Construction of sustainable civil engineering structures, whether simple or complex ones, requires profound knowledge about the characteristics of subsurface earth materials, particularly physical properties of the underlying rocks/soils, distribution of tectonic elements, contents of moisture or fluid within them , etc. Discontinuities in the form of bedding planes, joints, faults and folds highly determine the physical strength (deformation characteristic) of rocks. Similarly, properties of materials filling voids (openings), such as pure/mineralized water, air or both in unconsolidated soils or fractured rocks influence their physical characteristics. Therefore, the stability of civil engineering constructions depends on the correct assessment of the various physical and geotechnical properties of the underlying earth materials where the structures are intended to be constructed (Johnson, 1991).

On the other hand, constructions undertaken over formation with lacking bearing capacities often result in unexpected failures, manifested by cracks, settlements, displacements or total collapses. Particularly, those structures erected over areas where expansive soils (such as clays) are widely distributed demand special attention as their shrinking and swelling characteristics can easily cause damages due to their property variations as a result of moisture / fluid content fluctuations associated with seasonal changes.

Therefore, geotechnical investigation of any construction sites is essential to obtain reliable inputs that enable to develop economically and technically feasible structural designs incorporating mitigation measures to anticipated geo-hazard events.

Like elsewhere in the world, also in Ethiopia public officials require geotechnical investigation data acquired in accordance with the Ethiopian Building construction Code with accompanying recommendations prior to issuing a building permit in order to protect the safety of the public the surrounding environment (Dagnachew D, 2011).

Unlike drilling, pitting and trenching, geophysical methods are environmentally safe and also do not cause any substantial damages/ concerns to the communities. Geophysical measurement responds to change in the physical, chemical, mechanical, elastic, radioactivity or thermal properties of the underlying earth materials. Because of such diverse characteristics usually one or more of the properties correspond to certain features of earth materials, i.e., contact, discontinuity (fracture/fault zones).Unlike direct sampling, such as drilling or pitting and sending samples to laboratories for analyses, geophysical methods respond to different parameter in different ways and deliver information in a short time with minimum expenses.

For engineering applications electrical resistivity and magnetic methods are widely used to map the subsurface structures. These methods depend on the acoustic impedance, ground resistivity and magnetic susceptibility contrast of the subsurface materials respectively.

The unique tectonic setting of Ethiopia results in complex geological and geo-morphological setups where along with these and continuously deteriorating environmental conditions, the country is very vulnerable for such geo-hazard risks, as volcanic, seismic, landslide and alike. Every year Ethiopia allocates quite a substantial amount of budget to the expansion of infrastructures: roads, bridges, dams and building complexes. Particularly, to expand access to education the construction of universities is taking place in different parts of the country and among these is the Injibara University.

To study the foundation conditions at site and evaluate its suitability for erecting a four story building to be used as dormitory for student, subsurface investigations were carried out employing electrical resistivity, magnetic and seismic refraction methods with an ultimate objective of generating inputs for civil applications.

Description of the Study Area

The research area, Injibara University site, is situated, in Awi Zone, Amhara Regional State about 447km NW of Addis Ababa (Figure1.). It is bounded by UTM coordinate 271602-272203m Easting and1211020- 1211378m Northing, and characterized by flat to gently sloping topography bounded by mountains and small hills from the western and southern side. It has an average elevation of about 2552m amsl and located only about 1.5 km SW of the city center, just on the Injibara-Chagni asphalt road.

Figure 1: Location map of the study area.

Methodologies

The success of any research work is highly determined by appropriateness and feasibility of the methodologies employed to achieve the objectives. In this case, to deal with both the general and specific objectives stated earlier, both primary and secondary data are used. Secondary data sources obtained from different government offices, literatures related to geophysics, geological, hydro- and engineering geological investigations are thoroughly reviewed.

In general, the methodology used in this research work involved three steps, which are classified as desk study, field data collection and Data processing and analysis.

Desk Study

Identifying and reviewing of previous data/information that have relevance to the current research work was the first step, which includes the following activities:

Identification and interpretation of topographic and geological maps.

Reviewing published and unpublished maps, reports to get insight into the geological, hydro-geological, engineering geological and geophysical peculiarities of the study area.

Interpretation and develop conceptual models that serve as input to prepare refined field survey methodology.

