VES soundings Kajiado Town - SamSamWater...VES soundings Kajiado Town Technical Interpretation and...

P a g i n a | 1

VES soundings Kajiado Town Technical Interpretation and inversion results of all available VES soundings in and near Kajiado Town up to

date April 2017

by Harry Rolf , SamSamWater Foundation, Netherlands

November, 2017

SamSamWater Foundation http://www.samsamwater.com/index.php

P a g i n a | 2

Introduction This document comprises the technical interpretations (inversions) of all VES soundings -as far as available up to date - in and

around Kajiado town.

It includes:

• 22 VES soundings from fieldwork for ISGEAG and Sponge city projects February 2017 and April 2017.

• Re-interpretation of VES Soundings as found in existing survey reports .

Unfortunately only a very limited number of seven (7) survey reports could be made available of which 3 are lacking

measured field data. There must have been many more geophysical surveys, but the availability reports often

appears to be a major organisational challenge in Kenya.

Each interpretation includes all relevant administrative data (location, reference to report, etc.), the field measurements and the

interpretation (inversion) results using SchlumBG software1. In case of the existing survey reports it concerns a re-interpretation

of the data and a comparison to the interpretation as given in the report.

The inversion, layer-model results are given with their 90% uncertainty range as calculated in the SchlumBG software. Some of

the existing survey reports do not comprise the measured data; in those cases the measurements are generated by forward

modelling but the re-interpretation is questionable.

Reading guide First, the locations of all VES soundings are presented on a map (next page Figure 1) and the inversion (layer model) results are

summarised in Tables 1,2 and 3.

Next, for each VES sounding a short (2-3 pages) interpretation report is included. These reports are presented in three blocks:

1) all soundings of the 2017 Sponge City/ ISGEAG surveys (up to page 53)

2) the re-interpretations of soundings as found in existing survey reports that include the measured data and

3) the soundings from reports that lack the measured data.

Interpretation report content Each VES interpretation report contains 6 sections:

General:

• giving the aim, date and location (map) and reference to the survey report2

VES information:

• WGS84 coordinates, line direction, electrode reach, VES array, land elevation and estimated depth to groundwater

Measured data:

• Table and graph of measurements and data remarks

VES interpretation:

• giving the equivalent inversion model(s) having the best fit with the minimum nr. of layers

Results:

• giving the interpreted layer model parameters (depth of the top of the layer and layer resistivity) including their 90%

confidence range

• note: the layer depth gives the depth of the top (!) of the layer

• ‘basement’ is defined when layers have resistivity > 1000 Ωm

Major conclusion

1 SchlumBG for Windows, C.J. Hemker and V.E.A.Post, http://www.microfem.com 2 For the Sponge City soundings the reader is referred to ‘Geophysical surveys for Sponge City project Kajiado, Kenya;

SamSamWater, Harry Rolf, November 2017’

For the ISGEAG soundings in Kajiado a research report is being prepared by Acacia Water (probably available in February 2018)

P a g i n a | 3

Figure 1 VES locations in and around Kajiado Town. The ‘existing survey report’ locations are given by name, the

ISGEAG/Sponge city VES locations are numbered.

P a g i n a | 4

TABLE 1 summarized results of Sponge City/ ISGEAG VES soundings

Sponge City/ ISGEAG soundings February and April 2017

coordinates (WGS84) Electr RMS REMARKSname/ VES LAT LONG Elev sp. error TOP Rho confidence TOP confidence Rho confidence TOP confidence Rho confidence TOP confidence Rho

location nr. (S) (East) (mas) AB/2 (m) x10-2 (m) (Ωm) (m) (Ωm) (m) (Ωm) (m) (Ωm)

Gulley C VES1 1 01°49'39.5'' 36°46'31.2'' 1690 320 3.6 0 26 (23 - 29) 2 (1 - 4) 50 (40 - 80) 15 (12 - 30) >>>1000 discontinuities in meas.

Gulley C VES3 3 01°49'26.8'' 36°46'27.8'' 1710 320 3.5 0 90 (80 - 120) 1.7 (1,5 - 2) 30 (23 - 40) 50 (40 - 60) discontinuities in meas.

Gulley C VES4 4 01°49'34.7'' 36°46'22.9' 1685 320 9.2 0 2 (1 - 5) 2.5 (2-8_ 50 (30 - 1000) 4 < 8 >>1000 discontinuities in meas.

Gulley C VES7 7 01°49'33.0'' 36°46'24.3' 1690 400 1.2 0 30 (25 - 60) 2.5 (2 - 3) 300 (290 - 320) 90 (80 - 100)

Gulley C VES13 13 01°49'34.4'' 36°46'23.4' 1682 63 5 0 10 (6 - 12) 4.5 (4 - 5) >>1000 on gulley river bed

Gulley C VES16 16 01°49'31.9'' 36°46'25.2'' 1687 500 3 0.1 250 (220 -280) 11 (9 - 15) 57 l(arge) 15 (large) 500 (250-1000) 76 (40-160) (1000-1500) some discontinuity

VES2 (road) 2 01°49'55.1'' 36°46'22.0'' 1720 320 unreliable sounding unreliable (reason ?)

Gulley B VES9 9 01°50'17.3'' 36°46'17.2'' 1710 500 1.2 0 290 (250 - 300) 1 (1 - 2) 145 (130 - 150) 9 (8 - 13) 90 (85 - 110) 55 (50 - 70) (700 - 900)

Town VES5 5 01°50'21.7'' 36°46'59.1'' 1720 130 1.7 0 170 (150 - 200) 0.5 (0 - 1) 65 (60 - 70) 9 (8 - 12) 190 (150 - 300) ? >40 short AB line

Town VES6 6 01°50'24.1'' 36°47'28.8'' 1735 130 1.6 0 5 (1 - 12) 3 (2 - 4) 43 (40 - 55) 45 (20 - 90) short AB line large disc.

Town VES8 8 01°50'27.8'' 36°47'25.3'' 1732 63 1.8 0 53 (52 - 56) 11 (9 - 13) 100 (90 - 110) ? >75 short AB line large disc.

Kajiado River:

Kaj. R VES10,12 10,12 01°48'50.9'' 36°46'02.3'' 1680 500 2 0 35 (35 - 50) 1 (1 - 2) 48 (44 - 70) 12 (10 - 15) 400 (300 - 500) 130 (120 - 200) >>>1000 two perpendicual VES's

Kajiado R VES11 11 01°49'46.3'' 36°45'50.6'' 1670 500 1.5 0 320 (150 - 500) 0.7 (0- 1) 4 (large) 0.8 (large) 130 (100 - 200) 35 (25 - 43) (>3000)

VES11 layer4: 27 (large) 25 ( 1000

Kajiado Ri VES15 15 01°49'03.5'' 36°45'59.8'' 1692 500 1.5 0 480 (340 - 700) 7 (2 - 12) 200 (185 - 210) 80 (65 - 95) >800 large discontinuity

Kajiado R VES17 17 01°49'11.0'' 36°46'03.8'' 1675 250 4 0 60 (large) 0.3 (0 - 3) 20 (12 - 40) 13 (11 - 15) >>>1000

Kajiado R VES18 18 01°49'11.1'' 36°46'03.5'' 1675 160 2.3 0 600 (300 - 1500) 1.2 (1 - 3) 3 (1 - 10) 1.7 (large) 240 (50 - 600) 80 >30 discontinuity

Kajiado R VES19 19 01°49'06.2'' 36°46'11.6'' 1675 500 1.9 0 1200 (large) 0.7 (0 - 5) 80 (30 - 150) 9 (3 - 30) 500 (300 - 800) 33 (>25, uncertain) major disconinuity

Kajiado R VES20 20 01°49'03.7'' 36°46'04.3'' 1680 200 1.7 0 800 (250 - 2600) 1.2 (1 - 4) 5 (large) 2.4 (large) 300 (140 - 700) 90 (>45, uncertain) short AB reach

Interpreted geo-electric resistivity model

surface layer Layer 2 Layer 3 Basement (Rho≈≥1000

P a g i n a | 5

TABLE 2 summarized results re-interpreted VES soundings from existing survey reports

