# Remote Monitoring of the Impact of Oil Spills on Vegetation in the Niger Delta, Nigeria
## 2 Materials and Methods
### Methods
#### 2.3.2 Selection of Oil Spill Sites and Determining the Impact of Oil Spills on Vegetation  
The locations of the selected oil spill and non-spill control sites (see Figure 2) across the study area were used to extract the corresponding Landsat NDVI values for subsequent analysis. All the sites were selected to ensure they did not fall within the wedge-shaped scan-to-scan gaps caused by the Landsat 7 ETM+ Scan Line Corrector (SLC) failure on 31 May 2003 [53]. Firstly, to determine the impacts of both oil spill volume and the time  
Figure 2: (**a**) Spatial distribution of oil spill sample points for determining the effect of oil spills on the different vegetation types and (**b**) locations of spills and no-spills sites for temporal monitoring of oil spill impacts on vegetation. gap between oil spills and subsequent observations on the vegetation health, oil spill sample sites totalling 55 for dense vegetation, 60 for sparse vegetation, and 61 for mangrove vegetation were identified with oil spill volumes ranging from 10 to 5000 bbl. The vegetation cover class for each spill sample site was determined using high-resolution Google Earth time series imagery to determine the oil spill point that fell on a particular vegetation type. The corresponding post-spill NDVI value was extracted (Figure 3) from within a \(1\times 1\) pixel window around each site from the first available Landsat image acquired after each spill. A \(1\times 1\) window was used since the areal extent of oil spills < 225 bbl is typically smaller than that of a single Landsat pixel (900 m\({}^{2}\)). The extracted post-spill NDVI values for the sample sites were then correlated with both spill volume and time gap since the oil spill occurred in order to determine their effect on vegetation health. The time gap was computed as the difference between the date of a recorded oil spill and the date of the corresponding post-spill NDVI observation. Figure 3 shows Google Maps images of some oil spill sites with their corresponding control sites for some selected dense, sparse, and mangrove vegetation, respectively. Secondly, to investigate whether the health of vegetation exposed to oil differs from vegetation at sites not exposed to oil, pair-wise comparisons of NDVI at spill sites (SSs) with those at no-spill control sites (CSs) were performed. Spill sites were selected for each of the three land cover types: eight for dense vegetation, eight for sparse vegetation, and six for mangroves (Figure 2b). Sites with oil spill volumes in the range of 225-2550 bbl were specifically selected for this analysis based on the minimum volume for which oil pollution is readily detectable in the region [32]. At each of the oil spill locations, the average NDVI value within a \(3\times 3\) pixel window around the site was calculated because oil from spills at this volume range may migrate from the point of source, thereby also affecting neighbouring surroundings [14; 31] The same sampling approach was also used for no-spill sites. For each site, a time series of NDVI values was extracted for a period covering just before the spill and 4-10 years afterwards, depending on when the oil spills occurred. However, in this case, instead of using the absolute NDVI values to determine the temporal response in the health of the vegetation, changes in NDVI at SSs and CSs were calculated relative to the NDVI value just prior to the timings of spills. Utilising the change in NDVI accounts for the possibility that some sites may have inherently higher NDVI values than those at other sites if exposed to different health-affecting external factors. The potential effects of such external factors on the vegetation's health were further mitigated by carefully selecting SS and CS pairs with corresponding conditions, such as water availability, sunlight, soil composition, and climate (Figure 2).