Fieldwork, which is the most important part of this research work, has been executed. Under this step all, the necessary primary geological and geophysical data have been collected as follows:

Acquisition of magnetic data at average station spacing of 15-20m following selected traverses and at random points considering accessibility conditions employing the GSM-19T proton precision magnetometer.

The magnetic data were collected by establishing a base station and at each station magnetometer reading, position, time and elevation were recorded. Each base station was recovered (revisited) every an hour.

Electrical resistivity sounding using SARIS Terrameter with schlumberger electrode arrays. The interval between successive sounding stations varies from 45-190m.

Seismic refraction data acquisition was carried out employing 48-channel Dolang Seismograph. In this research only 24 and 12-channels with vertical geophones depending the accessibility of the site, where active construction is taking place. The geophones were laid down at a spacing varies from 4 to10m. Seismic refraction data have been collected along seven spreads with the spread length varying from 60 to 120m.

The primary data collected from the field work was processed and analyzed using different updated software packages. These important and widely used software include Oasis Montaj V6.4.2, Arc GIS 10.1, Win Resist, SeisImager/2D, Surfer 10, MapInfo and IPI2win.Using these software packages qualitative and quantitative interpreted data interpretation were made that finally enabled to produce the results  in the form of geophysical anomaly maps, sections and profiles.

Different geophysical instruments were utilized to carry out the current research.

Saris Terrameter

Scintrex Automatic Resistivity Imaging System (SARIS) used for electrical resistivity sounding survey data acquisition. It is a portable terrameter with a digital readout that automatically displays apparent resistivity (in W-m), current (mA), voltage difference (mV) and standard deviation (SD). The accessories included stainless steel electrodes used as current and potential electrodes and reels with wire.

Proton Precession Magnetometer

The GSM-19T proton precession magnetometer is widely used modern equipment with high accuracy (1nT) and resolution (0.2nT). It measures the earth’s total magnetic field (T).

The geology of the study area and its surrounding is dominated by the following rock units, scoraceous basalt, Vesicular basalt, scoria, trachyte and recent alluvial deposits.

Vesicular Basalt

This rock unit is covered large part of the study area and it is clearly out cropped at Ayo river bank and Mesni,Sutena and Zerket stream banks and beds. Ridges surrounding Injibara town are also covered by this unit. Weathering and fracturing are affecting this unit and it is not filled by secondary minerals. Previously drilled well data result indicates this unit gives potential amount of water to the wells.

Scoraceous Basalt

Dark to brown color, weathering, fracturing and vesiculation are the major characteristics of this unit. This unit is dominantly observed at Ayo river bank and Sutena stream bed.

Scoria

This unit is found in western part from the study area and it is clearly out cropped along Injibara-Chagni road cut. It is weathered and its thickness reaches up to 15 meter. This unit is important for ground water storage and conductance, however it is not suitable for engineering foundation purpose.

Trachyte

This unit is found in western ridges of Injibara town that cover small area and it is characterized as fractured, weathered and reddish in color.

Alluvial Deposits

Alluvial deposits are covering low land plain part of the study area and it consists of clay, silt, sand, gravel, cobbles and boulders which are basaltic origin. The clay unit covers large part of the study area. Gravels, cobbles and boulders are dominant on the beds of streams and its thickness varies from place to place.

Figure 2: Geological map of the study area and its surroundings.

The dominant geological structures observed on the area are lineaments, local fault, and medium to large spacing fractures with three major sets trending N-S, SE-NW and NE-SW direction.

 Data Acquisition, Processing and Presentation

The data were acquired and analyzed using different updated software to get the relevant information for subsurface condition of the building site. The detailed data collection, processing, presentation and procedure for the present study are given below.

Figure 3: Distribution of geophysical survey stations with the location of a nearby borehole.

Field Data Acquisition and Instrumentation

The electrical resistivity survey was carried out employing Vertical Electrical Sounding techniques

Vertical Electrical Sounding (VES)

In this case the Schlumberger array was used to study the vertical resistivity variation by systematically expanding the separation between the current electrodes maintaining the center of array fixed for a number of successive measurements. This configuration is less sensitive for lateral in homogeneities. A total of thirteen vertical electrical sounding survey stations along four profiles were conducted to understand the subsurface conditions of the site (Figure4.1). This technique was implemented by injecting electrical current (I) into the ground by means of two current electrodes and measuring the resulting potential difference (DV) by another pair of potential electrodes placed close to the center of the array. Using the measured values of I and DV, the apparent resistivity (ρa) at the measurement point were determined using the Ohm’s law:

                              Where

And k referred to as array coefficient or geometric factor

That takes an account of the mutual arrangement the current and potential electrodes;

The survey lines were oriented in NW-SE and NE-SW directions and in each case the maximum spacing of the current electrodes (AB/2) was 150m. At AB/2=20 and 30m repeated readings were taken at two different potential electrode positions (MN/2) in order to examine the data quality and also assess any possible effect of anisotropic character of the underlying formations.