TABLE 3 summarized results VES soundings from existing survey reports, lacking measured data

re-interpreted from existing survey reports

coordinates (WGS84) Electrode RMS remarks

name/ VES nr. LAT LONG Elev spacing error TOP Rho confidence TOP confidence Rho confidence TOP confidence Rho confidence TOP confidence Rho

location (S) (East) (mas) AB/2 (m) x10-2 (m) (Ωm) (m) (Ωm) (m) (Ωm) (m) (Ωm)

MTTI VES1 MTTI 01°51'16.0'' 36°46'55.3'' 1720 200 1.87 0 350 (300-400) 1.5 (1 - 2) 100 (20 - 500) 25 (10 - 40) 200 (100 - 300) ?? > 80 detail coordinates missing

MTTI VES2 MTTI 01°51'16.0'' 36°46'55.3'' 1720 160 0.95 0 10 (8 - 12) 1.5 (1 - 2) 75 (70 - 80) 25 (20 - 30) 340 (220 - 460 ?? > 80

MTTI VES3 MTTI 01°51'16.0'' 36°46'55.3'' 1720 200 1.88 0 15 (10 - 20) 3 (1 - 5) 85 (70 - 100) 10 (8 - 14) 210 (190 - 240) ?? >130

MTTI VES4 MTTI 01°51'16.0'' 36°46'55.3'' 1720 200 1.72 0 120 (90 - 160) 1.5 (1 - 2) 35 (28 - 40) 9 (5 - 10) 200 (130 - 230) ?? >50 or deep correctness of last 3 points ?

Masai Stores Masai St 01°50'52.9'' 36°47'24.7'' 1730 200 1.28 0 75 (70 - 80) 1.5 (1- 2) 52 (50 - 55) 20 (15 - 25) 175 (100 - 400) 80 (50 - 130) >>1000 survey report model gives BAD FIT

Osman town Osman 01°50'45.0'' 36°47'25.0'' 1730 250 1.31 0 14 (8 - 27) 0.7 (0- 3) 22 (21 - 23) 10 (10 - 11) 105 (93 - 130) ? >90

Osman Kaj River Osman 01°49'26.0'' 36°45'54.5'' 1670 250 1.9 0 4 (3 - 5) 5 (4 - 6) 40 (15 - 50) 30 (15 - 40) >>1000 inconsistent interpretation in report


surface layer Layer 2 Layer 3 Basement (Rho≈≥1000


coordinates (WGS84) Electrode remarks

Name/ VES nr. LAT LONG Elev

spacing RMS

error TOP Rho TOP Rho TOP Rho TOP Rho TOP Rho TOP Rho

Location (S) (East) (mas) AB/2 (m) x10-2 (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm)

Itinyika 01°50'40.0'' 36°45'40.0'' 1660 ? ? 0 70 2 30 8 100 16 200 50 80 100 700 field data missing

Empeut VES1 01°50'3.54'' 36°47'42.0'' 1720 ? ? 0 79 0.8 17 1.4 53 19 1150 36 81 100 1000 field data missing

Empeut VES2 01°50'0.97'' 36°47'40.4'' 1720 ? ? 0 16 1 30 11 93 42 1000 field data missing

Parmet 01°49'05.0'' 36°45'40.0'' 1685 ? ? 0 90 2 15 7 100 40 250 80 125 125 1200 field data missing

Layer 4surface layerLayer 2 Layer 3 Layer 5 Basement ≥1000 Ωm

P a g i n a | 6

Content

Interpretation report index

Page nr.

Sponge City and ISGEAG soundings:

VES 1 --------------------------------------------------------------- 7

VES 2 --------------------------------------------------------------- 9

VES 3 --------------------------------------------------------------- 11

VES 4 --------------------------------------------------------------- 13

VES 5 --------------------------------------------------------------- 15

VES 6 --------------------------------------------------------------- 17

VES 7 --------------------------------------------------------------- 19

VES 8 --------------------------------------------------------------- 21

VES 9 --------------------------------------------------------------- 23

VES 10/12 --------------------------------------------------------------- 25

VES 11 --------------------------------------------------------------- 28

VES 13 --------------------------------------------------------------- 31

VES 14 --------------------------------------------------------------- 33

VES 15 --------------------------------------------------------------- 35

VES 16 --------------------------------------------------------------- 37

VES 17/18 --------------------------------------------------------------- 39

VES 19 --------------------------------------------------------------- 44

VES 20 --------------------------------------------------------------- 47

VES 21 --------------------------------------------------------------- 50

mini VES --------------------------------------------------------------- 52

From existing survey reports:

MTTI --------------------------------------------------------------- 55

Masai Stores --------------------------------------------------------------- 60

Mr. Osman (Town) --------------------------------------------------------------- 63

Mr. Osman (Kajiado River)--------------------------------------------------------------- 65

---------------------------------------------------------------

Existing reports lacking field data:

Itinyika --------------------------------------------------------------- 68

Empeut --------------------------------------------------------------- 70

Mr. Parmet --------------------------------------------------------------- 73

P a g i n a | 7

Kajiado VES sounding nr.1 Technical interpretation report

Harry Rolf for SamSamWater Foundation 17/02/2017

Client : Acacia Water

General

VES sounding for Sponge City project

Aim: finding locations for infiltration (highly weathered or fractured rock)

Location name: South bank of Gully C. This is one of the VES’s and HEP lines conducted close to this gully, see report Rolf 2017,

Geophysical surveys for Sponge City project Kajiado, Kenya.

Sounding Date : 15/02/2017

Location

VES sounding information

Location: (WGS82): 360 46’31.2” E and Latitude 010 49’39.5” S

Direction of sounding: 110 gr SE (the VES line has crossed the gully which may have caused anomaly in the measurements)

AB reach: 320 m

Array: Schlumberger

Elevation: 1690 mas

Estimated depth to Water: 30 m

Measured data

Remarks: There obviously was a lateral change, which in this case may be caused by the gully (geology change) that was crossed

at the South east part of the line (at 65 m from the centerpoint)

DATA table VES 1

MN/2 AB/2 Rho Rho shifted

0.5 1.6 27 27

2 27 27

2.5 28 28

3.2 32 32

4 31 31

5 32 32

6.3 38 38

8 43 43

10 46 46

0.5 13 52 52

5 13 58

16 63 56

20 63 59

25 69 61.7

5 32 95 85

10 32 71

40 97 114

50 126 150

10 63 163 193

25 63 126

80 182 280

100 220 335

25 130 268 407

50 130 194

160 230 483

200 226 475

250 225 472

50 320 190 399

100 320

400

500

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 8

VES interpretation Inversion results (SchlumBG modelling)

Possible model concepts

3 layers shifted data (leaving out the last bending down measurements) very high resistivity basement

Results including 90% confidence levels

Depth (m) Rho (Ωm )

topsoil ------ 23 - 29

Overburden 1 -4 40 – 80

Shallow aquifer ------- ---------

Basement (ρ>1000) 12 - 30 >>>1000 (high resistivity)

Conclusion

• Overburden on top of fresh basement rock at 12 – 30m depth

The bending of the measurements beyond AB/2 = 100 is not (yet) explained.

The VES line crossed a major anomaly, probably when crossing the gully.

1 10 100 L/2

10

1e+2

1e+3

1e+4

1e+5

1e+6

Rho

P a g i n a | 9




General

VES sounding for Sponge City project.


Location name: along the road down to the Kajado River (in between gulley C and gulley B, at the beginning of HEP1 and HEP2

profiles)

Sounding date : 13/02/2017

Location


Location: (WGS82): 360 46’ 22.0” E and Latitude 010 49’ 55.1” S

Direction of sounding: SE -N W along the road

AB reach: 640 m

Array: Schlumberger

Altitude: 1718 mas

Depth to groundwater

(if any) estimated at: 70 mbgl

Measured data

Remarks: The measurements are discontinuous. The difference between the shifted and unshifted measurements is

considerable. There are some clear outliers e.g. at AB/2 =30 m. The cause of these varying measurement quality is not clear. It

may be caused by crossed powerlines and fences along the road.

