When more than one person is buried in an avalanche, the searching transceiver receives multiple signals. If you have a limited number of rescuers (e.g., probably four or fewer), the best strategy is usually to use your transceiver to find the closest victim, dig him up, turn off his transceiver, and then search for the next victim. With limited resources, and with the realization that more than half of avalanche victims are dead within 15 minutes, it's best to put all your effort into quickly saving one victim rather than slowly attempting to save two. If you do have enough people to extricate multiple victims, then it's best to locate the first victim with your transceiver and, while other rescuers begin unburying the first victim, search for the next victim.
Special search techniques are required when searching for subsequent victims, because your transceiver will be receiving beeps from more than one transceiver. Consider two cars at an intersection with their turn signals on. If the cadence of the turn signals is not identical, there will be times when the lights of both cars are blinking at the same time, periods when they are partially overlapping, and periods when the lights are blinking independently. This also happens with two transceivers. There are times when both transceivers are transmitting a "beep" at the same time ("signal overlap"), times when they are partially overlapping, and times when the beeps do not overlap. Unfortunately, when the beeps overlap, the searching transceiver receives this as one beep. This makes it difficult for the searching avalanche transceiver to distinguish between multiple victims.
Articles about avalanche rescue often claim that avalanches with multiple victims are rare, I guess everything is relative, but between 10% and 30% of avalanche accidents do involve multiple victims.
Some avalanche transceivers analyze the multiple signals and attempt to to estimate the number of, and the location of, the victims. These transceivers then allow you to "suppress" a specific signal. This is done by "ignoring" a signal based on the previously analyzed signals. The buried beacon will continue to transmit a signal, but the searching transceiver will try to ignore that transmission and guide you to the next-closest transceiver. Manufactures refer to the process of ignoring a signal as either "marking," "masking," "suppressing," or "flagging."
Transceivers use various techniques to analyze the multiple, occasionally overlapping, signals. Although the algorithms are proprietary, some transceivers, like the Pulse, appear to focus on the cadence in addition to the signal strength of the multiple signals. Transceivers that use this approach take a little longer to analyze the multiple signals, but tend to do a better job at reporting the number of transmitters and at suppressing the individual signals. Other transceivers, like the Tracker3, appear to focus more only on the signal strength. These transceivers tend to do well when there are only two transceivers and less well when there are more than two transceivers.
When there are 3 or 4 victims, it becomes almost impossible for the searching transceiver to separate the signals due to signal overlap. It is also more difficult for the transceiver to separate the signals if the victims are located close to each other, because it is harder for the searching transceiver to distinguish the signals based on their strength. And finally, digital transceivers can be confused when receiving transmissions from older (single-antenna analog transceivers) that emit a continuous carrier signal.
Although none of the transceivers are perfect at suppressing a signal, they do a pretty good job. If your avalanche transceiver mistakenly returns you to a victim you previously suppressed, you should repeat the suppression process. You should always locate the victim with your probe before suppressing the signal.
Testing a transceiver's ability to suppress a signal is complicated because there are so many variables (e.g., the amount of time the signals overlap, the individual cadence of the beacons, the distance between the transceivers, how quickly the searcher moves toward the second transceiver after suppressing the first, the starting location of the search, etc). The individual transceiver reviews provide details on how to search for multiple burials using that model of transceiver. It is very important that you read the user's manual that comes with your beacon and that you practice using your transceiver's multiple burial features—none of these techniques are intuitive enough to figure out after the avalanche strikes. You can read the details of multiple burial tests here.
In addition to using a transceiver's built-in electronics to suppress a transmitter, you can use "generic" multiple burial search techniques locate multiple victims. The advantage of these generic techniques is they can be used with any avalanche transceiver. The disadvantage is they require significantly more training and practice.
The expanding circle technique works well and is relatively easy to learn. The disadvantage is you have to cover a lot of ground and walk uphill to complete the circles.
In this illustration, the red line represents your search path to the first victim. You would then move 3 meters from the victim and circle the victim as shown by the smallest blue circle. As you walk around the victim on this 3-meter circle, your transceiver will continue to point to the first victim and display a distance of approximately 3 meters.
After completing the first circle without a significant change in the displayed distance, you would enlarge the circle by 3 meters. While walking on this second circle you should expect the distance to be approximately 6 meters (because you are now six meters from the first victim).
In this example, as you get closer to the second victim than the first, the distance displayed on your transceiver will drop from the expected 6 meters down to approximately 1 meter. You should then use the coarse and fine search techniques to locate the second victim.
The distances displayed by your transceiver in this example, i.e., 3 meters when on the 3-meter circle and 6 meters when on the 6-meter circle, are very rough estimates—don't take them too literally. Mathematically, the distance would also include the depth of the first victim and, realistically, the distances displayed by avalanche transceivers aren't that accurate. It's more important that you watch for a significant change in the distance (and possibly a change in the direction indicator). It's critical that you understand the concept of the expanding circle technique which is that by making 3-meter circles, at some point you will end up closer to the second victim than to the first victim.
This technique is similar to the expanding circle technique in that you search within 3 meters. It is physically easier than the expanding circle technique, because you don't have to walk up hill (that makes it especially helpful on steeper terrain). Conceptually it is more difficult, because the distance between you and the first victim changes (whereas in the expanding circles, you remain the same distance as you circle the victim).
In this example "micro search strip" illustration, the red line shows your initial search path to the first victim. You should then move a few meters up hill and continue searching. Your directional indicator will continue to point to the first victim and the distance will decrease as you move further way from the first victim.
When you zigzag back, the distance will decrease as you approach the first victim and then increase as you pass the first victim.
When you eventually get closer to the second victim than the first victim (this is the point where the blue line changes to a dotted line in this illustration), your transceiver will point toward the second victim and the distance indicator will decrease to display the distance to the second victim. When the displayed distance is close to the width of your micro search strips (i.e., about 3 meters), you can use the fine search technique to locate the second victim.
As with the expanding circle technique, don't take the displayed distances and direction indicator too literally. It's more important that you watch for a significant, unexpected change in the distance (and possibly a change in the direction indicator). It's critical that you understand the concept of the micro search strip technique which is that by making 3-meter search strips, at some point you will end up closer to the second victim than to the first victim.
Details on the micro search strip technique can be found here.