Figure 4: Field setup of the Vertical Electrical Sounding data acquisition.

The Sintrex-Made SARIS (Sintrex Automatic Resistivity Imaging System) having a maximum power of 100W was used both for resistivity sounding and profiling. This instrument automatically displays ρ_a values along with Standard Deviation (SD), Current (I) and Primary Voltage (V_p) on its digital screen.

The locations, i.e., Easting, Northing and elevations, of each sounding stations were determined employing the Garmin GPSmap 62 receiver,which provides readings with an accuracy of ±2-3m.

The apparent resistivity values obtained from the instrument were plotted on a bi- logarithmic graph sheets. In this case,ρa values on the ordinate (y-axis) and AB/2 valueson the abscissa (x-axis) were plotted. Data were processed using IPI2win and Win RESIST software programs in order to determine the layer parameters (resistivity and thickness/depth) that were used as input for further processing and plotting using Oasis montaj, Surfer and MapInfo softwares and then interpretations carried out. During data processing using IPI2win and WinRESIST software iterations continued until best fitting was attained between the practical and theoretical curves, and in this case an RMS error of 1.7-4% was achieved. Then the values were used to construct the geo-electric sections and based on which data interpretations were made. Generally, good correlation between the field data and the interpreted model sections are obtained and this may be assured by an RMS error of 1.7 to 4% obtained for the sounding data. Several reinterpretations for modeled soundings curves were performed to get better model parameters and the iteration process was finalized when the root mean square (RMS) errors was less than 5%.

In the sounding curves, a 3 to 4 layer of the subsurface is seen and it well represents the subsurface condition of the construction site with the AB/2=150 m used for the survey. The layer parameters acquired by the Win Resist and IPI2win softwares for each sounding point were then used to construct the geoelectric sections of each Profile using MapInfo, Surfer 10 and Oasis montaji softwares. In addition, pseudo-depth section maps and sliced-stack map are constructed using Surfer 10, Oasis montaj(6.4.2) softwares from the apparent resistivity data collected in the field.

Figure 5: Samples of interpreted VES curves.

Magnetic method is a geophysical technique that measures the total magnetic field intensity of the Earth. The magnetic method is capable of mapping various geologic features, such as igneous intrusions, faults, some geologic contacts and lithology. In the present study Magnetic method is employed for subsurface study to outline lithological contacts and structural zones which could be associated with faults and fractures that may affect the building foundation.

Magnetic methods comprise of three major activities;

• Measurement of the specific field value at the ground (Data acquisition)
• Processing of the measured data (Data processing)
• Interpretation of the processed data qualitatively and quantitatively in terms of the rock property variations within the known geology (Data interpretation)

The land magnetic data acquisition in the area were done by using GSM-19T proton precession magnetometer for acquiring primary data for this research. The total magnetic data collected during the field work was 153 data points. At the field survey three magnetic readings were taken for the base station and then average of this readings were used for the diurnal correction. All the magnetic data were collected for one day during the morning time.

Figure 6: Magnetic data acquisition using the GSM-19T magnetometer.

At the very beginning, before establishing base stations the first thing that was done is to look for appropriate location with very less magnetic noises such as cars, houses with iron roof, roads, power lines and others and a point close to the survey area. At each station the magnetometer reading, recording time, location in UTM coordinate and elevation (meter) were recorded. For this study the base station reading were reoccupied within one hour interval for the correction of diurnal variation. The positions of the survey stations and the station elevations were determined using the GPS receiver, Garmin. Hence we have one base station located at a UTM coordinate of 271672 Easting and 1211169 Northing. In each time magnetic data reading was started and ended up at this base station.The distribution of magnetic data in the study area is shown in (figure 7).

The magnetic data were collected and processed for effective interpretation of the subsurface condition of the building site. Observed magnetic field data, coordinate data, elevation data and time were manually entered into a computer after the completion of the field work using Microsoft excel worksheet. Before processing the data the field magnetic data should undergo different corrections.