DATA table VES 2


0.5 1.6 276 276

2 270 270

2.5 264 264

3.2 274 274

4 274 274

5 277 277

6.3 284 284

8 257 257

10 228 228

0.5 13 191 191

5 13 225

16 192 164

20 185 157

25 209 178

5 32 221 187

10 32 220

40 282 240

50 243 207

10 63 257 220

25 63 219

80 252 252

100 281 282

25 130 318 319

50 130 292

160 326 299

200 364 333

250 404 370

50 320 404 371

100 320

400

500

VES measurements

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 10

VES interpretation

Inversion results (SchlumBG modelling)


3 or 5 layer model with shifted or unshifted data

Basement layer (taken at ρ>1000 ohmm) is uncertain

\

5 layer unshifted

3 layer unshifted

4 layer shifted data


Depth (m) **) Rho (ohmm)

topsoil (1000) >40 (actually unknown)

*)Please note the topsoil Rho can be just a phony representing electrode contacts, electric leakings

**) the top of the layer

Conclusion

• High uncertainty, actually the sounding is unreliable

• The (dry) overburden layers are in the order of 160 - 250ohmm.

• At the level of in between 25 – 75 m there is a change to a somewhat higher resistivity (380 – 500 ohmm)

• No basement rock within 40 m depth

1 10 100 L/2

1e+2

1e+3

1 10 100 L/2

1e+2

1e+3

1 10 100 L/2

10

1e+2

1e+3

P a g i n a | 11




General



Location name: on the Gully bed (Northern runoff gulley C) close to a solar borehole (Mr. Rehoba)


Location


Location: (WGS82): 36046’27.8” E and Latitude 01049’26.8” S

Direction of sounding: NE at the north end of HEP line 1 (in the HEP line direction!)

AB reach: 640 m

Array: Schlumberger

Elevation: 1710 mas

Depth to groundwater (if any)

Estimated at: 50 m

Measured data

Remarks: Major lateral changes and outliers. The sounding direction was across the direction of geology changes

Data shifted acc. to Earth Water Consultant’s methodology.

DATA table VES 3


0.5 1.6 62 62

2 73 73

2.5 85 85

3.2 89 89

4 90 90

5 77 77

6.3 78 78

8 68 68

10 61 61

0.5 13 49 49

5 13 76

16 57 37

20 48 31

25

5 32 48 31

10 32 58

40 64 34

50 70 37

10 63 53 28

25 63 74

80 104 36.64

100 123 47.2

25 130 177 67.6

50 130 134

160 152 76.5

200 189 95.1

250 202 101.5

50 320 275 138

100 320

400

500

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 12

Interpretation



4 layer model with shifted data

4 layer unshifted data


Depth (m) **) Rho (Ωm )

topsoil (1000) 40 - 60 >1000

*) the topsoil Rho can be phony representing electrode contacts, electric leakings

**) the top of the layer

Conclusion

• Overburden relatively low resistivity 23 - 40

• Basement at 40 – 60 m

Strong lateral changes indicating geology changes in NE direction

1 10 100 L/2

10

1e+2

1e+3

1 10 100 L/2

10

1e+2

1e+3

P a g i n a | 13



General



Location name: across Gulley C at HEPline1 (in the direction of HEP line!!)


Location


Location: (WGS82): 36°46'22.90"E and Latitude 1°49'34.70"S

Direction of sounding: NNE (across the structure, centre is just South the gulley bed)

AB reach: 320 m

Array: Schlumberger

Elevation 1685 mas

Groundwater (if any)

Depth estimated at: 20m

Measured data

Remarks: There obviously are large lateral changes, both from structural changes in geology, but also because the VES

centerpoint was stationed close to the bed of the gulley, having clay and loamy sand deposits (explaining the low resistivity at

small AB/2 range. The shifted curve is remarkable due the method of shifting. Therefore , the shifted line was only used to up to

AB/2 = 20m in the inversion.

DATA table VES4


0.5 1.6 18 18

2 18 18

2.5 17 17

3.2 15 15

4 13 13

5 12 12

6.3 12 12

8 13 13

10 14 14

0.5 13 20 20

5 13 29

16 37 26

20 49 34

25 65 44

5 32 82

10 32 57

40 95 63

50 89 59

10 63 91 60

25 63 131

80 131 63

100 145 69

25 130 151

50 130 374 69.21429

160 385 71

200 380 70

250 464 86

50 320 584 108

100 320

400

500

VES measurements

10

100

1000

1 10 100 1000AB/2

Rho

Rho

Rho shifted

P a g i n a | 14

VES interpretation



4 layer model with shifted (up to 20 m) data

The model fit is not good (RMSE 9.28E-02) due to the many lateral changes

4 layer model , unshifted data


Depth (m) Rho (ohmm)

topsoil (1000)

P a g i n a | 15

Kajiado VES sounding nr.5 Technical report



General



Location name: street in Kajiado town (originally suggested for HEP line3)


Location



Direction of sounding: 2620 West-SouthWest

AB reach: 260m (short due to buildings and risky power lines)

Array: Schlumberger

Elevation: 1720 mas

Groundwater depth (if any)

Estimated at: 55 mbgl

Measured data

Remarks: Shifting is reasonable (K3= 1.177) , minor lateral changes within this (short) reach

DATA table VES5

MN/2 AB/2 Rho SHIFTED

0.5 1.6 122

2 102

2.5 93

3.2 79

4 72

5 68

6.3 72

8 70

10 72

0.5 13 75

5 13 72

16 77 81

20 100 104

25 111 116

5 32 122

10 32 125 127

40 133 135

50 148 150

10 63 170 167

25 63 142

80 185 219

100 199 234

25 130 237 279

50 130

160

200

250

50 320

100 320

400

500

VES measurements

10

100

1000

1 10 100 1000AB/2

Rho

Rho

SHIFTED

P a g i n a | 16

VES interpretation



3 or 4 layer model with shifted data

Basement layer (assuming ρ>1000 ohmm) very uncertain

3 layer model

4 layer model


Depth (m) Rho (Ωm)

(top)

topsoil (1000) >40

*)Please note the topsoil Rho can be just a phony layer, representing electrode contacts, electric leaking

Conclusion

• Overburden is like 65 (60 – 70) Ωm.

• Less weathered/fractured rock starts at ≈10 m

• Fresh basement rock unknown, but at least deeper than 40m

• No indications for substantial structural changes within the (short) AB reach

1 10 100 L/2

10

1e+2

1e+3

1 10 100 L/2

10

1e+2

1e+3

P a g i n a | 17




General



Location name: at the location in Kajiado Town, close to a Sponge City test infiltration pit along roadwater runoff gully


Location



Direction of sounding: East-Northeast

AB reach: 160 m (short due to buildings/powerlines)

Array: Schlumberger

Elevation: 1735 mas

Estimated depth to groundwater (if any): 75 mbgl

Measured data

Remarks: Short AB electrode reach due to building restrictions. Measurements show large discontinuity at potential change at

13m. The inversion of the unshifted data is too inaccurate. Inversion only done on the shifted data. This however is tricky

because over the large shift in the data. The shift may be caused by serious fracturing, or….. ??

DATA table VES 6


0.5 1.6 10.2 10.2

2 9.8 9.8

2.5 9.9 9.9

3.2 10.3 10.3

4 11 11

5 12.4 12.4

6.3 15 15

8 15.5 15.5

10 19.6 19.6

0.5 13 20.8 20.8

5 13 36.7

16 43.5 25

20 53.3 30

25 58.3 33

5 32 54.7 35

10 32

40 72.2 41

50 84 48

10 63 87

25 63 94 53

80 123 70

100

25 130

50 130

160

200

250

50 320

100 320

400

500

VES measurements

10

100

1000

1 10 100AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 18

VES interpretation



3 or 4 layer model with shifted data only

3 layer model

4 layer model



(top)

topsoil ------ 9 - 12 *)

topsoil2 0.5 – 4 1 - 10

Overburden 2 - 4 38 - 54

Basement 20 – 90 >> 1000

*)Please note the topsoil Rho can be just a phony representing electrode contacts, electric leaking

Conclusion

• Overburden is like 45 (38 - 54) Ωm, top starting at 2 – 4 m

• Fresh basement rock below 20 m, not deeper than 90 m

• VES is rather uncertain due to large shifts in the measured data

• Considerable shift in the measurements may indicate fracture zone??

