Seismic imaging to manage salt water intrusion in Benin, West Africa: Challenges in an urban, coastal environment John H. Bradford*and Kyle Lindsay, Department of Geosciences, Boise State University Summary The coastal city of Cotonou in Bénin, West Africa, is a large population center that is facing a serious threat to the sustainability of its fresh water supply. It relies on the Godomey aquifer for domestic water supply. The aquifer is undergoing saltwater intrusion and this problem is likely to worsen without significant steps to improve management of the water supply. The continuity of the aquifer and saltwater flow paths are poorly understood but this information is critical to ensure sustainable access to fresh water in this growing urban center. In January, 2012, we began a two year geophysical investigation with the prime objective of using seismic reflection to image the primary aquifer units. Logistics in this congested urban environment are complicated and seismic coverage is sparse because of limited access. Despite these problems, our initial results show that the aquifer units are substantially more complicated than previously thought; critically, there is an ~60m deep paleo-channel that cuts through a substantial portion of the aquifer system and truncates multiple aquifer/aquitard boundaries. This channel likely has a major impact on the aquifer hydrology that has not previously been taken into account. Introduction The city of Cotonou in Bénin, West Africa (Figure 1) is a large and growing population center. The sole source of drinking water for Cotonou is the Godomey aquifer. The city s primary water well field lies roughly 5 km from the Atlantic coast, but adjacent to Lake Nokoué (Figure 1). The well field consists of 20 wells which withdraw water from one or more of three semi-confined aquifers ranging in depth from 30 m 180 m. The growth of the urban population has necessitated an increase in the pumping rate from the Godomey well field. Increased pumping has been accompanied with an increase of salinity in the wells. In an effort to understand this large, complex system, an initial groundwater model was developed (Boukari et al., 2008) and then improved through interpretation of the structure of the local geology (Silliman et al., 2010). Combined field observations and numerical modeling indicate that Lake Nokoué is the likely source of saline intrusion into the Godomey aquifer system. Silliman et al. (2011) conclude that the current groundwater model and field characterization efforts have not yet reached the level where sound, data driven management Figure 1: Map showing Bénin, West Africa, the location of the Godomey well field, and the study area. The Godomey well field is the primary source of drinking water for the city of Cotonou. The two seismic profiles discussed in the text are highlighted in blue. SEG Houston 2013 Annual Meeting Page 5212
decisions can be made. Substantially greater characterization, coupled with refined groundwater modeling, is needed. In January, 2012, we began a two year project with the objective of using seismic reflection to image the primary aquifer units in the critical area where the well-field capture zone intersects the southwest portion of Lake Nokoué. Seismic Data Acquisition and Processing Our initial field project consisted of land seismic reflection acquisition along a set of profiles that ranged in length from < 200m to greater than 1.5 km (Figure 1). Logistics for the seismic reflection were complicated by conducting the Figure 2: Streets in the city consist of variable surfaces and are congested. Restricted accessways and safety concerns limited our seismic coverage. Figure 3: This photo shows a typical roadway used for acquisition. Limited access required that seismic lines be placed among small roadside businesses and homes. Heavy foot and vehicle traffic produced substantial coherent seismic noise. work in this congested urban area (Figure 2). Road surfaces were highly variable and many were not amenable to planting geophones. Heavy traffic and the associated safety concerns for the seismic crew further limited our seismic coverage. Where seismic acquisition was feasible, the roads typically ran through crowded neighborhoods with many road side shops along with heavy foot and vehicle traffic (Figure 3). These challenging conditions limited our seismic coverage to somewhat sparse and irregular coverage grid (Figure 1). Data were acquired with 5, 24 channel Geode seismographs, 3 m geophone spacing, and 6 m source spacing in an off-end geometry. The source was a 10 kg sledge hammer. A total of 6.5 linear km of data were acquired along the 13 profiles shown in Figure 1. Figure 4: Along Line 5b (Figure 1), shots 1027 and 1032 are separated by just 15 m, but show sharply different data quality. In the raw records, shot 1027 is heavily contaminated with traffic noise, while shot 1032 is nearly free of external coherent noise. A spectral balancing filter (S-balance) largely attenuates the low-frequency ground roll and other coherent events, but little coherent reflection energy is evident in shot 1027. SEG Houston 2013 Annual Meeting Page 5213
