Lesson 6: Multiple Burials

Solving multiple burial search problems can be challenging because all transceivers transmit on a single transmit frequency. This results in the potential for overlapping signals. Manufactures attempt various approaches to differentiate between multiple signals, and all must be learned and practiced in order to reliably solve multiple-burial situations. They are:

Signal separation functions (marking)
Scan functions
Alternate search functions, including analog modes
Scroll list of buried subjects

Frequency of Multiple Burials

It is not absolutely clear from literature how frequently multiple burials occur, however it is obvious that most avalanche incidents with multiple full burials result in one or more fatalities. The importance of avoiding multiple-burial situations by applying safe travel techniques cannot be overstated.

When to Apply the Appropriate Search Strategy: The Mental Map Concept

Searchers must constantly evaluate the situation during a multiple burial situation. Key pieces of information on the mental map are:

Size and nature of search area
Total number of burials
Number of signals in receiving range
Number of signals in close proximity (<15 meters)
Number of signals in very close proximity (<2 meters)
Available number of rescuers

In most situations, a standard search mode with a marking function will be the most appropriate search strategy. Alternate search strategies may only be necessary when close-proximity burials (<15 meters) are suspected and the device’s marking functions have failed. When subjects are spaced farther apart, searching with the standard search-strip width is usually sufficient, and the search strategy is conducted as a series of single-burial searches.

Information on Number of Burials

At the start of the search, it is unlikely that all transceiver signals are within the searcher’s receiving range. Information on the number of received signals (and within what range) strongly depends on the features of the transceiver:

Analog Only: A single-antenna analog transceiver always indicates all received signals. The number and strength of the beeps indicates the range and number of buried subjects. The searcher must be trained and have practiced enough to interpret the data and separate the signals.

Digital/Analog: Digital devices with an analog mode indicate the distance and direction of all received signals, and provide the raw analog beep sound for all received signals. The distance indication gives important information about the radius of the multiple-burial problem. For instance, if the searcher can clearly distinguish two beep sounds and the distance indication reads 10 meters, then the searcher knows with certainty that there are two buried subjects within 10-15 meters.

Digital Only: Digital devices without an analog mode are the most challenging from which to obtain exact information on the number and range of the received signals. Distance and direction may jump back and forth between two signals of roughly even strength. The user must practice in order to interpret the variable distance indications that are often present during a multiple burial.

Marking Function: If the device provides a list of buried subjects or a multiple-burial indication, then the device will only provide distance and direction information for the burial that is highlighted in the list.

Device-Specific Transceiver Functions to Assist with Multiple Burials

Multiple-Burial Indication. Most modern transceivers indicate whether multiple signals are received; however, the information displayed varies between devices. Some include an LED that only indicates that multiple signals exist, while others show the exact number of signals with distance information.

Scan Functions. Digital transceivers usually display only the strongest received signal, even if there are multiple overlapping signals. Some transceivers provide a scan function, which allows the searcher to see all signals within the receiving range. These functions may be helpful for the searcher to create a mental map of the search area to help determine the best search strategy.

Marker Functions. A marking function allows the processor of the transceiver to suppress the signal of a located transceiver in a multiple-burial scenario. In order for the transceiver to recognize and suppress a signal, the signal must have properties that make it unique and consistent. The algorithm of the digital processor needs a certain amount of time to clearly recognize the signal as distinct, which may result in a delay between the initial acquisition of the analog signal and the recognition of separate signals.

Signal-Overlap Indication. Some devices provide an indication that a signal-overlap situation is occurring. This “stop”, “stand still,” or frozen display message means that the device cannot derive any useful information from the overlapped signal. The searcher must stop searching and wait for the overlap condition to resolve itself before continuing with the search.

Analog Mode. Some devices provide access to raw analog sounds. This function can be used as support for alternate search strategies.

W-Link Support. Pulse Barryvox and Arva Pro-W use a second high-speed data channel to exchange information between devices that support W-link. Such information includes identifier data that increases reliability of signal separation and marking.

Vital Data. Barryvox Pulse provides Vital Data information from other Vital Data capable transmitters. The pulse is equipped with motion sensors that detect subtle movement (signs of life), or Vital Data. This data can be used as criteria in the remote triage of multiple burials.

Problems with Marking Features

Marking functions of transceivers are not 100% reliable. Common problems encountered when attempting to mark a signal may include:

Transceivers not recognizing all buried transceivers.
A signal unable to be “marked” due to overlapping signals or an inability to recognize a distinct signal.
The performance of the marking feature decreasing when there are four or more signals.
A marked signal becoming unmarked, resulting in the same burial being found again.
Reduced receiver range once a signal has been marked.
Misleading directional arrows once a signal has been marked and other signals are present.
Non-error-tolerant marking strategies; failure to mark a found device may result in the inability to continue with an additional coarse or signal search.

Signal Overlap. Overlapping signals are one of the biggest challenges for transceivers in multiple burials. When signals are overlapped, no useful information about the second signal is available to the device. The only option is to wait until the signal overlap resolves itself, which can take up valuable time.

Signal overlap explains most of the above-listed shortcomings. Other reasons that cause issues in recognizing multiple signals are:

Variations in frequencies (older devices).
Receiver band width (may not recognize a device with a drifted frequency).
Constant carrier signal as a result of local interference or analog technology (may be interpreted as a distant signal).
Ineffective search strategy (failure to apply systematic search pattern, or poor balance between search speed and search precision during the different phases of the search)

Alternate Search Strategies

Multiple Single Burials. In many multiple-burial scenarios, the spacing between subjects is large enough (>10 meters) so that no specialized technique is necessary. Instead, a series of single transceiver searches are undertaken. In this case, it is important that the entire potential search area has been scanned with a standard search-strip width.