1 10 100 L/2

1

10

1e+2

1e+3

1e+4

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 19




General



Location name: parallel to the Gully C (North bank) on HEPline1


Location



Direction of sounding: 300gr SEast

AB reach: 800 m

Array: Schlumberger

Elevation 1690 mas



Measured data

Remarks: only minor lateral changes

DATA table VES 7


0.5 1.6 96 96

2 82 82

2.5 76 76

3.2 80 80

4 87 87

5 95 95

6.3 111 111

8 138 138

10 146 146

0.5 13 170 170

5 13 195

16 224 196

20 238 208

25 251 219

5 32 269

10 32 273 236

40 278 274

50 277 273

10 63 289 285

25 63 276

80 299 308

100 309 318

25 130 370 381

50 130 344

160 410 455

200 458 508

250 497 552

50 320 623 691

100 320 548

400 613 772

500

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 20

VES interpretation



4 layer model with both shifted and unshifted data

4 layer model, shifted data

4 layer model, unshifted data



topsoil (1000) 80 – 100 1300 – 1800

*)Please note the topsoil Rho can be a phony layer representing electrode contacts, electric resistance

Conclusion

• Basement at 90 m (80 – 100) overlain by overburden 300 Ωm (290 -320)

• No structural changes in 300 gr SE direction

1 10 100 L/2

10

1e+2

1e+3

1 10 100 L/2

10

1e+2

1e+3 VES7

north of the gully

(parrallel )

P a g i n a | 21




General



Location name: in Kajiado town, behind the Mosque at school premises


Location



Direction of sounding: 2900 Northwest

AB reach: 126 m (short due to school buildings)

Array: Schlumberger

Elevation: 1732mas


Estimated at: ??

Measured data

Remarks: Discontinuity at AB/2 32m is considerable, maybe caused by e.g close-by buildings?

DATA table VES 8


0.5 1.6 34 34

2 29 29

2.5 33 33

3.2 37 37

4 41 41

5 43 43

6.3 49 49

8 50 50

10 51 51

0.5 13 52 52

5 13 53

16 56 56

20 62 62

25 66 66

5 32 71

10 32 83 71

40 85 73

50 94 80

10 63 93 80

25 63

80

100

25 130

50 130

160

200

250

50 320

100 320

400

500

VES measurements

10

100

1 10 100AB/2 (m)

Rh

o (

oh

mm

)

Rho

Rho shifted

P a g i n a | 22

VES interpretation



3 layer model with shifted or unshifted data

Basement layer (assuming ρ>1000 ohmm) beyond Depth of Investigation


3 layer model, shifted data



topsoil (1000) >75

*)Please note the topsoil Rho can be just a phony layer representing electrode contacts, electric leakings

Conclusion

• Overburden is like 54 (52-56) Ωm.

• Weathered/Fractured rock (≈ 100 Ωm) starts at 10 (9-13) m.

• Fresh basement rock unknown, but at least deeper than 75m

• VES is too short to see any evidence of fracturing/structural lateral changes, though the measured shift at AB/2=32m may

indicate a geological anomaly.

1 10 100 L/2

1

10

1e+2

Rho

1 10 100 L/2

1

10

1e+2

Rho

P a g i n a | 23




General


Aim: finding locations for infiltration (highly wheathered or fractured rock)

Location name: end of HEP line2, SW of the Gully B orthogonal to HEPline


Location



Direction of sounding: SEast- NW

AB reach: 1000 m

Array: Schlumberger

Elevation 1710 mas



Measured data

Remarks: The shifting is wrong due to an error outlier measurement at AB/2 = 63. Beyond that point (63) , the unshifted data

were used in the inversion.

DATA table VES 9


0.5 1.6 231 231

2 228 228

2.5 201 201

3.2 176 176

4 169 169

5 151 151

6.3 153 153

8 144 144

10 141 141

0.5 13 130 130

5 13 153

16 143 121

20 135 116

25 123 105

5 32 125

10 32 131 107

40 132 107

50 135 110

10 63 83 67

25 63 126

80 151 81

100 171 92

25 130 210 112

50 130 189

160 244 145

200 274 162

250 317 187

50 320 377 223

100 320 357

400 430 268

500 466 291

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 24

VES interpretation




4 layer shifted data




topsoil ------ 250 - 300 *)

overburden 1 - 2 130 - 150

Shallow aquifer 8 - 15 85 - 110

Basement 50 - 70 700 – 900 (just below 1000)

*)Please note the topsoil Rho can be phony layer representing electrode contacts, electric leaking

Conclusion

• Basement at ≈60 m (50 – 70) overlain by shallow aquifer and overburden

• Basement resistivity is relatively low (just below 1000 Ωm).

• Minor/ no structural changes in SE direction

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

1e+2

1e+3

P a g i n a | 25

Kajiado VES sounding nr.10 and 12 Technical interpretation report



General

Two perpendicular VES soundings ISGEAG geophysical project .

Aim: reconnaissance to find interesting locations to test contemporary geophysical methods

Location name: NW of Kajiado town Two perpendicular VES soundings in between 2 branches of Kajiado river (Kajiado river

branch and Imutatin river branch in the West)


Location


Location: (WGS82): 360 46’ 02.3” E and Latitude 01048’ 50.9” S

Direction of sounding: North - South (Ves10) and East West (Ves 12) ,

AB reach: 1000 m

Array: Schlumberger

Elevation: 1680 mas


Estimated at: 15 - 20 m

P a g i n a | 26

Measured data VES10

Measured data VES12

Remarks: Apart from the (unexplained) discontinuity in VES12 (at AB/2=13 m) there is no big difference in lateral changes

between the two soundings.

It is not clear whether the discontinuity in VES12 actually due to a change in geology or a measurement error

DATA table VES 10


0.5 1.6 42.8 42.8

2 38.7 38.7

2.5 43.2 43.2

3.2 44.4 44.4

4 51.2 51.2

5 45.9 45.9

6.3 49.5 49.5

8 50.9 50.9

10 51.1 51.1

0.5 13 49.1 49.1

5 13 45.5

16 51.2 55.3

20 59.8 64.6

25 72.7 78.6

5 32 91.6

10 32 84.1 99

40 103.9 122.3

50 127.5 150.2

10 63 164 193

25 63 131

80 157 231

100 192 283

25 130 247 364

50 130 256

160 307 436

200 378 537

250 445 632

50 320 513 728

100 320 553

400 654 861

500 928 958

VES measurements

10

100

1000

10000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

Rho

Rho shifted

DATA table VES 12


0.5 1.6 64 64

2 60 60

2.5 64 64

3.2 69 69

4 75 75

5 75 75

6.3 82 82

8 84 84

10 87 87

0.5 13 98 98

5 13 56

16 60 104

20 70 122

25 82 144

5 32 101

10 32 93 177

40 113 215

50 143 272

10 63 179 341

25 63 173

80 198 391

100 218 430

25 130 227 448

50 130 230

160 279 544

200 344 671

250 381 743

50 320 497 969

100 320 485

400 628 1254

500 765 1528

VES measurements

10

100

1000

10000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

Rho

Rho shifted

P a g i n a | 27

VES interpretation




VES10 Unshifted 5 layer

VES12 5layer model , first measurements shifted down



topsoil (1000) 120 - 200 >>>>1000 Ωm

Conclusion

• Overburden layer accurately 44 – 70 Ωm

• Shallow 300 – 500 Ωm aquifer and

• deep, high resistive basement > 120m (like 150m)

• Apart from the unexplained shift in (the first - shallow - part of VES12) there are no indications for large structural changes

in both directions.

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

10

1e+2

1e+3

1e+4

1e+5

1e+6

Rho

P a g i n a | 28




General

VES sounding for VIA Water ISGEAG project.

Aim: finding locations , fracture zones for the ISGEAG geophysical methods project

Location name: on the Kajiado river bed South of Haraf Borehole

Date : 17/02/2017

Location



Direction of sounding: SSWest on the river bed ,

AB reach: 1000 m

Array: Schlumberger

Elevation: 1670 mas


Estimated at: 20 m

VES interpretation

Measured data

Remarks: just minor discontinuity (lateral changes)

DATA table VES 11


0.5 1.6 110.95 110.95

2 80.9 80.9

2.5 73.6 73.6

3.2 69.8 69.8

4 72.5 72.5

5 78 78

6.3 89.8 89.8

8 96.3 96.3

10 101.3 101.3

0.5 13 111 111

5 13 109

16 112 114

20 118 120

25 116 118

5 32 116 117

10 32 118

40 121 120

50 125 125

10 63 139 139

25 63 134

80 162 168

100 197 204

25 130 291 301

50 130 268

160 319 358

200 400 448

250 490 550

50 320 606 680

100 320 565

400 705 850

500 794 957

VES measurements

10

100

1000

10000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

Rho

Rho shifted

P a g i n a | 29




4 layer model , unshifted data

5 layer model with shifted data



topsoil (1000) 35 (25-43)*) 5000 ((>3000)

*) see sensitivity analysis below

Conclusion

• The 5 layer model concept (basement overlain by a less resistive layer) has a significantly better fit, but the parameters are

uncertain

• Accepting the 5 layer concept, sensitivity analysis indicates basement depth in between 25 a 43 m

• No indications of fracturing (in this SSW direction)

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

1e+4

Rho

P a g i n a | 30

Sensitivity analysis

Sensitivity analysis on the 5 layer model (shifted data) shows the following:

The optimal RMSE fit is 1.51*10-2. Any solution within a 10% less good fit (RMSE < 1.66 *10-2) is acceptable

Minimizing (and fixing) the basement depth shows a (still mathematically acceptable) solution as:

Minimized basement depth (at 16m, RMSE = 1.56*10-2) mathematically possible, but it is most unlikely that the sounding can

‘see’ such a thin low resistivity layer.

