1. Introduction

2. Methodology

3. Data Acquisition and Reduction

3.2. Gravity Data Reduction

For most geophysical methods, it is necessary to apply a variety of corrections to the raw data obtained in the field in order to reduce or prepare the data for further enhancement and interpretation [12]. In gravity work, more than in any other branch of geophysics, large and in principle calculable effects are produced by sources that are not of direct geological interest [13]. The gravity data reduction processes was aimed at transforming the raw data sequentially in to a data table of final complete Bouguer anomaly values by correcting the variations in the earth’s gravitational field which don’t result from the differences of density in the underlying rocks. Accordingly, all the stations occupied in this paper have been referred to the International Gravity Standardization Net71 (IGSN 71) gravity datum. The theoretical gravity has been computed using the 1967 gravity formula (Geodetic reference system 1967). All the necessary standard correction types have been made, based on the standard gravity data correction procedures.

Figure 2. Distribution map of gravity data of the study area. Where (LZ) Lake Ziway, (AT)Adami Tulu (AL), AlutoLangano, and (LL) LakeLangano.

Figure 2. Distribution map of gravity data of the study area. Where (LZ) Lake Ziway, (AT)Adami Tulu (AL), AlutoLangano, and (LL) LakeLangano.

4. Result and Interpretations

4.1. Complete Bouguer Anomaly Map

The Bouguer gravity anomaly reflects changes in mass distribution below the surface, except the Bouguer anomaly has had an additional correction, removing the effect of mass above sea level datum on land [14]. The variation of the Bouguer anomaly should reflect the lateral variation in density, such that a high density feature in a lower density medium should give rise to a positive Bouguer anomaly. Conversely, a low density feature in a high density medium should result in a negative Bouguer anomaly [16]. A first analysis of the complete Bouguer anomaly map (Figure 3) indicates that the gravity responses reflect the heterogeneity in the subsurface density distribution beneath the survey area. The observed gravity anomaly values range from a minimum of -198 mgal to a maximum of -177 mgal in the study area. The anomaly map further reveals three distinct anomaly zones, which include the medium to very high central anomaly zone which is oriented in a NNW to SSE direction and a medium to very low negative anomaly zones located in the Northeastern, eastern and south eastern part of the study area.

The positive peak (-177 mgal) which corresponds to the location of the Aluto volcanic center may thought to be caused by high density volcanic products. The positive peak (-177 mgal) which corresponds to the location of the Adami Tulu locality is thought to be caused by high density rhyolite intrusions. The relatively low anomaly (-198 mgal) observed over the study area is caused by low density sedimentary depositions occurring at the southern shore of Lake Ziway and northern shore of Lake Langano. The medium negative gravity anomaly (≈ -185 mgal) observed over the summit of the Aluto volcanic complex being superposed on the positive one is thought to be caused by the sedimentary deposit of the caldera and the volcanic ash and pyroclastic materials on the top of the mountain. Since the complete Bouguer anomaly includes effects from the sources of a local and shallow nature as well as regional and deep seated structures, these effects are described in the following section.

Figure 3. Complete Bouguer anomaly map. Where (LZ) Lake Ziway, (AT) Adami Tulu, (AL) Aluto Langano, and (LL) Lake Langano.

Figure 3. Complete Bouguer anomaly map. Where (LZ) Lake Ziway, (AT) Adami Tulu, (AL) Aluto Langano, and (LL) Lake Langano.

4.2. Regional Gravity Anomaly Map

The complete Bouguer anomaly map consists the sum of the effects of features of long wave length, deep seated (regional anomaly) and the near surface, short wave length (residual anomaly) components. The separation of these components is important to determine the effects of the deep seated and the shallow depth sources contributing for the complete Bouguer anomaly. Despite the availability of various methods of anomaly separation, a low pass filtering with a cutoff wavelength 10 km was applied to the complete Bouguer anomaly for generating the regional Bouguer anomaly map using geosoft oasis montaj (V 6.4.2) based on the following series of steps. During filtering, increasing the separation between the source and detector cause a decrease in amplitude and increase in wave length of the response, where the rate of attenuation of amplitude is wave-length dependent (i.e. Shorter wavelengths, high frequencies associated with near-surface sources attenuate more rapidly with height than the longer wavelengths and lower frequencies sources. Hence, shorter wave length, shallow-sourced responses plus any short wave length noises are suppressed and longer wave length responses dominate the filtered data [12]. The Regional gravity anomaly map (Figure 4) reveals three prominent anomaly zones which are similar to those of the complete Bouguer anomaly map (Figure 3). The only difference between them is that the anomaly features observed in regional anomaly map are smooth and elongated this indicates the effect of deep seated geologic structures.