Figure 7: Magnetic data distribution of the survey area.

Variation of the Earth’s magnetic field with time, due to the rotation of the earth and with respect to the solar wind, which may last several hours to one day, is called diurnal variation. The magnetic data processing started by correcting the raw data for diurnal variation. In spite of great improvements in instrumentation, Diurnal variation will be corrected and data enhancement techniques are applied in order to extract maximum information from the magnetic data. Repeated readings were taken every one hour of the magnetic measurement at the base station. After the data collection, the diurnal effect was calculated and the magnetic data were filtered.

The formula applied for diurnal correction is 

 Where M₁ and M₂ are magnetic field readings at the base station at the beginning and at the end of the magnetic survey, T₁ and T₂ are the corresponding time respectively. M_c is the corrected magnetic data, M_i is magnetic data observed along the traverse, T_i is the corresponding time.

 Figure 8: Total magnetic field intensity anomaly map (a) and magnetic profile plot for survey profile one (b).

Magnetic survey involves measurement of the sum of magnetic field produced by both local and regional magnetic fields. The regional magnetic field, often referred to as geomagnetic field needed to be subtracted from the acquired total magnetic field to obtain the magnetic field anomaly (residual) caused by local source.

After subtracting the diurnal effect from the original magnetic data observed, the geomagnetic field was calculated using the mathematical model of earth magnetic field called the International Geomagnetic Reference Frame (IGRF) model 2005 in Geosoft Oasis Montaj 6.4.2 software. This model is calculated based on the dates, elevation and geographical location (latitudes and longitudes) of the observed magnetic data with the generated average geomagnetic field of 36150nT. The IGRF values were subtracted from the observed magnetic values for each station to determine the residual magnetic field due to anomalous contribution from local magnetic sources in the area.

The corrected magnetic data were then gridded and the results were then contoured using computerized mapping and processing software (Geosoft Oasis montaj version 6.4.2). To produce different maps for both potential methods magmap application were found to be very important. The corrected magnetic data were plotted, after which the possible noisy data were removed. For effective interpretation of the obtained magnetic data, further enhancements were carried out using various filtering techniques.

The commonly used data enhancement techniques including the analytical signal method and tilt derivative method are employed here. As a result, the analytical signal magnetic map and the tilt derivative magnetic map are produced from values of total magnetic anomaly map compiled for the study area.

In this thesis work interpretation has been made based on integration of results from electrical sounding, seismic refraction and magnetic data with the help of a borehole lithological log and dug test pits data. The borehole data helps to understand the vertical geological section of the study area and to correlate these different units with the electrical and seismic refraction velocity model sections. The depth of the boreholes used for lithological correlation is 296m; whereas the depths of the geoelectric section and seismic velocity model 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 byAmhara 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.

In addition, apparent resistivity pseudo depth section and sliced stacked map were prepared from the resistivity sounding data to examine the general picture of the subsurface to larger depths.

 This section includes the interpretation of geo-electric sections, apparent resistivity sliced-stacked map, Pseudo depth section maps p-wave velocity models, magnetic anomaly maps, analytical signal map, tilt derivative map which have been developed using different geophysical plotting and processing softwares.

In the following sections, interpretation of the electrical resistivity, seismic refraction and magnetic data for each survey was done. The thorough interpretation of the study goes as follows:

Interpretation of Resistivity Survey Data

Resistivity Sounding Data

The resistivity sounding survey have been conducted at thirteen sounding points along four profiles with half current electrode spacing (AB/2) of 150m. The individual VES are interpreted to get the layer parameters (resistivity and thickness of the subsurface layers) using win resist and IPI2win.

 Surfer (Version 10), Oasis montaji (6.4.2) and MapInfo softwares were used to plot the presented pseudo-sections, slice-stacked map and geo-electric section based on which both qualitative and quantitative interpretations are conducted. The VES data are presented in the form of apparent resistivity pseudo depth sections, geoelectric section and slice- stacked map have been discussed separately in the following sections.

Profile-1

Pseudo-Depth Section Map

Pseudodepth maps are prepared by taking raw apparent resistivity data show resistivity variation of the subsurface both in lateral and vertical directions without introducing the bias of data filtering. They do not reflect the actual depths of anomaly sources however they are good means of displaying the vertical distributions of resistivity values and also can be used as guide when the geo-electric sections are produced.