And maximizing basement depth gives a (maximal) solution as follows:

Maximum basement depth solution (basement at 43m RMSE = 1.65*10-2)

From this we conclude that basement depth is somewhere in between ≈25m and 43m

1 10 100 L/2

1

10

1e+2

1e+3

1e+4

Rho

RMSE = 1.56

1 10 100 L/2

10

1e+2

1e+3

1e+4

Rho

RMSE = 1.65

P a g i n a | 31




General



Location name: on the Gully bed (Northern runoff-gully c) in the centre of HEP profile 1


Location


Location: (WGS82): 360 46’ 23.4” E and Latitude 010549’ 34.4” S

Direction of sounding: SEast on top of the gulley bed ,

AB reach: 126 m

Array: Schlumberger

Elevation: 1682 mas


Estimated at: 20 m

Measured data

Remarks: Only minor discontinuities (at AB/2=32m)

DATA table VES 13


0.5 1.6 34 34

2 15.3 15.3

2.5 14 14

3.2 14.2 14.2

4 14.1 14.1

5 15.2 15.2

6.3 17.3 17.3

8 21.9 21.9

10 26.2 26.2

0.5 13 35.5 35.5

5 13 32.9

16 40.5 43.7

20 52.5 56.7

25 76.6 82.7

5 32 106.1

10 32 95.1 114.6

40 126.9 154.1

50 171 206

10 63 229.5 276.6

25 63

80

100

25 130

50 130

160

200

250

50 320

100 320

400

500

VES measurements

10

100

1000

10000

1 10 100AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 32

VES interpretation




3 layer Unshifted data

4 layer Shifted data



topsoil (1000) 4 – 5 >>>1000

Conclusion

• Topsoil is representing the clayey sand deposits on the gully bed

• Thickness of these deposits is 3 – 4 m

• Right below the gully bed there is high resistive fresh basement rock at ≈ 4 m depth

• No signs of lateral changes is this direction , no indications of fracturing (in this direction)

Remark:

Other experiments (visual and logger data) prove that water infiltrates at this location from the gully into the subsoil. There

must be major permeable fractures right to the North or South gulley bank allowing this vertical infiltration

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

1e+4

1e+5

Rho

P a g i n a | 33




General


Aim: underground at West side of HEP8 line (having a constant resistivity of ≈60 ohmm)

Location name: Kajiado River, close to Osiligi farm at the West end of ISGEAG HEP profile 8

Date : 18/04/2017

Location



Direction of sounding: N-South

AB reach: 400 m

Array: Schlumberger

Elevation: 1675 mas


Estimated at: 20 m

Measured data

Remarks: Only minor discontinuity at AB/2=130m

DATA table VES14

# MN/2 AB/2 Rho Rho shifted

0.5 1.6 18.6

2 20.4

2.5 20

3.2 18.1

4 15.3

5 12.6

6.3 10.4

8 8.4

10 7.5

0.5 13 8.5

5 13 8

16 9.7

20 12

25 13.9

5 32 18.2

10 32 17.1

40 23.1

50 30.9

10 63 43.2

25 63 46.6

80 65

100 85.4

25 130 116.6

50 130 100.8

160 128.5

200 174.3

250

50 320

100 320

400

500

VES measurements

1

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

P a g i n a | 34

VES interpretation



4 or 5 layer model unshifted data

4 layer model , clay right on top of very high resistive basement

5 layer model, having an ‘in between’ layer below the clay overlaying basement

Results including 90% confidence levels (

Depth (m) Rho (Ωm)

(top of the layer)

topsoil (>>1000

Conclusion

• Clay layer on top of shallow very high resistive rock (hardcap?)

1 10 100 L/2

0.1

1

10

1e+2

1e+3

1e+4

1e+5

1e+6

Rho

P a g i n a | 35




General


Aim: to confirm ERT2 results at Kajiado River

Location name: 65 m East of the ERT profile centre

Date : 21/04/2017

Location


Location: (WGS84): 360 45’ 59.8” E 010 49’ 03.5” S

Direction of sounding: 195 azimuth S-N

AB reach: 1000 m

Array: Schlumberger

Elevation: 1692 mas


Estimated at: 30 m

Measured data

Remarks: major discontinuities. The south line was close to powerline at AB/2=250 m (or even passed the river bed?)

DATA table VES15


0.5 1.6 308 355.432

2 331 381.974

2.5 367 423.518

3.2 394 454.676

4 380 438.52

5 389 448.906

6.3 384 443.136

8 382 440.828

10 352 406.208

0.5 13 337 388.898

5 13 389

16 348 348

20 334 334

25 278 278

5 32 249 249

10 32 258 257

40 248 248

50 227 227

10 63 237 237

25 63 222 222

80 242 242

100 265 265

25 130 303 303

50 130 293 293

160 325 325

200 389 389

250 384

50 320 404

100 320 441 539.784

400 532 651.168

500 591 723.384

VES measurements

100

1000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

Rho

Rho shifted

P a g i n a | 36

VES interpretation



4 layer unshifted or shifted

4 layer model unshifted data

4 layer model shifted data

Results including 90% confidence levels (

Depth (m) Rho (Ωm)

(top of the layer)

topsoil 0 (340 – 700)

Overburden 7 (2 – 12) ( 185 – 210)

Basement (ρ>1000) 80 (65 – 95) >800

Conclusion

• Shallow aquifer

• Basement (not very high resistive) at 80m

Discontinuities may be caused by anomalies or by passing powerline or river bed

1 10 100 L/2

1e+2

1e+3

1 10 100 L/2

10

1e+2

1e+3

P a g i n a | 37




General


Aim: to confirm ERT1 anomaly results at Gully C

Location name: , Kajiado Osiligi , Northern run-off Gully C about 130 m North East of the Gulley bed, on HEPline/ERT line 1

Date : 23/04/2017

Location map


Location: (WGS84): 360 46’ 25.2” E 010 49’ 31.9” S

Direction of sounding: 295 GR azimuth SE-NW

AB reach: 1000 m

Array: Schlumberger

Elevation: 1687 mas


Estimated at: 25 m

Measured data

DATA table VES16


0.5 1.6 37 37

2 42.4 42.4

2.5 49.2 49.2

3.2 58.9 58.9

4 73.5 73.5

5 88.9 88.9

6.3 104 104

8 116 116

10 131 131

0.5 13 147

5 13 129 147

16 153 174.35

20 161 183.47

25 161.4 183.92

5 32 174

10 32 203 198.28

40 215 210

50 239 233.44

10 63 277

25 63 279 270.56

80 315 305.47

100 353 342.32

25 130 422

50 130 349 409.23

160 389 456.13

200 421 493.66

250 494 579.26

50 320 606

100 320 527 710.59

400 584 787.44

500 599 807.67

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

Rho

Rho shifted

P a g i n a | 38

VES interpretation



4 or 5 layer model shifted data

(the unshifted data have too many discontinuity)

4 layer model shifted data

5 layer model , shifted data

There is reason to believe, and to accept the 5 layer model (it corresponds with the findings in the ERT tomography), the

uncertainty though is high.


Depth (m) Rho (Ωm)

(top of the layer)

topsoil (1000) 76 (40 – 160) 1250 (1000 – 1500)

*) large: meaning that uncertainty is high and not quantifiable

Conclusion

• Basement depth uncertain; shallow aquifer in the order of 500 ohmm possibly below a (57 ohmm) sub-layer

1 10 100 L/2

1

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 39

Kajiado VES sounding nr.17 and 18 Technical interpretation report



General

VES sounding for VIA Water ISGEAG project Kajiado.

