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Comparing soil from a crime scene with a sample of soil that may have ransferred to a suspect may allow forensic scientists to create a link between the crime scene and the suspect. Many soils can be distinguished from other samples by their color and texture. While visually looking at the soil samples can show differences, it is important to note that soil vflll look darker when it is wet so color comparisons should be done when the soil samples are dry. Microscopic analysis can also reveal the presence or absence of any vegetation or animal material.
Artificial materials like glass particles are also more easily seen under a microscope. A forensic geologist may use a high- owered microscope to examine the sample for rock and mineral materials that can further tell investigators whether samples show similarity to each other. Due to the wide range of materials that can be found in soil, there are many aspects or points that forensic geologists can compare for two soil samples. The more similarities in these points, the greater likelihood that the two soil samples come from the same place.
However, the range of soil that meets these similarities has to be taken into consideration. For example, if the soil around a crime scene is similar for a mile around the crime scene, it is ess useful than if the soil is only similar within the crime scene or in a particular spot in the crime scene. Rare minerals and rocks or artificial debris can narrow down possible soil matches. Soil samples are typically collected from the crime scene within a 100-yard radius at various spots. This helps show the variation in the soil at the crime scene.
If soil is found on a suspect, that soil will be collected as well as from any alibi locations that the suspect might give. The samples collected will only be surface materials, as the soil may show differences if a deeper sample is taken. Each sample is generally bout a tablespoon or two in volume and each sample should be clearly labeled as to where the location was taken from. Forensic scientists take great care in collecting and preserving soil samples. If soil is found on the bottom of a shoe, for example, the soil is left on the shoe and the entire shoe is taken to the crime lab.
The object must also be stored in such a way that any pieces of dirt that may fall off the object are retained. For example, a piece of clothing with a dried mud spot may be placed in a paper bag, which would collect and save any of the mud that might come off the clothing. Impressions At some crime scenes, impressions from shoes, tires, or other objects may be left. These can vary widely in the quality of the impression, from deep ruts in the ground from a car going through mud to a fine, thin impression of a shoe on a dusty floor.
As with tool marks, impressions can give either individual or class characteristics, depending on the impression and what made it. Photographs of the impression are the first step for crime investigators. A scale, like a ruler, should be included in them to help show the size of the impression. With the right light, the photographs can pick up some of the etail in the impressions. Like dealing with tool marks, forensic scientists may collect the whole object that the impression is in. However, this is not always possible, particularly when the impression is left in the dirt by a tire or shoe.
In these cases, a cast is made of the impression. To do this, the forensic scientist places a frame around the impression to hold the casting material that is poured in. The cast may be made of silicone rubber or dental stone. In the past, plaster of Paris was often used to make casts, but this substance has been widely discontinued for casting since it more easily crumbles than some f the other options. In general, casts need time to sit and cure before they are able to be analyzed. This can take up to 24 hours.
An impression may give investigators some class characteristics to work from, such as the type of tire that an impression is from or a particular brand of shoe. In some cases, the impression may come from a source that has some unique features to it from wear. If the impression can be matched to a known sample impression through its individual characteristics, the forensic scientist can testify that they came from the same source. In the comparison of the two samples, it is mportant for the forensic scientist to look for both individual and class characteristics.
For shoe impressions, a forensic scientist looks at the width, length, size, and the tread. Shoes that have wear patterns may have uneven wear, broken tread, or gouges on the soles that might help investigators find the shoe that made the impression. When an impression is found in dust, forensic scientists may use an electrostatic dust print lifter. With this method, a large negative charge is applied to a plastic coated metal sheet. The dust print will appear as a mirror image on the plastic coated sheet. This technique ay be used on rough surfaces.