The apparent resistivity pseudo depth section (Figure 9) is prepared from the VES surveys carried out along Profile-1 includes VES3, VES1, VES2, VES7, VES8 and VES13 from NW to SE respectively.

The pseudo depth section (Figure 5.1) indicates that the top most part of the study area has higher resistivity value that ranges from 450?-m to750?-m. This is likely to be the response of the compacted top soil composed of the mixture of clay, silt, sand, gravel and sediments derived by river action. In between VES-8 and VES-13 high resistivity value is observed extending to shallow depth to deep. Furthermore, relatively medium resistivity values are shown at deep and covering most of the area below each VES points. This intermediate resistive layer is found at higher depth under VES1, VES2, VES7 and VES8. However, very low resistive formation is found at shallow depth on these VES points. Therefore, much attention should be given in this area in the designing of building foundation. In general, the pseudo depth map along profile one shows that in between VES2 and VES7 the study area has very low resistivity value which extends up to 100m depth. These area has a resistivity value <50?m and this could be the response of the weathered and fractured vesicular basalt. An intermediate resistivity value ranging from 100?m to 300?m covers the area at shallow depth beneath VES3, VES8 and VES13.

Figure 9: Apparent resistivity pseudodepth section map of profile-1.

Geoelectric Section

The geoelectric section of profile-1 is obtained from the interpreted layer parameters of VES1, VES2, VES3, VES7, VES8 and VES13 in the study area as shown in figure5.2.The geoelectric section shows three layers. The top layer that has variable resistivity ranging from about 97-427?-m and the thickness of this layer varies about from 2-3.5m and it is more likely to be a mixture of  clay, silty and sandy soil from the lithologic log and dug pit test data. The lateral variation in resistivity is believed to be come due to variation of compactness along the profile. The variation in compactness may come from the movement of cars and animal etc. The second layer, which is over lain by the top soil, has relatively very low resistivity value ranging from 33Wm to 60Wm and thickness varying 5-27m beneath the VES points. This layer has a maximum thickness beneath VES2 and VES7 and is inferred as highly weathered and fractured vesicular basalt. The third layer possesses relatively high resistivity value ranging from 167-1269? which could be the response of the basaltic formation. This formation is found at shallow depth at about 10m in the northwestern and southeastern part of the study area respectively, while the depth of this unit in the near central part geoelectric section just beneath VES2 and VES7 goes up to 27m.

Figure 10: Geo-electric section map along profile-1.

Profile-2

Pseudodepth Section Map

Profile-2 of the vertical electrical sounding survey consisting of four VES points VES4, VES5, VES6 and VES9 which is almost parallel to profile one has been carried out. The distance between the four sounding points is different from one VES to the other to have a profile length of about 292m. The generalized electrical picture of the subsurface of this profile displayed by the pseudodepth section map as given in Figure 11. This psuedodepth section gives the electrical resistivity variation of the subsurface. The top part of the section has relatively high resistivity ranging 230-340?m, underlain by very low resistivity formation ranging 60-110?m beneath VES4 and VES5 and moderately high resistivity 120-210?m below VES6 and VES9.In this section map we can generalize that the northwestern part of the profile possess very low resistivity values, therefore it is not recommended to set the foundation on this area or special attention should be taken.

Figure 11: Apparent resistivity pseudodepth section along Profile-2.

Geoelectric Sectction

Profile-2 ofthe geoelectric section was obtained from the model parameters of sounding points along profile-2(VES4, VES5, VES6 and VES9).Well log and dug pit test data located around the survey area were used to constrain thickness and resistivity values these sounding points. The individual VES points VES4, VES5, VES6 and VES9 have been interpreted to obtain the layer parameter resistivity and thickness of the individual layers using a combination of the IPI2win and WinResist softwares and the results of these are given in chapter four and Appendices of the thesis. The geoelectric section constructed from interpretation of the VES along this profile (Profile-2) from the interpreted layer parameters of VES4, VES5, VES6 and VES9 is given in Figure 5.4.The first layer in this section is the top dry soil which is composed of clay, silt and sandy soil shows relatively high resistivity ranges from 190-351 W-m and having a thickness of 2m.The second layer in the geoelectric section exhibits relatively low resistivity values that could be due to weathered and fractured vesicular basalt because of higher moisture content relative to the overlying and underlying layers. From the geoelectric section, the resistivity of this layer is seen to be vary from 39-70W-m and its depth extends to 18m.The resistivity values of the third layer are in the range of 173- 363W-m, this relatively higher resistivity response could be due to the weathered basalt which is considered to be the competent bedrock in the survey area.

