In this work, interpretation has been made based on the results of seismic refraction with the help of a borehole lithological log data. The borehole data helps to understand the vertical geological section of the study area and to correlate these different units with the seismic refraction velocity model sections. The depth of the boreholes used for lithological correlation is 296m; whereas the depths of the seismic velocity models are about maximum depth 20-35m, i.e. the depth of the geophysical sections is smaller as compared to the depth of the borehole depth. A borehole used in the interpretation of the geophysical data is found near the boundary of the study area drilled by Amhara design and supervision works enterprise collaboration with Amhara water, irrigation and energy bureau for water supply purpose to Injibara University for the coming year consumption.
Interpretation of seismic refraction data
Data were processed using the software programs PickWin95 and Plotrefa from the SeisImager software package. These programs allow cross-sectional areas of the subsurface beneath each spread to be plotted, thus modeling the bedrock interface using a time-term inversion method was employed. A three-layer model was employed to represent the basalt bedrock, vesicular basalt and a thin top soil layer. Raw field data were imported into PickWin95 and then before picking the first arrival times, band pass frequency filter using upper and lower cut off frequency of 409.6HZ and 56.95HZ respectively was applied to remove the noise and improve signal to noise ratio. The first arrival times were picked up using the auto-pick option of the system for all records and few adjustments were made manually where it seems necessary. This was performed for each of the shot points along the spreads. Then, the first arrival data were imported into Plotrefa, and a plot of time versus distance (1/v) was generated (Fig. 1.6). Plotrefa automatically checks reciprocal times for multiple shot locations. It is best if the root mean square (RMS) error is less than 5%. Most of the data points in the spreads have RMS values below 5%; however, there are some data points with higher RMS errors.
Layers are assigned by identifying crossover points, which occur where the slope of 1/v changes. The crossover point separating vesicular basalt and basalt is minor, but the change in slope between the top soil and bedrock is distinct. After the layer assignment, a time-term inversion model can be run. Velocity is calculated and the model depth is inferred. Velocity models for each spreads were generated using different first arrival picks in order to gain an understanding of model sensitivity.
The model produced using the above software packages were interpreted according to the area geology and the parameters determined from the model. Because of noise data of the seismic refraction spread three, first arrival picking of p-wave travel times was very difficult. Therefore, spread three was not considered in the processing and interpretation part.
Velocity model for spread-1
The seismic refraction velocity model for spread one is presented as in figure1.7. This is 92m long profile which runs NW-SE direction. The model is generated using time-term inversion method. The velocity model represents seismic velocities between 280m/s and 1758m/s. The top most layer shows low p-wave velocity varies 280-520m/s and is about 1.5-3m thick with slight difference along the spread. The velocity indicates that the top layer is composed of soil deposits. The second layer is located at a depth of about2-3m.The velocity of this layer ranges 1400-1355m/s and from the lithological log in the study area this layer is probably weathered and fractured vesicular basalt. The p-wave velocity in the third layer is relatively high and from local geology and lithological log this layer is possibly moderately weathered and fractured basalt. This moderately weathered and fractured basalt is regarded as the bed rock in the building site. The velocity of the model shows that the third layer is relatively strong rock type. Its velocity is 1758m/s. The depth of this layer is varies along the spread. It extends relatively high depth to the left ends of the model (figure1.7).
Velocity model for Spread-2
The seismic refraction of spread two lie in the same line as that of spread one laying in NW-SE direction and its velocity model is presented in figure1.8 with total spread length of 88m.From the velocity model generated from this profile, the time-term inversion model show three p-wave velocity layers. The model presents seismic velocities between 458m/s and 1500m/s. The top layer shows low p-wave velocity of 458m/s and the thickness of this layer is almost similar along the spread. The p-wave velocity indicates that the top layer consists soil deposits of clay, silt and sand. The p-wave velocity of the second layer is varying 900m/s-1250m/s and from the lithological log in the study area this layer more likely to be slightly weathered and highly fractured vesicular basalt. The p-wave velocity in the third layer is relatively high 1500m/s and from the lithological log this layer is probably moderately weathered and fractured basalt. This moderately weathered and fractured basalt is regarded as the bed rock of the site. The velocity of the model shows that the third layer is relatively competent rock formation. The depth of this layer is different at the left and right ends of the velocity model. It is relatively deeper in the left side of the velocity model than towards the right end. From the velocity model we can see that the velocity change among each layer is not gradational i.e. sharp or abrupt.
Velocity model for spread-4
In spread four, the first Layer in the velocity model essentially shows (fig.1.9) low-velocity material overlying on medium velocity layer. This 300m/s indicates that the top layer is dominated by soil deposits of clay, silt and sand with thickness ranging from 2-3m.
The velocity of the second Layer with p-wave velocity 1200m/s indicates moderately zone which is possibly weathered and highly fractured vesicular basalt from the lithological log near by the boundary of the study area. The depth for this layer extends up to about 12m.
The third layer which has a relatively high p-wave velocity material 3000m/s at the base of the velocity model is interpreted as to represent moderately weathered and fractured basalt and it is considered as bed rock in the area. This layer is suggested relatively good for setting civil structures.
Velocity model for spread-5
The seismic refraction of spread five laying in E-W direction which crosses profile four of the electrical resistivity sounding survey. As shown in velocity model (fig.1.10) the thickness of the first top layer is vary 2-3m with seismic velocity of 255m/s. Information from the lithologic log and the velocity value this layer is more likely made up of top soils of clay, silt, and sand. The second layer with p-wave velocity value 1092m/s is corresponding to weathered and highly fractured vesicular basalt. This layer from the geoelectric section has low resistivity due to its high moisture content. The third layer has relatively high p-wave velocity 2225m/s and it is found at a depth up to about 10-12m. From the calculated velocity and borehole information, this layer may be moderately weathered and fractured basaltic formation. This layer is relatively competent and therefore it is regarded as the bed rock in the study area.
Spread-6 Velocity model
Spread six is parallel to profile four of the resistivity sounding survey which lies along SW-NE direction. The seismic velocity model of this spread is shown in figure1.11.The top Layer of the velocity model for spread six is about 3-4 m thick with average P-wave velocity of 510m/s. Layer 2 and 3 have average velocities of 948m/s and 2555m/s respectively. The third Layer is buried about 11-13 m deep in the spread line. The second layer of spread six has relatively low velocity as compared to other spreads in the survey area. This may due to the presence of completely weathered and fractured vesicular basalt. Some intercalation of clay material in the vesicular basalt.
Velocity model for spread-7
Velocity model generated for spread seven located to the northwestern end of the survey area as shown in figure 4.5. As the p-wave velocity, model figure 1.12 shows the thickness of the top most layer varies from 2-3m with velocity of 273m/s. This very low p-wave velocity indicates the top layer is composed of clay, silt and sand deposits. From the lithological log in the study area the second layer is more likely to be weathered and highly fractured vesicular basalt with p-wave velocity varies 1400-1802m/s. This layer is mapped at depth ranges 3-15m deep with irregular morphology. Moderately weathered and fractured basalt is mapped below this layer with velocity of 2527m/s. The p-wave velocity of this layer suggests that it is competent enough to be the bed rock of the study area and it is located at a depth of about 20-25m.From the velocity model we can point out that for each layer velocity it is changed abruptly, it does not show gradational change. Relatively this bed rock is one the most competent one. The competent rock formation is relatively at shallow depth in the left side of the velocity model as compared to right end of the spread.