After successful completion of the 1st field experiment in Kajiado Town (now available on the SamSam Water website) the second geophysical field campaign was performed in the coastal area of Kwale from 15 - 31 of March, 2018. Kwale is a typical coastal geology with fresh-salt water intrusion issues and Pleistocene sediments on Jurrasic transgression overlaying older formations of terrestrial (Mazeras) sandstones deposited under arid circumstances (Trias period). Besides continued co-operation with ISGEAG partners KenGen and Amref Health Africa, this geophysical field campaign took place in close collaboration with local partners BASE Titanium Ltd and Rural Focus. In contrary to the fieldwork in Kajiado, the project team could relate geophysical measurements to an extensive and reliable dataset of BASE Titanium Ltd in relation to their boreholes and various groundwater monitoring wells.
BASE Titanium is especially interested in the depth of the Jurrasic deposits, the transition to the Mazeras sandstones, and how far this extends to the East. With vertical electrical soundings (VESes) this zone is indistinguishable, and can only be identified with expensive exploration drilling.
Besides BASE Titanium Ltd and the dedicated ISGEAG partners (Amref Health Africa, KenGen, Acacia Water and SamSam Water Foundation), students of both University of Nairobi (UoN) and the Kenya Water Institute (KEWI) were again involved. Furthermore the new WalkTem of ABEM Sweden was hired, while representatives of the Gro for gooD project, Rural Focus and the local Kenyan Forest Service (KFS) were involved as well. The input of Amref Health Africa (Kenya office) proved again its added value and was indispensable in terms of project management, logistics and local communication & liaison.
The new ABEM WalkTem could be hired from ABEM against a discount price to promote its application and functioning in East Africa. ABEM is very much interested in the results and introduction of WalkTem in Kenya/East Africa, and want to be involved in for example seminars given in the region. This advanced equipment version of time-domain electromagnetics (TDEM) provides very exact measurements with 1-metre accuracy on (sharp) changes in resistivity based on geological and fresh-salt water boundaries.
Summarizing the results of geophysical measurements:
- Due to the good reference data of Base Titanium it has been demonstrated that new geophysical methods such as TDEM, audio-magnetotellurics (AMT) and electrical resistivity tomography (ERT) provide an important improvement to explore the subsurface at greater depths;
- First analysis shows that the vertical resolution of TDEM WalkTem is better than ERT Wenner. Even with small transmittor loops (40x40m) the WalkTem goes to depths of 150 m bgl with a very high resolution because of its so called dual-moment wherein deep and shallow is measured at the same time. A third possibility is high-moment with which very high current strength can be measured. All these 3 measurements can be combined in an inversion leading to the high resolution;
- Especially the new TDEM WalkTem appears to be very useful for practice and application in coastal zones. The WalkTem equipment is is easy and convenient to use (user-friendly). In a day approximately 6 - 8 measurements can be done, depending on field circumstances (obstacles, no cables, etc.). Especially the built-in software which can generate a first inversion in the field is very important to adjust the concept during the measurement and, if necessary, change the measurement strategy;
- Coastal areas with low resistivities in the subsurface lends itself very well for the application of TDEM, whilst the limitations of VES just come more to light (on top of the limited exploration depth of 1/6 of the AB, typically 50 - 70m bgl), making it a less suitable method for coastal areas;
- Again, knowledge and skills are essential for all methods, both when used in the field and when interpreting a geo-hydrological concept. Geophysics shouls always be compared with available borehole logs and broader hydrogeological insights from existing information;
- When using the conventional VES and horizontal electrical profiling (HEP) methods, it has again been established that the current practice is poor: the execution is routinely and without consideration for the quality and significance of understanding the groundwater situation;
- Indications of the probing depth/depth of interaction (DOI) and vertical resolutions per method are:
- VES, to approximately 70 m bgl, reasonable resolution;
- ERT, to approx. 100 m bgl, reasonable resolution;
- TDEM WalkTem, up to 150 m bgl, good resolution;
- TDEM Zonge, up to 200 m bgl, reasonable resolution;
- AMT, up to great depth >500 m bgl, moderate resolution.
Especially TDEM WalkTem has proven its added value and greater accuracy to great depths. Based on these geophysical survey results it appears that the coastal area around Kwale is at enormous risk of salinisation of the groundwater. There appear to be substantial groundwater abstractions around the Kwale International Sugar Company Ltd (KISCOL) areas, where sugar is locally grown, and does not appear to be structurally monitored. At various locations salinisation of boreholes have been confirmed with the TDEM/WalkTem measurements.
It is advised to setup a measurement campaign for the entire coastal area as quickly as possible, for example with SKYTEM. Based on the results, deep probings will have to be carried out at specific locations (TDEM / AMT) as well as investigation of possible deep groundwater exploration (e.g. the Neogene aquifer). Following the results, a monitoring program will have to be set up in the salinization areas. The area otherwise runs great social and economic risks.
The ISGEAG partners are currently busy interpreting and integrating all the fieldwork data and drafting the report. A final draf is envisioned to be distributed early September. Furthermore, for the third and last geophysical field campaign the geologically volcanics of Naivasha are selected. The project team is currently in the process of planning for this two-week field campaign.