Figure 12: Geoelectric section map of profile-2.

Profile-3

Pseudo Depth Section

In profile-3, three sounding points has been carried out and the pseudo-depth section developed from VES10, VES11 and VES12 that lie on the surveyprofile is given in (Figure 13).

Figure 13: Pseudodepth section along profile-3.

The figure shows that, the vertical resistivity variation increases as one goes to deep except just beneath each VES points at shallow depth. The highly resistive region at the top of the section shows that the presence of compacted top soil, while the shallower level which is dominated by a low resistivity values, interpreted as a likely response of weathered and fractured vesicular basalt. The deeper level with high resistivity value from 190-250 Ohm-m is may be the presence of massive basalt formation which has an implication for building foundation in the survey area.

Geo-Electric Section

The geo-electric section of profile three was constructed from model parameter of VES point data along profile three (VES10, VES11 and VES12). The resistivity parameters found from one dimensional inversion from IPI2win and win resist software of each VES point data were used to prepare geoelectric section shown below in (Figure 14). The geoelectric section shows that the shallow subsurface lithological units found along line three which are represented by three VES points. The three geoelectric units have a relative resistivity in accordance with the following patterns, r2<r1<r3 with resistivity values r1 (198-286) ?-m, r2 (44-53) ?-m, r3 (266-445) ?-m.

The first geoelectric layer that has resistivity r1 (198-286) ?-m and the thickness varies about from 1-4m is characterized as top soil composed of clay, silt and sand. The second geoelectric layer is marked by resistivity values r2 (44-53) ?-m is likely correlated with highly weathered and fractured vesicular basalt. The resistivity of the third geoelectric layer that corresponds to moderately weathered and fractured massive basalt has a resistivity value r3 (266-445) ?-m. From the section, the third layer is relatively a competent bed rock in the study area. This geoelectric layer is at shallow depth in the northwestern flank relative to the southeastern one. Therefore, along this profile setting the building foundation in northwestern part is more advisable in order to minimize risk. The lateral variation in resistivity along the first, second and third horizon possibly associated to the variation in degree of weathering, intensity of fracturing and geological structures.

Figure 14: Geoelectric section map along profile-3.

Profile-4

Pseudo depth section

This section is constructed using the VES points VES7, VES6 and VES11that lie on the traverse line four as given in figure 15.According to this figure, the resistivity of the section increases with increasing depth of the investigation in between VES6 and VES11. The central part of the section is dominated by a high resistivity value extends to from shallow depth to deep, while the shallow zone of the section beneath VES7 and VES11 is dominated by low resistivity value. In the pseudo-depth section (figure 15) we can observe that southwestern and northeastern flank is dominated by low resistivity and high resistivity formation respectively. As a result, along this profile it recommended that to set the building foundation on the northeastern flank of the study area and use southwestern flank for parking purpose.

Figure 15: Apparent resistivity pseudodepth section map along profile-4.

Geoelectric section

The resistivity sounding geoelectric section along profile-4 is constructed from the interpreted layer parameters of VES7, VES6 and VES11. It shows that the area is underlined by different layers beneath each point and the resulting geoelectric section is presented in (figure 16). The geoelectric section figure 5.8shows, VES7, VES6 and VES11 have three layers with different thickness of each layer. The top most part of the geoelectric section has resistivity values that range from 103 to190?-m with thickness varying from 1-2m. This layer is probably related to dry top soil mixture of clay, silt and sand.

The second layer has relatively low resistivity values ranging 35-58 Ohm-m which may possibly the response of the highly weathered and fractured vesicular basalt. This horizon shows different thickness beneath each VES points ranging 10-18m. The resistivity of the third layer ranges from 339-1200Ωm which may be the response of moderately weathered and fractured basalt. It lies at a depth of 25m in the vicinity of beneath VES7, at a depth of about 10m beneath VES6 and under VES11 the depth about 18m. Along this profile line possibly this layer would be the competent bedrock in the study area.

Figure 16: Geoelectric section map along profile-4.

