Seismic and geoelectric tomography surveys of dams in Albania

Seismic and geoelectric tomography 

surveys of dams in Albania 

A. FRASHERI, P.NISHANI, L.KAPLLANI, E.XINXO, and B. ÇANGA, Polytechnic University of Tirana, Albania 

F. D HIMA, Institute of Hydrotechnical Studies and Design, Tirana, Albania 

Albania’s hydroelectric power system 

has many large dams of concrete 

and/or rock fill with central clay core. 

Albania also has about 600 dams 

which control the reservoirs of the 

irrigation system. 

It is now necessary to reassess the 

condition of the dams. Geophysical 

data are important in determining 

the stability of the materials and 

rocks. 

Methods. Many geophysical methods 

were applied for in-situ investigation 

of the dams. 

Seismic tomography was used on 

the concrete along the galleries of the 

dams and between the galleries and 

the surfaces of the dams. The tomography 

was combined with refraction 

seismic profiling on the surfaces of 

the dams and in galleries of the concrete 

dams. Seismic was also used to 

investigate the grout curtain in the 

riverbeds under the dams. Geophone 

lines were 0.5-43 m, depending on 

the object’s size and the required 

depth of investigation. Seismic waves 

were generated by a mechanical 

source. Scintrex’s 12-channel ECHO- 

2 was the recording system. 

Seismic data were also used to 

calculate physical-mechanical properties 

for the soil, rocks, and concrete 

of the dams (e.g., Poisson coefficients, 

elasticity dynamic modulus, bulk 

modulus, rigidity modulus). 

Geoelectric tomography was 

used to investigate the clay core of the 

dams’ raw materials. The resistivity 

part of the geoelectric tomography 

used multiple gradient arrays (maximum 

spacing = 360 m) which provided 

a survey depth of 50-70 m. 

Self-potential surveys were carried 

out downstream in the area of 

raw materials to study the effects of 

water filtering through it. 

Results. The seismic and geoelectric 

tomography results in this paper are 

from surveys at the Ulza and Vau Dejes 

hydroelectric power plants. Ulza, constructed 

in 1957, can generate 25.6 MW. 

The concrete dam has a crest length of 

340 m and maximum height of 64 m. 

Vau Dejes, constructed in 1971, can 

generate 250 MW. Its Qyrsaqi dam has 

Vau Dejes hydroelectric power plant dam. (1) Concrete section; (2) gravelfill 

section. 

Concrete dam at the Ulza hydropower plant. 

Figure 1. Seismic tomography results; western edge of the concrete section 

of Vau Dejes dam. 

1384 THE LEADING EDGE DECEMBER 1999 DECEMBER 1999 THE LEADING EDGE 0000 

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Figure 2. Quality of concrete in galleries according to seismic investigation 

2 m under the gallery floor. Concrete section of Qyrsaqi dam. 

Figure 3. Quality of concrete in galleries according to seismic investigation 

0.5 m under the gallery floor. Concrete section of Qyrsaqi dam. 

Figure 4. Seismic section of the concrete section of Qyrsaqi dam. 

Direction of seismic tomography 

Figure 5. Concrete blocks with 

different quality at Ulza dam. 

a concrete section and a gravel fill with 

central clay core section. The dam has 

a crest length of 480 m and maximum 

height of 79 m. Geoelectric tomography 

was performed only in the rawmaterials 

section. 

Tomographic data at 30 m and 53.5 

m in the concrete section of Qyrsaqi 

dam showed that generally, the concrete 

has P-wave velocity greater than 

4000 m/s and shear-wave velocity 

greater than 1900 m/s (Figure 1). But 

at the left edge of the dam, V P 

decreases to less than 4000 m/s. 

S-wave velocity also decreases, showing 

that this sector has weaker physical 

and mechanical properties. 

These results were compared 

with seismic profiling. The concrete 

bottom of inspection galleries generally 

has good physical-mechanical 

properties. But in some sectors of the 

galleries, a superficial layer of up to 

1 m is attached, which is weak 

mechanically (Figure 2). This shows 

that the concrete will deteriorate due 

to the water’s effect or that these sectors 

were constructed with concrete 

of poor quality. 

