  Gravitational-wave bursts are observable as bright clusters of pixels in
spectrograms of strain power. Clustering algorithms can be used to identify
candidate gravitational-wave events. Clusters are often identified by grouping
together seed pixels in which the power exceeds some threshold. If the
gravitational-wave signal is long-lived, however, the excess power may be
spread out over many pixels, none of which are bright enough to become seeds.
Without seeds, the problem of detection through clustering becomes more
complicated. In this paper we investigate seedless clustering algorithms in
searches for long-lived narrowband gravitational-wave bursts. Using four
astrophysically motivated test waveforms, we compare a seedless clustering
algorithm to two algorithms using seeds. We find that the seedless algorithm
can detect gravitational-wave signals (at fixed false-alarm and false-dismissal
rate) at distances between 150-200% greater than those achieved with the
seed-based clustering algorithms, corresponding to significantly increased
detection volumes: 420-740%. This improvement in sensitivity may extend the
reach of second-generation detectors such as Advanced LIGO and Advanced Virgo
deeper into astrophysically interesting distances.