From this geoelectric section, the thickness of the first layer becomes thin towards northeastern and the second layer has increased in thickness towards SW and NE direction from the center of the survey area. This is may be due to sliding problem, accumulation of clay soils and slide deposits at this location. Therefore, in this area setting the building foundation in between VES6 and VES11 is more preferable because the competent bed rock is relatively found at shallow depth and the thickness of the weak zone is small as compared to the southwestern flank of the part of the survey building site.

From the different AB/2 apparent resistivity, value considerably varies from 10-750 Ohm-m as shown below.The stacked apparent resistivity pseudo section map (Figure 17) shows that the sub-surface resistivity section of the entire study area, which was constructed by extracting apparent resistivity, values of all profiles at the same depth of investigations. In order to obtain a morereliable and realistic representation of the possible subsurface structures, five slices (AB/2=1.5, 9, 30, 66 and 100m) were selected from the electrical apparent resistivity sounding data to characterizes the sub-surface. The choice of such spacing depends on the variability between them and to show the vertical and lateral variations of resistivity over the study area.

Figure 17: Sliced-Stacked apparent resistivity map at different AB/2 of the construction site.

From the sliced-stacked map (figure 17), it is found that  most northern, northeastern and eastern part of the study area is characterized by with high resistivity ranging from 450-750?-m in the study area. On the other hand, relatively low resistivity value characterizes the southern, southwestern and western part of the survey.

Based on this map (figure5.9) we can see the study area into three resistivity zones (Zone-1, Zone-2 and Zone-3).

Zone-1 is characterized by relatively very high apparent resistivity value which ranges from 450Ω-m to 750 Ω-m. This high resistivity value is possibly the response of the basaltic formation in the survey area. This formation dominates the northern, northeastern, eastern and southeastern part and it is extends to a high depth on these flanks.

Zone-2 is characterized by relatively intermediate apparent resistivity value ranging from 150Ω-m to400Ω-m.This zone covers most of the central part of the study area and its coverage increases towards the East and the West.

Zone-3 this zone is characterized by relatively low resistivity, which covers the southern and southeastern part of the construction site. It is found at all depths with variable lateral extension.

In general this map shows that the southern, western and southwestern part of the construction site comprises subsurface formation which has low resistivity as a result of the presence of water saturated, weathered and fractured vesicular basalt. As a result, it is not recommended for foundation purpose and/or it requires special attention in design and construction. However, most of the eastern and northeastern part of the survey site covered by higher resistive formation. This higher resistivity values are the responses of relatively competent formations as a result it is advisable to set the building foundation in the northern, eastern and northeastern part of the study area. This result is more or less similar with the results drawn from the geoelectric section map, inversion and velocity models.

Interpretation of magnetic data

Magnetic data can be interpreted both qualitatively and quantitatively. The qualitative process is largely map-based and dominates the early stages of a study. Qualitative interpretation involves recognition of the nature of discrete anomalous bodies including intrusions and faults.

 Magnetic method has many applications in engineering studies is to locate contacts between different lithological units and geological structures that display magnetic contrasts such as faults or dykes. To interpret the magnetic data in terms of such subsurface indications, the magnetic data are presented in different forms. As a result, different magnetic intensity/anomaly maps that are assumed to be relevant for this thesis work were produced following the appropriate reduction procedures. All the magnetic anomaly maps were generated using Geosoft Oasis Montaj (Version 6.4.2) software.

The image of the total magnetic field anomaly map of the study area is shown in figure 18.

The map is produced from the difference between the diurnally corrected total magnetic field and the expected value of the IGRF in the study area. The map is used to see the overall subsurface structure.

 The anomaly map of the survey area can classified in to three labels as A, B and C.

Label-A is characterized by very high magnetic anomaly and it covers the southern, northern and northeastern part of the study area.                                                                                                                                                                                      

Label-B, which have intermediate magnetic anomaly covers most of the central part, southeastern and northern of the study area. The area represented by label-C shows very low magnetic anomaly and it covers the northwestern, western and some portion of the central part of the area. In addition, this zone shows NW-SE trend on the survey area. This NW-SE trending zone with low magnetic anomaly is interpreted as weak or contact zone on the analytical signal and tilt angle derivative maps.

Figure 18: Total magnetic field anomaly map.

