Metal Detectors in Evidence Search and Recovery
By Sam Chan
The captain calls—a shooting just occurred. There are multiple victims, and the suspect is in custody. Investigators and crime lab personnel are on scene, but they cannot locate the spent casings. They will secure the site overnight, and the search team must be there at 6:30 a.m. with metal detectors.
When the team members arrive, investigators brief them on the circumstances of the shooting and describe where they believe it occurred. No information exists on the trajectory of the spent casings. The task is to search for .40 caliber casings in the 40' by 40' front yard of the residence.
The yard is well maintained. As team members approach the front of the house, they see the garage on the left, with shrubbery of varying sizes lining its wall. On the right is a neatly trimmed lawn. There is an apparent bloodstain on the grass.
The search team decides to tackle the most difficult search areas first, starting with the wall of the garage. If the casings landed in the bushes, they will be difficult to locate; the nails and copper wires in the wall will complicate the search. Team members decide to pull the shrubs away from the wall and search the ground. They find numerous roofing nails and other trash near the garage wall, but no casings.
Reserve Sergeant Chan serves with the Orange County, California, Sheriff's Department.
Next is the lawn. The team must search it systematically to ensure complete coverage. With two detectors, operators start at opposite corners and work their way up and down the lawn, ending in the middle.
As a detector approaches the stain on the lawn, it emits a tone. To avoid touching the casing with their hands, team members use a handheld pinpointer detector to locate it in the grass. Then, the crime lab technician documents and recovers it.
Team members search the rest of the lawn but do not find any more spent casings.
Creating a Search Team
A metal detector serves as an invaluable tool in the search for and recovery of metallic evidence. Some law enforcement agencies rely on external groups to provide this service, but various concerns about crime scene integrity—for example, the chain of custody for evidence and required documentation of the search—can make this practice problematic. To ensure admissibility in court, individuals operating the detectors must be vetted, properly trained, and accountable to the agency. This reduces the chance of missing or mishandled important evidence.
In response to this issue, in 2014 the Orange County, California, Sheriff’s Department (OCSD) established a metal detector search team. The agency made important considerations regarding equipment selection and the development of training for metal detector use. Other departments can use this information in their decision-making process when creating similar programs.
OCSD’s metal detector search team comprises part of the search and rescue reserve unit (SRRU), one of many units in the reserve bureau. Except for the lieutenant and sergeant, the reserve bureau consists of a mix of reserve deputies—sworn law enforcement officers—and nonsworn professional service responders (PSRs). Both reserve deputies and PSRs are volunteers who passed background investigations before joining the department.
SRRU searches for lost people, crime scene evidence, and potential weapons in jail facilities. Because most weapons contain some amount of metal, a metal detector proves important for evidence searches. In 2014, SRRU began to standardize equipment and formalize training for metal detector operators.
When the metal detector search team was created, detectors available to SRRU consisted of models that had appeared on the market around 2001. Since that time, detectors’ electronics, coil design, target-discrimination logic, and operator interface have improved in various ways.1 These upgrades translate into better depth penetration to locate evidence buried deeper in the soil; greater ability to determine the type and size of a target; and improved displays and controls, making detectors easier to learn and use.
To select an ideal ground-search metal detector, departments must consider the design and features they will find most useful. There are two main types of metal detectors on the market today: induction balance (IB) and pulse induction (PI).2 Each has benefits and drawbacks. For example, the PI design is superior in saltwater environments, so a metal detector designed for underwater use typically uses PI. However, in general, the IB design is easier to use and provides the best overall performance for ground-search applications.
One of the advantages of the IB design is its ability to determine the relative conductivity of the target and present that information on an analog or digital display. This target identification helps searches for a particular type of metal, especially if there exists a sample target from which to calibrate. The detector’s target discrimination enables it to provide an audio tone only when the target ID is within a specified range.
For example, when searching for a brass casing, the team can scan a sample to determine the ID range. Then, they can set the discrimination so the detector will only emit a tone when it detects that range of target ID. This saves a lot of time, especially in an environment full of trash. However, operators must use target discrimination cautiously. Target masking can cause users to bypass a valid target if they are too aggressive in setting discrimination.3
Another important feature is ground balancing.4 Depending on the geology of the area, soil may be conductive and produce a false signal. Salt or brackish water also can cause this problem. Certain metal detectors have features to adjust ground balance either manually or automatically.
IB detectors interact poorly with other detectors operating at the same frequency. This presents an issue for agencies that want to equip their units with more than one metal detector. Using multiple makes of detectors may make training and proficiency more difficult to accomplish. One solution is to look for a model that can make minor adjustments to its operating frequency to avoid interference. This ability also helps when searching near power lines or radio frequency transmission sources.
