# Remote Monitoring of the Impact of Oil Spills on Vegetation in the Niger Delta, Nigeria
## 1 Introduction
Among authors that have used remotely sensed vegetation indices to monitor the impact of oil spills on the vegetation area [2, 26]. The Niger Delta region has experienced oil production-related vegetation degradation through two main pathways: oil spills onto the land and gas flares into the atmosphere. For more than four decades, oil exploration and production activities have left a severely degraded environment in the Niger Delta region [27]. However, in recent times, the issue of oil spills in the Niger Delta was exacerbated due to militant activities in early 2006. A substantial amount of crude oil pipeline vandalism in the Niger Delta was carried out by militant groups with the excuse of fighting for better environmental management and development of the region [28]. Approximately 80,000-300,000 barrels (bbl)/day--valued at USD six billion--have been spilt into the environment in the Niger Delta in recent years [29]. Detecting oil spills and their impacts in polluted environs such as mangrove forests can be challenging using in situ measurements and laboratory-based analysis techniques due to security challenges and difficult terrain. As a more practical alternative, satellite remote sensing presents itself as an effective tool for assessing and monitoring vegetation health and status in polluted areas, especially over vast extents [30]. Despite the obvious potential offered, studies attempting to monitor the impact of oil spills in the Niger Delta region using satellite-derived vegetation indices are relatively scarce. Previous studies examining the effect of oil spills on the health of vegetation in this region using satellite-derived vegetation indices include [PERSON] (1998) and [PERSON] et [PERSON]. [PERSON] (1999, 2000). These studies investigated the influence of oil spill volume and time gap (i.e., number of days between oil spill events and subsequent satellite observations) on the condition of mangrove forests in the Niger Delta region using various vegetation indices, with the NDVI producing a stronger correlation between the volume of the oil spilt and the NDVI values. Similarly, the study by [PERSON] et al. [PERSON] et al. (2010) used the best five performing vegetation indices from [PERSON] (1998) to compare the difference between vegetation at spill and non-spill sites. In addition, [PERSON] et al. [PERSON] et al. (2010) identified oil-impacted land in the region by using a random forest classifier in conjunction with Landsat 8-derived vegetation health indices. Further, [PERSON] et al. [PERSON] et al. (2010) monitored the impact of oil pollution on plant species biodiversity at a regional scale by integrating a Hyperion satellite-derived normalised difference vegetation vigour index (NDVVI) and field data. However, these previous studies focus solely on detecting oil spills or determining the impact of oil pollution on the health of only a single vegetation species (i.e., mangrove) or used the absolute NDVI values. Thus, the impact of exposure to oil on the health of other types of vegetation found in the Niger Delta region is still poorly understood. Furthermore, the studies that focus specifically on the health of the vegetation in the region typically only compare the absolute NDVI values for groups of spill and non-spill sites before and after oil spills. However, comparing absolute values in this way fails to account for differences in the influence of other external factors on the health of vegetation that may vary naturally between sites, such as differences in soil nutrients and water availability. The novelty in this research is, to the best of our knowledge, that this is the first study to monitor the impact of the oil spill on three different types of vegetation and to use change-in-NDVI values. Accordingly, there is a clear need for a more comprehensive understanding of the impact of oil spills on vegetation in the Niger Delta region.