In Ulza, seismic tomography 

(Figure 3) indicates that the concrete 

is also characterized by high seismic 

velocities (V P = 4300-5035 m/s and V S 

= 2412-2429 m/s). The elastic dynamic 

modulus is (3.27-3.60)10 5 

kG/cm 2 . According to the tomographic 

data, the upper levels of the 

dam have P-wave velocities that are, 

on average, higher than at the lower 

levels. However, S-wave velocities are 

equal in both areas. In addition, the 



SCHEME OF SEISMIC TOMOGRAPHY 

BETWEEN THE GALLERIES 

Figure 6. Seismic tomography results, Ulza concrete dam. 

SKETCH OF GEOPHONE LOCATIONS 

Figure 7. Seismic in-situ test results in inspection gallery at Ulza. 

average deviation of V S is almost half 

that of V P. These facts are an argument 

that V P in the lowest levels is lower 

than in the upper levels as a result of 

SCHEME OF SEISMIC TOMOGRAPHY 

IN DAM BODY 

constant high pressure. The concrete 

at lower levels also contains more 

water and its mechanical properties 

are weaker than the upper levels. In the 

V P velocity >4000 m/s 

V P velocity 3000-4000 m/s 

V P velocity 2000-3000 m/s 

Geophone 

V P and V S according to surface surveys 

Seismic offset 

Ultrabasic rock 

Dam concrete body 

tomogram it is possible to define a sector 

which is characterized by lower 

physical-mechanical properties than 

the surrounding environment. 


Shotpoint 

Seismic discontinuity 


Figure 8. Electrical resistivity real section and seismic sections of head 

refraction wave velocities V P and V S, Vau Dejes’ dam. 

Figure 9. Geoelectrical section of the gravelfill Qyrsaqi dam. 

At Ulza, the inner walls of the 

inspection gallery and the dam surface 

have a low elastic dynamic modulus—up 

to 74.000 kG/cm 2 . At this 

sector, much lake water has infiltrated 

the inner of the dam, showing 

again the impact of “aging” on concrete. 

Evaluation of the physical-mechanical 

properties of soil and rocks. The 

clay has lower resistivity at the center 

and west of Qyrsaqi dam than at 

the east. The seismic velocity is lower 

in the east because water has filtered 

into the clay’s core. 

The average resistivity of the 

dam’s core is about 100 ohm-m. 

However, in five sectors of the core, 

resistivity decreases up to 25 ohm-m. 

These anomalous spots are 3-45 m 

below the top of the dam (Figure 4). 

It is evident that three of these anomalies 

coincide horizontally. The anomaly 

under station 20 starts at a depth 

of 22 m, continues to 45 m, and dips 

toward the west. 

Beneath the superficial layer at 

the top of the dam, which has a thickness 

of 2-10 m and low seismic velocities 

(V P = 1080 m/s and V S = 550 

m/s), seismic tomography shows a 

second clay layer with thickness of 4- 

21 m which increases to the west, 

where the ground dam meets the concrete 

part of the dam. Seismic velocities 

are lower in the western part of the 

dam. The clay’s core, under the second 

layer, is characterized by higher seismic 

velocities (V P up to 2200 m/s and 

V S up to 800 m/s). The elastic dynamic 

modulus, calculated according to V S 

data, varies from 0.04 to 0.8810 5 

kG/cm 2 for the second clay core. 

The lower resistivity and lower 

seismic velocities are interpreted as 

being the result of water filtering 

through the clay core. This interpretation 

does not exclude the possibility of 

heterogeneity of the clay. The decrease 

of seismic velocities with depth shows 

that the clay’s core is compacted. 

Conclusions. Dams at Qyrsaqi and 

Ulza hydroelectric power plants have 

superficial concrete layers up to 1 m 

thick, which have very poor physicalmechanical 

properties. 

The central and western parts of 

the clay core at Qyrsaqi dam have 

some sectors with resistivity and seismic 

velocities that are lower than the 

side parts of the dam. The increases in 

physical properties are interpreted to 

result from water infiltration to the 

clay’s core. 

Suggestions for further reading. 

“Outlook on results of geophysical insitu 

test and monitoring of hydrotechnical 

constructions in Albania” by 

Frasheri et al. (1998 EAGE Meeting). 

“Ingenierurgeologische Untersuchunger 

für den Wasserbau im Fels” by 

Dzienvanski et al. Veb Deutscher Verlag 

für Grundstoffindustrie, Leipzig, 

Germany. LE 

Corresponding author: A. Frasheri, alfi@ 

inima.al

Seismic and geoelectric tomography surveys of dams in Albania

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