Aim: to confirm fracturing at HARAF Traders borehole at Kajiado River

The HARAF borehole is probably the best yielding borehole in Kajiado. It is believed to abstract water from a major regional

fracture zone close to Kajiado river. Unfortunately no documents (from the borehole survey, nor from borehole completion and

drillers’ log, could be made available.

Two VES soundings were conducted in two directions, close to the borehole, mainly to detect fracture structures. The positioning

of the VES lines was restricted by local powerlines and river beds.

The co-operation the owner Mr. Osman is highly appreciated

Location name: , Kajiado (Olkuajado) river close to HARAF Traders borehole

Souning date : 25/04/2017

Location

Location details. The center-points of the two VESs were 10 m apart, about 40 m NE of the HARAF traders borehole

P a g i n a | 40

VES 17 and 18 sounding information

Location: (WGS84): VES17: 360 46’ 03.8” E 010 49’ 11.0” S

VES18: 360 46’ 03.5” E 010 49’ 11.1” S

Direction of sounding: VES17: azimuth 360gr. S-N

VES18: azimuth 67gr ENE

AB reach: VES17: 500 m

VES18: 320 m

Array: Schlumberger

Elevation: 1675 mas

Depth to groundwater

Estimated at: 15 m (SWL from data logger observations in the borehole)

Measured data

Remarks:

Although the centerpoints were just 10 m apart, there is a striking difference in the field curves, both in the absolute level and

the discontinuity (AB/2 = 63m) in VES18.

VES17 was done parallel to the believed N-S fractured structure direction and VES18 was across the structure direction.

VES17 has passed the river bed on the south side at AB/2 = 150 m.

DATA table

MN/2 AB/2 VES17 VES18

0.5 1.6 18.3 139

2 17 166

2.5 16 191

3.2 16 200

4 17 189

5 17.9 170

6.3 19.2 141

8 20.4 98

10 21.9 84

0.5 13 24.9 80

5 13 26.7 73

16 30.1 77

20 35 91

25 42.3 108

5 32 52.1 122

10 32 56.3 112

40 64 134

50 75.4 148

10 63 93.6 179

25 63 89 127

80 125 171

100 165 171

25 130 236 214

50 130 236 211

160 282 249

200 349

250 558

50 320

100 320

400

500

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

VES17

VES18

P a g i n a | 41

VES interpretation VES17

VES17 Inversion results (SchlumBG modelling)


3 or 4 layer model unshifted data

3 layer unshifted

4 layer unshifted

VES17 Results including 90% confidence levels

Depth (m) Rho (Ωm)

(top of the layer)

topsoil (1000) 12 (10 – 15) >>>1000

Conclusion VES17

• High resistive basement at ≈10 – 15 m depth

• No indication of fracture zone (discontinuity)

1 10 100 L/2

10

1e+2

1e+3

1e+4

1e+5

1e+6

Rho

1 10 100 L/2

10

1e+2

1e+3

1e+4

1e+5

1e+6

Rho

P a g i n a | 42

VES18 Inversion results (SchlumBG modelling)


5 layer model unshifted or shifted data

5 layer unshifted

5 layer unshifted leaving out last measurements

5 layer with shifted data

1 10 100 L/2

1

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 43

VES18 Results including 90% confidence levels

Depth (m) Rho (Ωm)

(top of the layer)

topsoil (1000) 80 (>30)) >1000

Conclusion VES18

• Shallow clay layer

• Basement at 80 m

• Discontinuity at potential electrode change AB/2 = 63m

Comparing VES17 and VES18

Although the two soundings are just 10 m apart, there is a clear difference:.

• Basement in VES18 is deeper and there is a shallow clay layer.

• Basement resistivity in VES17 is higher.

This indicates a significant difference in lithology within short range. Remaining question is: why doesn’t VES18 show

discontinues measurements at the short range, and only at AB/2 = 63??

VES17 VES18

1 10 100 L/2

10

1e+2

1e+3

1e+4

1e+5

1e+6

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 44

Kajiado VES /IP sounding nr.19 Technical interpretation report



General


Aim: to confirm TDEM sounding at the east end of ERT line2

Location name: Kajiado on the East bank of Kajiado River (end of HEP line 8 and ERT line2)

Date : 26/04/2017

Location


Location: (WGS84): 360 46’ 11.6” E 010 49’ 06.2” S

Azimuth of sounding: 205 GR SSW

AB reach: 1000 m

Array: Schlumberger

Elevation: 1675 mas


estimated at: 15 m

P a g i n a | 45

Measured data Resistivity (Ωm) and IP (mS)

Remarks:

When the measured line showed large discontinuity 2 additional points (in blue) were measured with a larger MN/2 of 100m

instead of 50m (red points).

obviously a major geological change was reached at ≈ AB/2 100 m (or even at 63 m).

DATA table

MN/2 AB/2 VES19 MN/2 100

0.5 1.6 116

2 135

2.5 140

3.2 159

4 167

5 154

6.3 119

8 132

10 125

0.5 13 130

5 13 119

16 127

20 139

25 156

5 32 179

10 32 173

40 207

50 233

10 63 249

25 63 239

80 219

100 207

25 130 230

50 130 216

160 217

200 199 262

250 216 319

50 320 242

100 320 370

400 471

500 652

DATA table IP

MN/2 AB/2 VES19 MN/2 100

0.5 1.6 0.21

2 0.19

2.5 0.2

3.2 0.21

4 0.24

5 0.27

6.3 0.24

8 0.43

10 0.67

0.5 13 0.75

5 13 0.75

16 0.75

20 0.75

25 0.77

5 32 0.9

10 32 1.08

40 1.12

50 1.44

10 63 2.1

25 63 1.89

80 1.97

100 2.89

25 130 1.94

50 130 2.1

160 2.09

200 2.23 2.71

250 2.49 3.68

50 320 2.67

100 320 2.33

400 2.82

500 4.96

VES measurements

100

1000

1 10 100 1000AB/2 (m)

Rh

o (

oh

mm

)

MN/2 100

VES19

IP measurements

0

1

2

3

4

5

6

1 10 100 1000AB/2 (m)

IP (

mS

)

MN/2 100

VES19

P a g i n a | 46

VES interpretation

(IP interpretation is not discussed here)



4 layer only using the data up to AB/2 = 63 m

Or

6 layer using the full measurements and leaving out the 3 points with MN/2 50 at 200, 250 and 320 m

4 layer using only data up to AB/2 = 63m

6 layer model

Overall results including 90% confidence levels (between brackets)

Depth (m) Thickness (m) Rho (Ωm)

(top of the layer)

topsoil (10 500 (300 – 800)

Basement (ρ>1000) > 25 >1000

Alternative interpretation:

Low resistive layer 24 (large) 9 13 (large)

Basement 33 (large) >>> 1000

Conclusion

• Measurements show major discontinuity at AB/2 > 63 m indicating lateral lithological changes

• Basement deeper than 25 m, may be at 33m (with large uncertainty) underlaying a low resistivity weathered zone between

24 and 33 m.

To better detect anomalies it makes sense to measure parallel with alternative MN potential electrode distances

IP measurements show discontinuities at larger range as well. Shallow IP confirms that there are no shallow clay layers

1 10 100 L/2

10

1e+2

1e+3

1 10 100 L/2

10

1e+2

1e+3

1e+4

P a g i n a | 47

Kajiado VES nr.20 Technical interpretation report



General


Aim: to confirm ERT/TDEM soundings in the centre of ERT line2

Location name: ,Kajiado ,Ildamat, Osiligi , Kajiado River


Location


Location: (WGS84): 360 46’ 04.3” E 010 49’ 03.7” S

Azimuth of sounding: 7 degrees NS

AB reach: 400 m

Array: Schlumberger

Elevation: 1680 mas


estimated at: 15 m

Measured apparent resistivity data (Ωm)

Remarks:

Discontinuity at AB/2 = 63m. The AB reach is short; the sounding had to be stopped due to planned TDEM measurements at the

same location.