The last feature to consider for ground-search metal detectors is the availability of different sized search coils. One size coil does not fit all. For most searches, the default choice is the largest coil available for the unit. Such a coil provides the greatest coverage per sweep and the best depth penetration. In some cases, though, the terrain may not allow a large coil to get close to the ground. For these situations, users need a smaller coil.
“To ensure admissibility in court, individuals operating the detectors must be vetted, properly trained, and accountable to the agency.”
In addition to the ground-search detector, a handheld pinpointer detector is essential. The ground-search detectors isolate the general location of the target. A smaller and more precise detector assists in locating the target during excavation. The handheld detector does not need to have target ID or discrimination capabilities, but a means to adjust the sensitivity or rebalance is desirable.
All SRRU members have prior training in evidence search and proper conduct at a crime scene. This assisted in the development of the team because metal detector operation was the only extra training required.
Very little metal detector training exists with specific application to crime scenes. This issue, combined with budget constraints and the need to train a large cadre of operators, may make in-house training development necessary. The training needs to strike a balance between the theoretical and the practical. A thorough understanding of how detectors work enables an operator to make decisions in the field. OCSD’s training consists of classroom discussion to cover concepts and field exercises to provide hands-on experience with the detector.
The two most important theoretical concepts are relative conductivity and the effect of the search coil’s configuration. Because the target ID of an IB detector is based on relative conductivity, users need to know the different types of metals and their conductivity. Coil size and configuration affect the shape of the detection field. Operators need to understand this so they can make appropriate choices during a search.
On the practical side, training needs to focus on the specifics of the detector that the team will use. There are many buttons on the control panel of a modern metal detector. For those who get nervous about pushing buttons and changing settings, this can be intimidating. However, as with most electronic devices, there is a way to perform a factory reset, which will take the device back to a fresh-out-of-the-box state. This is one of the first things operators should learn.
Although metal detector manufacturers market some models for law enforcement, they originally design them for treasure hunters. Because of this, the detectors include specialized modes for different targets. Instructors must describe these modes, how to change them, and when to use them so that operators can justify such decisions.
The target ID and audio tone depend on the model of the detector. Instructors should demonstrate the response of the detector to different types of metal. This is most effective in a field trial with known targets that are typical of the metals that operators will encounter. An assortment of various ammunition casings will allow operators to see the difference in target ID between aluminum, brass, steel, and nickel-plated brass. A diverse collection of bullets—for example, lead, copper jacketed, undamaged, and deformed—also is useful for demonstration.
“The detector’s target discrimination enables it to provide an audio tone only when the target ID is within a specified range.”
One of the more difficult skills to learn is the proper swinging motion of the detector. It may seem simple, but it can be challenging to keep the detector coil flat and parallel to the ground—and not roll the edge of the coil up—through the full range of the sweep.
The orientation of the target relative to the detector coil has a significant impact on the signal return. This can be demonstrated using a long, thin nail. The detector will respond differently depending on whether it scans the nail parallel or perpendicular to its length.
This discrepancy always requires the operator to rescan an area in a direction perpendicular to the original scan. For example, if operators start to scan an area by sweeping the coil east to west, they should scan again with the coil sweeping north to south.
Once operators are comfortable detecting an isolated target, they must learn about target masking. This is the effect an undesirable target can have on a desirable target adjacent to it. For example, when team members searched near the garage in the earlier example, they found many roofing nails. If the spent casing had landed next to a nail, the combination of the nail and brass may have resulted in a different target ID, depending on the orientation of the two targets and the direction of the sweep.
A final training exercise is to let the operators locate and recover any targets they can. An old construction site or a dirt lot is the best location for this due to the variety of targets and minimal concern for damage to landscaping.
Metal detectors add enormous value to crime scene investigations by helping recover critical evidence, but many agencies do not take full advantage of them. Manufacturers have greatly improved the devices’ capabilities and simplified their operation in recent years. With appropriate equipment selection and training to ensure precise and methodical searches, it is within the means of most police departments to develop and maintain this skill in-house, instead of outsourcing it to external groups.
“If the spent casing had landed next to a nail, the combination of the nail and brass may have resulted in a different target ID, depending on the orientation of the two targets and the direction of the sweep.”
Reserve Sergeant Chan can be reached at firstname.lastname@example.org.
1 For a brief overview of metal detector terminology, see Dan Tennant, “The Metal Detector Glossary: What Does All This Stuff Mean?” Top Ten Reviews, September 5, 2013, accessed June 17, 2019, https://www.toptenreviews.com/the-metal-detector-glossary-what-does-all-this-stuff-mean.
2 Jeff Tyson, “How Metal Detectors Work,” HowStuffWorks, May 23, 2001, accessed June 17, 2019, https://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector.htm.
3 “Metal Detecting Techniques and Methods: How to Detect Masked Targets,” Metal Detecting World, accessed June 17, 2019, https://www.metaldetectingworld.com/masked_target_detecting_technique.shtml.