What is it? How can traces of blood be found and used in evidence? Is DNA chemistry really so powerful? What happens (biologically and chemically) if someone tries to poison me? What happens if I try to poison myself? How can we tell how long someone has been dead? What if they have been dead for a really long time? Can a little piece of a carpet fluff, or a single hair, convict someone? Was Emperor Napoleon murdered by the perfidious British, or killed by his wallpaper? *For Nanyang Technological University (NTU) students, please be noted that this course will no longer be eligible for credit transfer.

De la lección

Time of Death; Blood

Week 3A - 1 Recent Deaths10:39

Week 3A - 2 Decomposing Bodies I (Putrefaction)6:13

Week 3A - 3 Decomposing Bodies II (Forensic Entomology)11:50

Week 3A - 4 Analysis of Skeletal Remains I8:29

Week 3A - 5 Analysis of Skeletal Remains II11:37

Week 3A - 6 Ötzi Case; Summary3:36

Week 3B - 1 Blood13:02

Week 3B - 2 Tests for Blood6:54

Week 3B - 3 Precipitin Technology3:53

Week 3B - 4 Blood Spatter Analysis8:17

Week 3B - 5 Lord Lucan Case; Summary7:04

Conoce a los instructores

Roderick Bates
Associate Professor
Division of Chemistry & Biological Chemistry | School of Physical & Mathematical Sciences

[MUSIC]

So we've talked about blood biochemistry

and how we can use blood biochemistry to determine

where blood comes from, and who it belongs to.

So now let's talk about some blood physics and

see if we can work out how the blood got there.

So what we're going to do is take a look at some blood patterns,

and see if we can reconstruct the crime from the pattern of the blood.

So if there is blood on the suspect, from looking at how

the blood is distributed, can we understand how the crime was committed?

The technical term for this distribution of the blood on

the suspect in the crime scene is called blood spatter,

so what we're going to do is blood spatter analysis.

So, here are some typical marks in blood that might be found at a crime scene.

Both of these marks come from the transfer of blood.

So in this case, blood has got on an object

and then has been transferred to the place where it's found.

So the one on the left, for instance, clearly someone has

stepped in blood, and this is a partial bloody foot print.

The one on the right is a hand print in blood.

Someone has got blood on their hand, and their hand is

covered in blood, and then it's been smeared on the wall here.

The pattern, the way that the blood is distributed, can tell

you about where the blood came from and how it got there.

So if in a violent crime an artery is severed,

then there will be a very characteristic pattern of arterial spurting.

So, because arteries come directly from the

heart, the pressure in the artery varies

as the heart beats, and this means that blood will spurt from the artery.

So instead of a continuous flow, you will get a spurt with each

heartbeat, and this gives a splash of blood, in this case, on the wall.

And because it's a relatively large amount of blood coming

from the artery, it will then dribble down the wall.

This is called arterial spurting.

On the other hand, if someone has a much smaller wound,

and for instance, is standing there wondering what to

do, then you will get a pattern of blood drips.

Now, the exact pattern will depend on the surface in question,

and also how long the person stands there with blood dripping into the pool.

The pattern created by flying blood will also depend on

the violence of the event that caused that blood to fly.

If it's a relatively low energy event, for instance, an

ordinary fight, then you will get low velocity blood spatter.

This can also be from blood that is falling

from a wound, and also blood that is cast off.

For instance, if someone is struck with a weapon and the assailant

then pulls the weapon back to take a second blow, as he

pulls the weapon back, there will be some blood drops cast off

from the weapon, and these would also give you low velocity blood spatter.

Thinking about someone being attacked by a weapon,

for instance, being beaten with a baseball bat,

this is a relatively energetic event.

The blood that's flying around will move somewhat faster, and this

will give medium velocity spatter as shown in the picture here.

The most energetic events would, for instance, be associated with gun shots,

and this gives high velocity blood spatter, where you have a lot

of small droplets of blood moving very fast, a little bit like a mist.

So this is high velocity blood spatter.

So by looking at these patterns of blood, the forensic

scientist can determine something about what happened in the crime.

Now, let's consider a falling drop of blood.

A blood drop, as it falls, is roughly spherical.

And if it hits the floor at an angle of

90 degrees, then you will get a circular blood spatter mark.

But suppose that drop of blood doesn't hit the floor at 90 degrees,

suppose it hits the floor at an angle,

then you will not get a circular pattern on the floor,

you'll get this elongated shape.

And maybe, if the blood, as it hits the

floor, has enough energy, there will be a little splash.

So in front of the main elongated shape, there will be a

little extra dot of blood that bounced out from the main droplet.

So for instance, you may have a pattern like this.

So here the blood has come from the left, because you can

see the main shape followed by a smaller droplet on the right.

In fact, we can correlate the shape of these

blood droplets with the angle at which they hit the surface.

So when you have a 90 degree angle, we get this circular pattern,

and as you go through to a very acute angle, it becomes more and more elongated.

So with a very acute angle of ten

degrees, you get this very long blood droplet shape.

[BLANK_AUDIO]

Now, suppose at the crime scene you

have multiple blood droplets in different places.

We can work out which point in space was the origin

of those blood droplets by plotting lines backwards from the droplets.

Where all of the lines cross, which is called the point of convergence,

this would have been the source of the blood.

Now when we look at a diagram like this in two dimensions,

it looks very simple, but of course, crimes occur in three dimensions.

So, we have to do this kind of blood spatter analysis in three dimensions.

So if we can plot the lines back from the bloodstains, for instance, on the

wall to a point of convergence, then we

know in three dimensional space where the crime happened.

So in this example for instance,

plotting the lines back from the blood stain pattern on the wall gives us the

height above the floor at which this event that generated the blood happened.

So if that height above the floor is say, five feet,

then we'd know the victim was still standing at the time.

If that height above the floor is only a few inches, then of course,

we know the victim was already on the floor when they were being beaten.

And this is very important evidence for reconstructing the crime scene.

Now of course, these diagrams we're using here are very, very simplified.

And in a real situation, it would actually be

much more complicated to work out the point of convergence.

In fact, if you have a crime scene with

even more blood than this, it may just be impossible

because there's just too much blood at the

crime scene to be able to interpret the situation.

[BLANK_AUDIO]

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