One of the most important areas of study in wireless sensor networks (WSNs) is security. This work presents how to maximize data survivability in unattended WSNs (UWSNs) where sensed data cannot be off-loaded to a sink in real time. The characteristics of this network are the presence of intermittent sink and operation in possibly hostile settings. The major goal in this study is to improve the availability of the data in case of failure or compromise of sensor nodes resulting in the unavailability of or the loss of data. This work proposes an on-demand data replication method that can dynamically change the degree of replication to improve data survivability. Analytical results show improved data survivability with less storage consumptions than existing solutions in literature.