DATA table

VES20

MN/2 AB/2 Rho Rho shfted

0.5 1.6 86 86

2 103 103

2.5 116 116

3.2 119 119

4 125 125

5 121 121

6.3 112 112

8 86 86

10 79 79

0.5 13 72 72

5 13 74 74

16 72 72

20 71 71

25 78 78

5 32 102 102

10 32 106 106

40 127 127

50 150 150

10 63 179

25 63 151 179

80 174 206

100 196 232

25 130 246 292

50 130 234 277

160 289 343

200 338 401

250

50 320

100 320

400

500

VES measurements

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho shfted

Rho

P a g i n a | 48

VES interpretation



5 layer model with unshifted or shifted data

5 layer model, basement can be anywhere deeper than 55 m

5 layer model shifted data basement anywhere deeper than 45 m

Overall results including 90% confidence levels (


(top of the layer)

topsoil (1000) 90 (> 45) 2500 (>1000)

Conclusion

• Clay layer

• Basement depth uncertain but anywhere deeper than 45 m

1 10 100 L/2

1

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 49

ERT comparison

The ERT2 profile (ISGEAG research) shows a high resistive ‘hard cap’ at 25 m, overlying a less resistive (250 ohmm) zone and the

actual basement somewhere at 100m.

We forced the interpretation (by fixing layer parameters) to see if the VES data could correspond with such a solution.

The answer is YES it could (see graphs below), but any solution of such reverted low resistivity under a high resistivity is possible.

So this cannot be found without additional knowledge (the ERT info in our case).

Reverted low resistivity below a ‘hard cap’ high resistivity (VES20 6 layers, unshifted data)

Reverted low resistivity below a ‘hard cap’ high resistivity (VES20 6 layers, shifted data)

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

10

1e+2

1e+3

Rho

P a g i n a | 50

Kajiado VES /IP sounding nr.21 Technical interpretation report



General

VES sounding for VIA Water ISGEAG project, to verify an existing VES sounding

Aim: to confirm VES4 (clay on top of basement) from survey report (Kariuki for mr. Osman, HARAF

Location name: Kajiado, Ildamat, Osiligi , at HARAF farm West of Kajiado River.


Location


Location: (WGS84): 360 45’ 54.4” E 010 49’ 25.9” S

Azimuth of sounding: 7 degr. NSouth

AB reach: 160 m

Array: Schlumberger

Elevation: 1670 mas


estimated at: 5 m

Measured data Resistivity (Ωm) and IP (mS)

Remarks:

discontinuity in measurements at AB/2 40m. indicating fracture/lateral change. Measurements > 32 m left out in the inversion.

DATA table VES21

MN/2 AB/2 Rho IP (mS)

0.5 1.6 23 0.23

2 23 0.23

2.5 21 0.2

3.2 19 0.19

4 16 0.23

5 14 0.47

6.3 13 0.46

8 13 1.12

10 14 0.94

0.5 13 18 0.47

5 13 19 0.46

16 24 0.82

20 29 0.85

25 36 1.48

5 32 56 1.1

10 32 52 1

40 56 1.73

50 65 1.93

10 63 82 2.04

25 63 76 1.94

80 99 2.48

100

25 130

50 130

160

200

250

50 320

100 320

400

500

VES measurements

0.1

1

10

100

1 10 100

AB/2 (m)

Rho (ohmm)

Rho

IP (mS)

P a g i n a | 51

VES interpretation



3 layer Measurements > 32m left out



(top of the layer)

topsoil (1000) 3.4 >>>>1000

Conclusion

• This verification VES clearly shows a shallow clay layer on top of high resistive (basement) rock

• Discontinue measurements (>40m) indicate anomaly (fractured?)

The results of this VES can be compared with the VES4 results in the existing survey report, see

borehole survey for Mr. Osman, HARAF farm near Kajiado River, page @

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 52

Kajiado mini VES Technical interpretation report



General

Mini VES sounding for VIA Water ISGEAG project.

Aim: short range VES to test the resistivity of the clay topsoil and training purposes

Location name: ,Kajiado ,Ildamat, Kajiado River on West-East HEP/ERT profile


Location


Location: (WGS84): 1°49'3.50"S 36°46'8.04"E

Azimuth of sounding: Northeast - Southwest

AB reach: 64 m

Array: Schlumberger

Elevation: 1676 mas


estimated at: 15 m

Measured apparent resistivity data (Ωm)

Remarks:

The mini VES sounding was conducted at a location where compact clays were found (and confirmed by shallow hand drillings).

The electrode distances are very short, starting with potentials at only 5 cm and current electrodes at only 20 cm from the

centre point (see photo). The electrode distances were increased in small steps, from 20cm, 40cm,… up to AB/2 of 32 m.

The field curve shows apparent resistivity initially going down and consequently going up when the current starts passing the

bottom of the clay layer.

VES measurements

1

10

100

0.1 1 10 100AB/2 (m)

Appare

nt re

sis

tivity

(ohm

m)

DATA table

Mini VES

MN/2 AB/2 Rho

0.05 0.2 14

0.4 9.8

0.8 8.7

1.6 7.8

3 7.9

6

0.5 6 9.6

12 15

20 24

32 38

5 32 39

P a g i n a | 53

Foto ISGEAG field team conducting mini VES (a ‘veeeeery small ves’)

VES interpretation


The SchlumBG software assumes the AB/2 distances are in meters. SchlumBG in this case appears to have difficulties with the

data points within one meter. Since the inversion mathematics is linear with electrode distance, the distances are multiplied by

factor 10 to avoid the software problem. The layer model depth results are therefore in dm (decimetres).


3layer model (note: electrode distance in decimetres!



(top of the layer)

topsoil (1000 ohmm basement)

• According to the ‘rule of thumb’ the Depth of Investigation (1/6 of AB) in this case is like 10 m (AB = 64m)

1 10 100 L/2

1

10

1e+2

Rho

layer depth in dm!!

P a g i n a | 54

Existing survey reports The following section contains the re-interpretation of VES soundings from existing survey reports in the area of Kajiado

Town

P a g i n a | 55

MTTI Masai Technical Training institute survey re-interpretation of VES soundings report



General

Existing survey report: Hydrogeological survey for Masai Technical Training Institute, Kajiado. Report nr. AWS/1572/2008;

S.N.Njoroge. Aquawell Services Nairobi, February 2008

Aim: verification/re-interpretation of existing VES sounding data

Location


Location: (WGS82): unknown (general location MTTI compound = 36°46'55.3"E 1°51'16.1"S)

Direction of sounding: ?

AB reach: 400 m

Array: Schlumberger

Elevation: 1715 - 1730mas


Estimated at: ??

MTTI VES Measured data (for numbers, see report)

Remarks

VES4 was advised for drilling, the data may have been shifted to get smooth curves

VES Graphs MTTI (survey report)

10

100

1000

1 10 100 1000

AB/2 (m)

Resis

tivity

MTTI VES1MTTI VES3MTTI VES2MTTI VES4

Dry boreholes issue The MTTI survey comprised 2 HEP lines and 4 VES soundings. Unfortunately the precise locations are not given in the report. (in the

report the VES soundings all have the same coordinates)

Striking fact is that two borehole were initially drilled at the MTTI parcel, both were found dry.

According to a visit paid on 15 Feb 2017 (info Mr. Stephen Musyimi, (0725326932) the situation became more clear. The first dry

borehole (1) (): 36°46'58.83"E 1°51'9.42"S) presumably was drilled at location of VES4 (as advised in the survey report). The second

dry drilling, at 36°46'56.52"E 1°51'30.75"S most likely was at VES1 location. This drilling had a water strike at 70mbgl, but the water

‘disappeared’ when drilling continued to 90 mbgl)

Later (unknown date) a third, good yielding borehole was drilled at 36°47'2.71"E 1°51'9.63"S, just 120 m East of the dry borehole

(1). This borehole is presently known as the Masai Tech borehole. Unfortunately no technical borehole documents could be

made available.

