# Remote Monitoring of the Impact of Oil Spills on Vegetation in the Niger Delta, Nigeria
## 1 Introduction  
The impact of oil production activities on vegetation cover can be particularly devastating, resulting in either its degradation or complete destruction. This can occur due to changes in vegetation health and vigour following exposure to oil pollution [1, 2, 3], therefore necessitating effective monitoring of vegetation health to help identify and mitigate the impact of oil releases on the environment. The two main approaches for assessing and monitoring the health of vegetation are field-based [4] or through satellite remote sensing [5]. Key advantages of the field-based approach include the ability to acquire measurements at fine (i.e., leaf) scale and obtain under-canopy measurements and the flexibility in obtaining measurements whenever required. Despite these advantages, field-based approaches can consume considerable time, effort and resources, typically restricting their application to small areas and rendering them unsuitable for regional, national and global monitoring [6]. In addition, field-based approaches are challenging to implement in inhospitable locations, whether due to security concerns or inaccessible terrain. Progression in optical sensor technology has facilitated a great opportunity to understand vegetation health/quality at various spatiotemporal scales previously regarded as complex [7]. Remote sensing of vegetation has advanced significantly over the past half-century due to advances in technology and analyses improving the ability to retrieve useful plant biochemical, physiological and structural quantities across a range of spatial and temporal scales [8]. Satellite remote sensing has become an essential tool in vegetation health status monitoring because it provides a means of overcoming the limitations of field-based approaches [9]. For instance, it provides routinely acquired remote measurements over large areas, which permits regional, national and global monitoring, whilst overcoming the challenges of working on the ground [9] in inhospitable locations. Moreover, several satellite missions exist, e.g., Landsat, meaning that the vast historical archives of data that have been acquired enable long-term spatiotemporal analysis of vegetation health and other land cover changes. Accordingly, satellite remote sensing can play an important role in detecting and responding to oil spills and assessing the impact they have on the environment [1, 10, 11, 12, 13, 14]. More broadly, satellite remote sensing data have been used to forecast crop yield and assess natural forest expansion [15], biodiversity and conservation [16], and drought monitoring [17, 18] among other vegetation-related applications. Vegetation indices derived from remotely sensed (satellite) data can be an efficient tool for measuring vegetation status, growth, and biophysical parameters [19, 20]. Among the algorithms developed for remote estimation of biophysical characteristics of vegetation, the mathematical combination of visible and near-infrared reflectance bands is the most widely used [21]. These indices involve measuring electromagnetic energy reflected from vegetation canopies using optical sensors [19, 22]. These sensors can be either multi- or hyperspectral sensors, with the indices computed from the reflectance measured at specific wavelengths. In the case of the former, these indices can be grouped into broadband multispectral vegetation indices (BMVIs) and narrow-band multispectral vegetation indices (NMVIs), depending on the wavebands they use [23]. Time series of vegetation indices like the normalised difference vegetation index (NDVI) can be used to study vegetation dynamics [24], as they have proven to be a robust indicator of terrestrial vegetation productivity [25]. Such time series, therefore, provide a straightforward means of remotely determining and monitoring vegetation health and change from space [14], which is especially beneficial for assessing the impact of oil pollution in areas that are difficult to access on the ground.