Searching in Parallel with Multiple Searchers. This technique involves using as many searchers as possible, with the objective of narrowing each rescuer’s search-strip width.

Procedure:

The rescue leader adjusts the search-strip width according to the size of the deposit area and the number of available searchers.
Each searcher commits to the coarse and fine search only after his or her distance reading is lower than the designated search-strip width. This eliminates redundancy and ensures that each searcher locates a different subject.
If necessary, the search-strip width can be adjusted by the rescue leader after each subject is pinpointed.

Micro Search-Strip Method. The micro search-strip method is effective when there are two or more transmitters buried in less than 15 meters and when attempts to use the marking function are unsuccessful. Analog mode is beneficial in this search technique.

Procedure:

Start on a standard search-strip width. Once more than one signal is detected in close proximity and attempts to use the marking function are unsuccessful, the searcher switches to a micro search-strip method.
With a digital/analog or analog-only transceiver, the range must be set to low. With a digital-only transceiver, the searcher must ignore direction indication values and focus only on the distance indication.
The searcher walks back and forth on 2-5 meter-wide* micro-strips to pinpoint each transceiver individually. After the first signal is pinpointed, the searcher steps back to the point where the multiple signals in close proximity were detected, and starts with the micro-strip pattern.
Once additionally transceivers are located, the searcher goes back to where they left the strip and continues with the micro-strip search method.
Using a digital-only transceiver, the searcher only leaves the search strip to fine search when the distance indication drops below 5 meters. This approach takes practice as there is a strong tendency to follow the directional arrow.

*Determining the length and width of the micro-strips:

Length: As soon as the distance reading significantly increases above the largest distance of the indicated subjects.
Width: The distance between the closest burials. For example, three burials are indicated within 12 meters and two burials are indicated at 6-7 meters. The width of the micro-strip should be fairly small, at approximately 2 meters.

Three-Circle Method. The three-circle method utilizes a similar approach to the micro search-strip method, locating each transceiver one by one, by using a series of concentric circles around the first located transceiver. Each circle increases in radius by 3 metres. The probe (or other object) above the first located subject provides a visual guide to search along the three circles, each of which is approximately a probe length apart.

When using a digital-only transceiver, the searcher must ignore the already-pinpointed signal in the centre of the circle.
The three-circle method is more advantageous in larger search fields, as the search area is more clearly defined.

Micro-Box or Micro-Circle Method. When two or more burials are suspected in less than 2 meters, a box or circle with about 1-meter radius around the first pinpointed signal helps locate the remaining closely buried subjects. Again, it takes practice to ignore the signal that has already been pinpointed when using a transceiver in analog mode. A digital transceiver without an analog mode may jump back and forth between the two signals, or only indicate the first stronger one until the searcher is directly above the next transmitter. The micro-box and micro-circle methods can be combined with the micro search-strip and three-circle methods, as well as with all other standard search techniques.

Comparison of Alternate Search Strategies

The three-circle method should be the method of choice for areas greater than 30 meters in length or width. Experience has shown that determining the area to apply the micro-strip method in larger areas in not easy.

For smaller search areas less than 30 meters in length or width, the micro search-strip method is recommended. In smaller search areas with burials in very close proximity, the micro-strip method is more efficient. The width of the micro-strip can be adjusted to the spacing of the burials with the shortest distance apart. See the following table for a comparison of multiple-burial search methods:

Device-Specific Recommendations to Find Multiple Burials

Tracker DTS and Tracker 2: This device has an SP mode that helps recognize other transceivers in the scenario. This can be used to help find up to four transceivers, but takes practice and discipline to use. The multiple-burial LED indicator on the Tracker 2 can help identify when there is more than one signal in the general area and more than one in close proximity. At that point, the micro search-strip or three-circle method can be applied if searchers are not confident using the SP mode.

Barryvox Pulse: All searches should begin by using the standard mode with the marking function. If a “stand-still” message appears in the display, the device has detected a signal overlap. The best action is to stop and wait for the signal overlap to end, usually in five to ten seconds. If it doesn’t resolve itself, the marking function will not be reliable and an alternate search strategy should be employed.

If marking does not work, leave standard mode and go into backup mode. In backup mode, the analog sounds are available to the searcher and the device will show the distance indication to the strongest signal instead of showing the distance indication only to the highlighted burial. In backup mode, the searcher should use the micro search-strip or three-circle method.

Barryvox Opto 3000: A multiple-burial indicator light is built into this unit. Analog sounds automatically come on when a multiple burial is detected. The optional analog mode for multiple burials requires searchers to be comfortable with analog searching and using micro search-strip or three-circle methods.

Ortovox S1: This device can display up to three buried subjects on screen and has a marking function. If marking does not work, go into 4+ mode (which will automatically reduce the sensitivity down to about 5 meters) and turn on the analog beep sounds after the first burial has been marked. At this point, the micro search-strip or three-circle methods must be used. A “stop” message may appear if there is a signal overlap, and in some cases the transceiver will resolve itself in 5-10 seconds.

Ortovox 3+: This device has a multiple burial indicator for up to three signals with a marking function. If marking fails, try the micro search-strip or three-circle method. Ortovox also advises a method “sector” searching, which is like attacking the scenario from different quadrants.

Pieps DSP: This device has a multiple-burial indicator for up to three signals with a marking function. If marking does not work, try the micro search-strip or three-circle method.