Analytical Signal Map           

The total magnetic field anomaly map obtained in the survey area is used to produce the analytical signal map (figure 19) using Geosoft Oasis Montaj (6.4.2) software. The analytical signal the map shows the responses of anomalous bodies in the upper part of their sources. The analytic signal map shows maximum contrast over magnetic contacts or weak zones. In the study area, possibly two aligned weak zones or contacts are identified which aligned in the same direction NW-SE.

Figure 19: Magnetic analytical signal map.

Magnetic Tilt Derivative Map

The tilt derivative map is obtained by applying a tilt derivative filter to the magnetic analytical signal map using Geosoft OasisMontaj (v6.4.2) software. The map is used tolocate edges and geological boundaries in the study area. It produces a zero value over or near the source edges with positive values over the source and negative values outside the source. The tilt angle map (figure 20) shows the orientation and relative position of the weak zones or contacts in the surveyed area is demarcated on the map. The orientation and position of the weak zones indicated in the tilt angle map has a similar trend with that of the analytical signal map shown above.

Figure 20: Magnetic tilt derivative map.

Interpretation of Resistivity and Magnetic Data

Profile-1

Figure 21: Combined interpretation of Geoelectric section and magnetic anomaly of profile-1.

Variation of magnetic anomaly profile conducted along profile-1 ranges nearly from -41 to 78nT as shown figure 21a.These low and high peaks, which indicate the presence of weak zone or local fractures and high anomaly zones, appear at a distance of about 99m and 163m respectively. The low peak anomaly in the profile almost coincides with the discontinuities or local fractures displayed on the geoelectric section of this profile. Therefore, there is apositive correlation between magnetic anomaly plots with the geoelectric section of the same profile.

Integrated geophysical studies involving electrical resistivity and magnetic methods were carried out for engineering site characterization of the building construction site at Injibara University, southwest of Injibara town, Northwestern Ethiopia. Based on the results of collected data the following conclusions were made:

• Based on geo-electric propertycontrasts, i.e., 33-1,269?–m resistivity, three distance layers underlying the study area are delineated. The top soil, characterized by 97-441? -m resistivity is associated with the response of the upper layer composed of clay, silt and sand. The relatively wide resistivity variations are due to heterogeneous nature of these top soils in terms of their compositions, degree of compactions and moisture contents. Over the area, its thickness varies from about 1m at the SE part to 4m on NW flank of the study area. The second layer attributed to the highly weathered and fractured vesicular basalt is characterized by 33-70? –m resistivity, revealed somewhat undulating morphology. Such low resistivity rang (<70?-m) associated with this bed suggests that it is highly weathered with possible content of significant amount of fluid. The third layer in the study area is described by relatively high resistivity,on average >718?–m values which are interpreted as responses of a moderately weathered and fractured basaltic bedrock, which is assumed to acquire suitable geotechnical characteristics to bear loads from heavy civil engineering structures. The depth to the surface of this competent formation ranges from about 10m in the North West end and South East part to 27m near to the central part of the study area.
• The apparent resistivity pseudo-depth section maps show that NW part of the study area possesses relatively very low resistivity except profile-1 that may exhibit poor bearing capacity for the building set on. However, most pseudo-depth section maps indicated that the southeastern part of the study area shows high resistive formation, which may provide better bearing capacity for the building foundation.
• The sliced-stacked apparent resistivity section map indicates that the northeastern and southeastern portions of the study area may provide better bearing capacity for building foundation than the western and southwestern portion. Further, it is also anticipated that the strata with low resistivity may pose corrosive potential, which may severely affect the steel structures in building foundation. Therefore, while designing the footing for proposed buildings proper care must be taken to protect it from the possible corrosive effect within strata of low resistivity.
• The magnetic map shows high anomaly contrast. The analytical signal and the tilt derivative maps have clearly outlined localities of high magnetic gradient that probably are due to structural discontinues (weak zones) or may be lithologic contact zones.
• From this, it is possible to deduce that high magnetic anomaly responses are the results of relatively fresh igneous rocks whereas low magnetic anomaly responses are resulted due to weak zones and high degree of weathering in rocks. The dominant structural trend in the study area is inferred to have NW-SE orientation. These lineaments may have an adverse effect on the stability of building foundation.

Analytical signal map and tilt derivative of analytic signal show that the area associated with geological contacts or weak zone in the western and southwestern need special design for engineering foundation heavy civil structures.

There are no conflicts of interest associated with this paper. The authors would be fully responsible if the paper is found to violate any copyright law in the future. 

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