Figure 5: Line 10a (Figure 1) shows data quality variability similar to that found in Line 5b. In shot 1003, noise from a small grain mill is evident as a repetetive noise source. Shot 1002 shows noise from the same location, albeity with lower amplitude. High frequency reflections are evident in the raw shot 1002 record, but are obscurred in shot 1003. The spectral balancing filter enhances the reflections in both records, however shot 1002 is clearly cleaner. Along Lines 5b and 10a we imaged an incised paleochannel that likely has a major impact on the well-field hydrology and we will therefore focus on these two profiles for the remainder of the discussion. Optimal surface conditions produced good data quality despite the high levels of coherent noise. Vehicle traffic was the primary source of noise on Line 5a, but was highly variable (Figure 4). On Line 10a, the primary source of noise was a small grain mill which produced a repetitive impact near the center of the line (Figure 5). Pedestrian traffic was a constant but lower amplitude noise source along both lines. These sources of coherent noise masked reflections in the raw records, but a spectral balancing filter over the dominant band of reflection energy (40-300Hz) substantially enhanced the reflections and attenuated Figure 6: In Line 10a the eastern flank of the paleo channel is clearly observed as an erosonal surface cutting the aquifer system (red). The base of the aquifer system is defined by a high amplitude continuous reflection (blue). ground-roll and cultural noise. Additional processing steps included AGC (50 ms gate), a top mute to remove the first break refraction, NMO velocity analysis, elevation statics, and stacking. Results Despite the high levels of coherent noise, our simple processing flow produced high quality stacked sections showing reflections at up to 1 km depth, and revealing key characteristics of the Godomey aquifer. The base of the aquifer system is at ~180 m depth or 200-250 ms traveltime (Figures 6 and 7). High amplitude, laterally continuous reflections above 200 ms correlate with the thin clay aquitards identified in well lithologic logs. These thin clay aquitards separated by relative thick, sandy aquifers. While the data throughout the survey generally show significant variability of the interbedded aquifers and aquicludes, perhaps the feature with the greatest hydrologic significance is a paleo-channel imaged in Lines 5b and 10a (Figures 6 and 7). The channel is approximately 500 m wide and 60 m deep and cuts through the units which comprise the aquifer/aquiclude system of the larger Godomey aquifer (Figure 6). The channel runs roughly north-south along the eastern boundary of the well-field and and the western shore of Lake (Figure 8). Since the channel cuts multiple aquifer/aquiclude units it is clear that it must be included explicitly in the hydrologic model. Further, it lies between the primary recharge zone of Lake Nokoué and the well-field and will therefore have a major impact on mixing between multiple aquifer levels and the flow paths from the lake to the well-field. SEG Houston 2013 Annual Meeting Page 5214
Figure 7: Despite strong coherent noise in the shot records, stacking all offsets produced an excellent reflection image along Line 5b. The additional processing step of post-stack, phase-shift migration has also been applied to the data. The red line shows the base of an incised paleochannel. Blue line shows the base of the Godomey aquifer system at a depth of ~180 m. The channel also is roughly aligned with a lineation of high salinity shallow wells that extend inland from the coast roughly 5 km to the south (Figure 8). This correlation suggests the possibility that the channel is hydraulically connected to the ocean and is acting as a high permeability conduit to pull saline waters toward the well field, however, this is speculation at this stage and further study is required. Conclusions The Godomey aquifer system is substantially more complicated than previously mapped. These complexities must be included in the hydrologic model if it is to have value as a predictive management tool. Some large features such as the incised paleo-channel can be mapped directly, however, a geostatistical analysis will be required to extract information about the smaller scale heterogeneity. Acknowledgments Geoscientists without Borders is generously funding this work through a grant to Boise State University. We thank our collaborators Steve Silliman, Gonzaga University, along with Nicaise Yalo and Moussa Boukari at the University of Abomey-Calavi, Bénin. Figure 8: Blue lines show the two seismic profiles discussed in the text, and the red line indicates the interpreted axis of an incised paleochannel. Filled black circles indicate locations where elevated salinity levels were found in shallow groundwater samples. Modified from Silliman et al., 2010. SEG Houston 2013 Annual Meeting Page 5215
http://dx.doi.org/10.1190/segam2013-1481.1 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2013 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Boukari, M., P. Viane, and F. Azonsi, 2008, Three-dimensional modeling of a coastal sedimentary basin of southern Bénin (West Africa), in S. M. A. Adelana and A. M. MacDonald, eds., Applied groundwater studies in Africa: CRC Press, 437 456. Silliman, S. E., B. I. Borum, M. Boukari, N. Yalo, S. Orou-Pete, D. McInnis, C. Fertenbaugh, and A. Mullen, 2010, Issues of sustainability of coastal groundwater resources: Bénin, West Africa: Sustainability, 2, no. 8, 2652 2675, http://dx.doi.org/10.3390/su2082652. Silliman, S. E., M. Boukari, L. Lougbegnon, and F. Azonsi, 2011, Overview of a multifaceted research program in Bénin, West Africa: An International Year of Planet Earth groundwater project, in J. A. A. Jones, ed., Sustaining groundwater resources: Springer, 175 186. SEG Houston 2013 Annual Meeting Page 5216