Sequencing 27 Borehole Sites: Survey-to-Drill Scheduling and Geophysics Crew Planning at Program Scale

Twenty-seven sites are not twenty-seven drilling jobs. They are one survey job, one ranking job, and one logistics job, and the order matters more than the speed. A programme that drills before it ranks pays for dry holes with money it needed for the last five wells.

What a Countywide Survey Hands the Driller

The Turkana Batch II picture

In July 2026 the Horn of Africa Groundwater for Resilience Program finished a countywide geophysical and hydrogeological survey covering 27 proposed borehole sites under Batch II in Turkana County, Kenya, spread across seven sub-counties. The distribution is uneven in a way that matters to anyone holding a rig schedule: Turkana South carries 8 proposed sites, Turkana West 6, Loima 5, Turkana East 4, Turkana Central 2, while Turkana North and Kibish have 1 each. The survey work estimated aquifer depth, assessed potential yield, and was explicitly aimed at cutting the risk of drilling dry or low-yield boreholes. The programme behind it runs from 2022 to 2028 across five arid and semi-arid counties - Turkana, Marsabit, Mandera, Wajir, and Garissa - with a Kenya allocation of roughly 19.3 billion shillings and a stated target of reaching more than 1.5 million people. By late August 2026 a joint supervision team from the World Bank, the Water Sector Trust Fund, the national water ministry, and the county implementation unit was spending four days on six schemes in Turkana Central, West, North, and Loima, which is what the survey-to-drill handover looks like once rigs are turning.

Why 27 equal rows are not 27 equal jobs

On the survey register the 27 sites are 27 identical rows: a coordinate, a depth estimate, a yield estimate. On the ground they are not remotely equivalent, and the difference is mobilization. Eight sites in Turkana South can be drilled as one continuous campaign with a single rig move-in, one water bowser, one camp, and one set of community agreements. One site in Kibish costs the same mobilization for one borehole, and if the rig finishes and the next site is 200 km away in a different sub-county, the crew burns two to three days moving instead of drilling. Programme managers who read the register as a list and drill it top to bottom pay that mobilization tax up to seven times. The second asymmetry is hydrological rather than logistical. A cluster of eight sites in one sub-county usually sits over the same aquifer system, so the first hole drilled there calibrates the next seven and the yield estimate tightens with every completion. A single remote site carries no neighbours, no calibration, and no second chance if the first hole comes in dry.

Table 1. Turkana County Batch II proposed borehole sites by sub-county, as published with the July 2026 survey completion. Batch treatment is the scheduling reading, not the project structure.

Geophysics Crew Rates and the Survey Window

Crew output and what one point costs

Programme schedules are built on crew output, and the numbers are more stable than most planners expect. A resistivity crew running vertical electrical soundings completes 8-12 points per field day on open ground with vehicle access, which puts the 27 Turkana sites inside three to four working days of pure acquisition. Magnetic resonance sounding, the method that responds directly to water rather than to resistivity contrast, takes 30-60 minutes of measurement per point plus coil layout and de-rig on a 100 m loop, so a realistic field rate is 4-6 points per day rather than the 16 the measurement time alone suggests. Kenyan cost benchmarks published by drilling practitioners put a single VES sounding at 28,000-47,000 shillings, a full hydrogeological survey at 50,000-80,000 shillings, a Water Resources Authority permit near 22,000 shillings, and drilling at 6,500-9,500 shillings per metre, with a completed 150 m borehole excluding the pump at roughly 1,400,000 shillings. At about 129 shillings to the USD that is 220-365 USD per sounding, 390-620 USD for the survey report, 50-74 USD per drilled metre, and about 10,850 USD for a finished 150 m well - which puts the entire survey package under 6 percent of what the hole costs to drill.

Sizing the window for a 27-site programme

Acquisition time is not the window, and confusing the two is how a survey slips into the rainy season. A 27-site programme needs two to four weeks from crew arrival to signed report on the realistic path: three to four days of VES acquisition, two to three days of magnetic resonance on the shortlisted half, three to five days lost to access, community entry, and permit paperwork, and five to ten days for the hydrogeologist to model the soundings into depth and yield estimates and write the ranking. A single crew on a 30-well programme lands in the same 2-4 week band, which is why programmes of this size run two crews in parallel on different sub-counties rather than one crew working sequentially. The two crews must not share a window on the same sites: resistivity and magnetic resonance teams working the same ground on the same day produce interference on the magnetic method, and the sequencing fix is to keep them separated by either one sub-county or two days.

Table 2. Crew outputs are field-day rates on accessible ground; halve them on foot-access sites. Costs are Kenyan practitioner benchmarks converted at about 129 KES to the USD.

Ranking Sites into A B and C Bands

What puts a site in band A

Ranking is not a subjective exercise, and programmes that do it properly write the rule down before the survey starts. A band-A site carries agreement between two independent methods - a resistivity model showing a saturated low-resistivity layer at drillable depth plus a magnetic resonance response confirming mobile water at the same interval - supported by at least one producing borehole within a few kilometres on comparable geology, and a design depth inside what the contracted rig can reach with the casing programme budgeted. That combination puts yield probability above 85 percent. Band B carries one strong method plus supporting desk evidence and sits between 60 and 85 percent. Band C rests on a single method with no neighbouring well to calibrate it, or on a depth that pushes the rig past its contracted capability, and often lands below 60 percent. The published Kenyan comparison is stark: siting with modern geophysics runs above 85-90 percent success, while drilling without a survey sits near 50 percent.

