An investigation into the factors influencing the detectability of oil spills using spectral indices in an oil-polluted environment

Figures & data

Figure 1. The Niger Delta is shown in green colour within the map of Nigeria.

Figure 2. Landsat data in false colour composite (bands 4, 3, and 2) showing oil spill points and pipelines in the study area.

Table 1. Year of oil spills, sample points, and available image data.

Year of Spill	Sample Points	Acquisition Date
1985	2	17 January 1986
1986	9	19 December 1986
1998	7	21 February 1987
1999	11	29 November 1999
2000	10	17 December 2000
2002	6	8 January 2003
2004	4	26 November 2004
2006	5	19 January 2007
2007	1	19 January 2007
Total	56

Figure 3. Landsat-5 and -7 images in false colour composite (bands 4, 3, and 2) of the study area.

Table 2. Number of sample points, volume of oil spill, and time gap between oil spill and image dates.

Sample Points	Volume Of oil (bbl)	Time (Days)	NDVI	NDWI	Sample Points	Volume	Time (Days)	NDVI	NDWI
SP1	558	2	0.27	0.02	SP29	346	135	0.2	0.44
SP2	813	3	0.25	0.19	SP30	62	144	0.32	0.34
SP3	150	5	0.17	0.42	SP31	1042	158	0.24	0.22
SP4	352	11	0.18	0.22	SP32	200	161	0.33	0.35
SP5	28	12	0.19	0.44	SP33	1720	163	0.18	0.07
SP6	50	14	−0.09	0.33	SP34	150	169	0.09	0.19
SP7	2761	20	0.26	0.1	SP35	97	184	0.45	0.39
SP8	2	32	0.18	0.24	SP36	3500	198	0.17	0.38
SP9	54	33	0.17	0.42	SP37	269	206	0.13	0.35
SP10	318	35	0.45	0.34	SP38	96	221	0.32	0.35
SP11	1069	37	0.2	0.4	SP39	2578	234	0.04	0.44
SP12	63	45	0.15	0.25	SP40	232	237	0.34	0.43
SP13	400	46	0.42	0.26	SP41	126	254	0.06	0.22
SP14	40	47	0.11	0.07	SP42	2573	270	0.13	0.36
SP15	117	48	0.03	0.36	SP43	75	281	0.31	0.28
SP16	180	52	0.12	1.03	SP44	625	300	0.13	0.44
SP17	39	69	0.2	0.08	SP45	128	306	0.35	0.17
SP18	155	70	0.17	0.19	SP46	10	363	0.06	0.20
SP19	75	75	0.51	0.38	SP47	507	370	0.47	0.39
SP20	10	80	0.06	0.43	SP48	27	378	0.42	0.43
SP21	500	89	0.35	0.17	SP49	20	378	0.32	0.25
SP22	5	96	0.22	−0.08	SP50	785	474	0.23	0.14
SP23	184	97	0.37	0.29	SP51	468	560	0.40	0.31
SP24	32	99	0.34	−0.05	SP52	500	650	0.37	0.34
SP25	63	116	0.27	0.36	SP53	1000	707	0.48	0.40
SP26	9	118	0.24	0.31	SP54	807	708	0.48	0.41
SP27	221	127	0.31	0.32	SP55	358	819	0.38	0.39
SP28	1734	134	0.2	0.22	SP56	1505	844	0.33	0.34

Figure 4. Relationship between NDVI and NDWI and all volumes of oil spill.

Figure 5. Relationship between (a) NDVI and (b) NDWI and volume of oil spill where the volume was greater than 225 bbl.

Figure 6. Variation in relationship between NDVI and NDWI and volume of oil for different oil spill volume ranges: 6(a) and 6(b), 225–400 bbl; 6(c) and 6(d), 401–1000 bbl; and 6(e) and 6(f), 1001 bbl and above.

Table 3. Time gap between oil spill and image dates and NDVI and NDWI for 25 spill sites with volume of oil spill volumes of more than > 225 bbl.

Sample Points	Oil Spill Volume (bbl)	Time Gap (Days)	NDVI	NDWI
SP1	558	2	0.27	0.02
SP2	813	3	0.25	0.19
SP3	352	11	0.18	0.22
SP5	2761	20	0.26	0.10
SP6	318	35	0.25	0.34
SP7	1069	37	0.2	0.40
SP8	400	46	0.22	0.26
SP9	500	89	0.35	0.17
SP11	1734	134	0.2	0.22
SP12	1042	158	0.24	0.22
SP15	1720	163	0.18	0.07
SP16	3500	198	0.17	0.38
SP17	269	206	0.13	0.35
SP18	2578	234	0.04	0.44
SP19	232	237	0.34	0.43
SP20	2573	270	0.13	0.36
SP21	625	300	0.13	0.44
SP22	507	370	0.47	0.39
SP28	785	474	0.43	0.24
SP23	468	560	0.4	0.31
SP24	1000	707	0.48	0.40
SP25	807	708	0.48	0.41
SP26	358	819	0.38	0.39
SP27	1505	844	0.33	0.34

Figure 7. Relationship between time gap (number of days between oil spill and image date) and (a) NDVI and (b) NDWI in polluted sites.

Figure 8. The relationship between time [< 1 year 8(a) and 8(b) and 1–2 years 8(c) and 8(d)] and the two vegetation indices (NDVI and NDWI).