P a g i n a | 56

MTTI VES1 interpretation


Survey report MODEL VES1

(re-interpreted) 6 layer model

There may as well (giving the same fit) be basement (1000ohmm) at 80 m:

8 layer model with basement at 80m

Results VES1 (90% confidence levels)


topsoil (1000) anywhere > 80m

Conclusion

• Due to the undulations in the field curve, many solutions are possible

• Unknown to what extend the measured data are corrected and whether these undulations are realistic

• Up to 20 m (‘overburden’) a variety of low and mediate resistivities

• Basement may be anywhere below 80 m

P a g i n a | 57

MTTI VES 2 interpretation



VES2 MODEL acc. to the Survey report

5 layer model

There may as well (the same fit) be basement (1000ohmm) at 80 m:


Results MTT VES2 (90% confidence levels)



Conclusion

• Below (low resistive) topsoil there is an overburden zone of 70 -80 ohmm

• Resistivity of the shallow aquifer is depending on the actual depth of basement


P a g i n a | 58

MTTI VES 3 interpretation



Interpreted model from the SURVEY REPORT

5 layer model THE LAST 4 measurements are shifted (indicating lateral change)

There may as well (same fit) be basement (1000ohmm) at 130 m:


(Even when the last measurements are corrected for the shifts, basement is deeper than 130m)

Results (90% confidence levels)



Conclusion

• Below (low resistive) topsoil there is an overburden zone of 70 -100 ohmm



1 10 100 L/2

10

1e+2

1e+3 acc to survey report

P a g i n a | 59

MTTI VES 4 interpretation (location advised for drilling)


Survey report MODEL VES4

4 layer model

If the 3 (declining) last measurements were correct (!!), basement would be deep (as 200m or more)

5 layer model leaving out last (3) measurements (basement could even be at 50m) giving the same, good fit

Results (90% confidence levels)


topsoil (1000) can be deep but could as well be at 50m (depending on the last 3 measurements being y/n correct)

Conclusion

• Below topsoil there is an thin overburden zone of 30 ohmm


• Basement may be at 50m if the last declining measurements are false, if not basement is deep.

VES4 was advised for drilling and turned to be a dry borehole (drilling showed “just dust”). I tend to believe that the last 3

(declining) measurements were wrong.

1 10 100 L/2

10

1e+2

1e+3 SURVEY REPORT

MODEL

1 10 100 L/2

10

1e+2

1e+3

P a g i n a | 60

Kajiado Masai Stores survey Re-interpretation VES existing survey report



General

Existing survey report:

Hydrogeological/Geophysical survey report for one production borehole in LR.NO; 9923/262 forMasai Store Ltd Kajiado. Report

by E.Njaramba, NYIKA Associates Ltd Groundwater Consultants

Aim: verification/re-interpretation of existing VES sounding data

Location


VES

Location: (WGS82): 1°50'52.87"S 36°47'24.75"E

Direction of sounding: ?

AB reach: 400 m

Array: Schlumberger

Elevation: 1730 mas


Estimated at: ??

For measured data, see survey report

P a g i n a | 61

VES interpretation



Model and measurements from the survey report and apparent resistivity acc. to SchlumBG

REMARKABLE DIFFERENCES ON THE SAME DATA ???

Above: Model (fixed parameters) and measurements according to the survey report, entered in SchlumBG software does not

give a good fit.

Below: In the report, the same model on the same data shows a good fit,

WHAT COULD EXPLAIN THE GOOD FIT as given in the report (below)??? Can it be difference in the software used or…….??

1 10 100 L/2

10

1e+2

1e+3

Model from SURVEY

REPORT

P a g i n a | 62

Re-interpretation:

2 possible model concepts (comparable good fit)

4 layer model and 5 layer model

4 layer model

5 layer model , having an in-between lower resistive layer

Results (confidence levels unknown)

Depth top (m) Rho (Ωm )

topsoil (1000) 50 -130 >>> 1000

Conclusion

• Overburden in the order of 50 ohmm

• Shallow aquifer resistivity uncertain

• Top of Basement in between 50 and 130 m

In the survey report the last layer (> 64m) is interpreted as 390 Ωm. According to the re-interpretation the resistivity is higher

(> 1000 Ωm)

What is the explanation for the bad fit we are getting in SchlumBG for the interpreted model of the survey report?

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

10

1e+2

1e+3

Rho

P a g i n a | 63

Kajiado survey for Mr Osman in Town Re-interpretation existing VES



General

Survey report for Mr. Osman (HARAF):

Hydrogeological report survey for Mr. Hamud Mohammed Osman, carried out along Great North Road. S.N.Njoroge, Aqua Well

Services, Nairobi, report nr. AWS/2673A/2011, December 2011.

Aim: to verify and re-interpret existing survey report soundings

Location name: ,Kajiado Township block 1/229

Date : December 2011

Location


Location: (WGS84): approximately 360 47’ 25” E 010 50’ 45” S (coordinates in report are probably in ARC1960, not in

WGS84, coordinates estimated from map)

Azimuth of sounding: N-S

AB reach: 500 m

Array: Schlumberger

Elevation: 1733 mas


estimated at: ? m

VES interpretation

Measured data Resistivity (Ωm) SEE SURVEY REPORT

Remarks on the data:

Remarkable 1 point anomaly at AB/2 = 32 and Only one potential change (at 32m) ??


Model from the Survey Report 10 layers!! Basement (688 Ωm) at 200 m !

1 10 100 L/2

10

1e+2

1e+3

SURVEY

REPORT MODEL

P a g i n a | 64

Possible model concepts: 4 layer model

4.layer model basement at 134 m

4.layer model basement at 100 m, having he same GOOD fit



(top of the layer)

topsoil ( 90 105 (93 – 130)

Basement (ρ>1000) > 90

Conclusion

Instead of the 10 layer model in the survey report, a 4 layer model has the same good fit.

Basement depth uncertain, but deeper than 90 m

1 10 100 L/2

10

1e+2

1e+3

Rho

1 10 100 L/2

10

1e+2

1e+3

Rho

P a g i n a | 65

Borehole survey for Mr. Osman, HARAF farm near Kajiado River

Re-interpretation of soundings form existing survey report


General

Survey report:

Geophysical and hydrogeological investigation J.W.KARIUKI, Groundwater Max Ltd, report nr. GML- 029/07, Feb 2007.

Aim: re-interpretation of geophysical soundings, to understand high yielding HARAF Trades borehole., as part of ISGEAG

geophysical methods research project near Kajiado (Kajiado River)

Location name: Ildamat , Osiligi, 3 km NW of Kajiado Town

Sounding Date : Feb 2007

Location VES4 has been conducted close to ‘HARAF BH2’ close to the Kajiado River

LOCATIONS ARE NOT CLEAR. The location of VES4 (advised for drilling) is estimated from the hand-drawn map in the survey

report.

Note: the coordinates for the selected site are given in the report as 1 49 15 – 36 46 08. These could be in ARC 1960 grid system,

but, even when transferred to WGS84 (01 49’ 24.2” 36 46 10.9) the coordinates do not point at the location as given on the

hand-drawn map.

5 VES soundings were conducted in the survey, along 1 HEP profile line: VES4 was advised for drilling.

The re-interpretation results of only VES4 is given below. The other soundings are not (yet) re-interpreted.

VESnr.4 sounding information

Location: (WGS84): not given in report presumably: 1°49'25.90"S 36°45'54.40"E

Direction of sounding: not given

AB reach: 500 m

Array: Schlumberger

Elevation: 1670 mas


Estimated at: 5 m

P a g i n a | 66

Measured data

Remarks:

the south end of the line must have been close to the powerline at AB/2=250 m

The field curve already indicates a clay layer and shallow basement

VES re-interpretation



4 or 5 layer model

4 layer model , basement at 23 m

5 layer model, basement at 21 m

DATA table HARAF VES4


0.5 1.6

2 10.6

2.5 9.2

3.2 7.4

4 5.9

5 5.7

6.3 6.3

8 7.2

10 9.7

0.5 13 12.2

5 13

16 12.8

20 15.6

25 20.3

5 32 25.7

10 32

40 28

50 36.2

10 63 46.2

25 63

80 63

100 73.6

25 130 92.8

50 130

160 117.7

200 155.6

250

50 320

100 320

400

500

VES measurements

1

10

100

1000

1 10 100 1000AB/2 (m)

Rho (ohm

m)

Rho

Rho shifted

1 10 100 L/2

1

10

1e+2

1e+3

Rho

1 10 100 L/2

1

10

1e+2

1e+3

Rho

P a g i n a | 67

Re-interpreted results

Depth (m) Rho (Ωm)

Date post:	04-Feb-2021
Category:	Documents
Upload:	others
View:	4 times
Download:	0 times

VES soundings Kajiado Town - SamSamWater...VES soundings Kajiado Town Technical Interpretation and...

Documents