The budget arithmetic behind the order

The reason the order matters is that a dry hole is not a partial loss, it is the full cost of the hole with nothing to show. Take 27 sites at 200 m each and 50-74 USD per metre: one dry hole costs 10,000-14,800 USD, and each one consumes casing, screen, gravel, cement, development hours, and a pumping test before anyone can call it dry. Drilled in register order with no ranking, a programme sits near the 50 percent figure, so roughly 13 of 27 holes come in dry or below the yield threshold, sinking 130,000-190,000 USD. Drilled A-first, the same programme reports 8-12 percent dry, which is two to three holes and 20,000-44,000 USD. The whole survey package - 27 resistivity soundings at 220-365 USD, magnetic resonance on the shortlisted half, and the hydrogeological report - lands between 15,000 and 25,000 USD. Avoiding three dry holes covers the survey roughly four times over, and the saving is not a rounding error on a programme this size; it is the budget for the last five wells.

Table 3. Probability bands are programme rules of thumb, not geological certainties. The last two rows compare the same 27 sites at 200 m and 50-74 USD per metre.

Keeping the Rig Behind the Survey

Three weeks of lead and what it buys

The single scheduling rule that separates programmes which finish from programmes which explain delays is a fixed lead: survey starts three to four weeks before the first rig moves, and never shares a site with a crew that is still measuring. Three weeks buys three things. It buys a ranked register, so the first hole a rig drills is an A-band site rather than row one of a spreadsheet. It buys permit clearance, because in most jurisdictions the drilling authorization and the environmental screening are filed against the survey report and cannot start before it exists. And it buys the option to drop sites, which is the option programmes lose the moment a rig is standing on an unranked pad with a crew on day rate. On the other side of the same ledger, a rig that arrives two months early burns 1,800-2,600 USD per day waiting, and a rig that arrives after the survey but before the ranking drills the wrong holes at full cost.

Batching by cluster and mobilization cost

Once the register is ranked, the batching decision is arithmetic on mobilization. A rig drilling a 150-250 m water borehole spends two to four days making hole, one to two days on casing and screen, and one day on development and test pumping, so four to seven days per site including the move, which is four to six completed wells per rig per month. Twenty-seven sites on one rig is five to seven months; on two rigs running different sub-counties it is three to four months. Mobilization between sites in the same sub-county costs one to two days; moving between Turkana South and Kibish costs three to five days plus haulage. So the batching rule follows the register: Turkana South and its 8 sites is one campaign, West and Loima are one each, East and Central combine into a fourth, and North plus Kibish either pair into a fifth campaign or attach to another contract in the same county. The yield from batching is measurable: five or six campaigns instead of twenty-seven moves, and on a 13,000 USD average hole the mobilization tax falls from roughly 20 percent of the programme to under 8 percent.

The Handover Package and the Dry Season Countdown

Four fields every site must carry

The survey-to-drill handover fails more often on format than on geology, and the fix is a fixed four-field package per site that the hydrogeologist signs and the driller receives before mobilization. Field one is the coordinate, in WGS84 decimal degrees to five decimal places with the access note, because a site that cannot be reached by a rig is a survey result, not a drilling target. Field two is the design depth, stated as a target with a stop depth, so the driller knows where to case off and when to call the hydrogeologist rather than drilling on. Field three is the expected yield in cubic metres per day with the confidence band, which is what the pump specification and the community water demand are both built on. Field four is the formation risk note: basalt overburden that will hammer hard and slow, collapsing sand that needs mud or temporary casing, saline horizons to case off, and any fluoride or iron reading from neighbouring wells that will affect the completion. A package without field four is a package that turns a drilling problem into a completion problem six weeks later.

A dry-season countdown template

Programme calendars in northern Kenya are set by rainfall, not by contract signature. With the 2025 short rains and the 2026 long rains both coming in below average and drought conditions deepening through 2026, the working window is the dry stretch when roads are passable and boreholes are most urgently needed. The template runs backwards from that window. Ten weeks out the contract, the Water Resources Authority permit application, and the environmental screening are filed, because neither can proceed without a survey reference. Eight weeks out the resistivity crew mobilizes on the largest sub-county cluster. Six weeks out the magnetic resonance crew covers the shortlisted half and the hydrogeologist starts modelling. Five weeks out the ranked register is signed with the four-field package on every site. Four weeks out the first rig mobilizes onto an A-band cluster, which is the three-week lead this article keeps arguing for. Drilling then runs through the dry window, and the C-band sites are revisited only after the A and B completions have released the budget they were expected to.

Table 4. A backwards template for a 27-site programme on a dry-season window. Compress the survey column and the rig arrives before the ranking exists.

Everything in this article is sequencing logic that a programme team applies to its own survey data and its own rig fleet, and none of it depends on who supplies the equipment. Where the supply side does touch the schedule is lead time: a resistivity and magnetic resonance instrument package and a rig sized for 150-300 m holes have to be on the water before the countdown starts, because a survey that finishes on time and a rig stuck in transit produce the same delay as no survey at all. We build the detector and rig side of that package, with detector sets shipping in 7-10 days and rigs in 30-45 days, so a programme planning a dry-season window can order both against the same calendar and keep the survey three weeks ahead of the first hole.

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