Figure 4. Regional gravity anomaly map. Where (LZ) Lake ziway, (AT) Adami Tulu, (AL) AlutoLangano, and (LL) Lake Langano.

Figure 4. Regional gravity anomaly map. Where (LZ) Lake ziway, (AT) Adami Tulu, (AL) AlutoLangano, and (LL) Lake Langano.

4.4. Horizontal Gradient Gravity Map

The horizontal gradient is a mathematical anomaly enhancement method that can be computed by:

$$HG=HG(x,y)=\sqrt{{({\displaystyle \frac{\partial \Delta g}{\partial x}})}^2+{({\displaystyle \frac{\partial \Delta g}{\partial y}})}^2}$$

Where $\Delta g$ is the residual anomaly,

The horizontal gradient method is used to locate the boundaries of regions of contrasting density from gravity data. The method is based on the premise that the maximum horizontal gradient of the observed gravity anomaly, which is caused by a planar anomalous body, tends to overlie the edges of the body (Figure 6). The edges mark locations of the contacts indicating that the HG method is designed to inspect fault and contact features. Horizontal gradient map can be produced from gridded residual gravity anomaly map data using the geosoft oasis montaj software. The compiled HG map is judged to be simple, interesting and a reputable end result which reveals the anomaly texture and highlight anomaly-pattern discontinuities. The HG maxima identified on such maps can be inspected to assist in the location of linear structures such as faults and lineaments. The horizontal gradient gravity map (Figure 6) of the survey is compiled from the gridded residual gravity anomaly data. The map reveals lineated maximum horizontal gradient anomalies which are dominantly oriented in the N-S direction. Locations of the maximum horizontal gradient magnitude are shown with black lines superposed along their orientation (Figure 6). The high horizontal magnitude plots outline lateral mass heterogeneities which are thought to be the expressions of lithological and structural boundaries. The high gradient magnitudes plots trace linear features trending N-S. The linear features are attributed to faults and fractures, and their continuities are observed in the study area. Moreover the N-S trending geological structures (faults and fractures) revealed by the horizontal gradient map can be considered to be the major geologic structures that favor the flow of ground water from Lake Ziway toward Lake Langano.

Figure 6. Horizontal gradientgravity map of the study area. Black lines represent faults traced in the study area. LZ (Lake Ziway), LL (Lake Langano), LA (Lake Shala), Law (Lake Awasa), MAB (Munesa – Asela border fault).

Figure 6. Horizontal gradientgravity map of the study area. Black lines represent faults traced in the study area. LZ (Lake Ziway), LL (Lake Langano), LA (Lake Shala), Law (Lake Awasa), MAB (Munesa – Asela border fault).

4.5. Euler Deconvolution Gravity Map

The Euler deconvolution gravity data enhancement method is used to estimate the depth and location of geologic sources that generate gravity anomalies. The Euler deconvolution equation which is thought to operate for gravity data in 3D was developed by [17] given by:

$$(x-x_o){\displaystyle \frac{\partial \Delta g}{\partial x}}+(y-y_0){\displaystyle \frac{\partial \Delta g}{\partial y}}+(z-z_0){\displaystyle \frac{\partial \Delta g}{\partial z}}=n(\beta -g)$$

Where $(x_o,y_{0,},z_0)$ is the position of a source (source location) whose complete Bouguer gravity anomaly is computed at (x, y, z), β is the regionl gravity anomaly $\Delta g_R$ value, and n is the structural index (SI), which can be defined as the rate of attenuation of the anomaly with distance. The SI must be chosen using prior knowledge of the source geometry. For example, SI = 2 for a sphere, SI = 1 for a horizontal cylinder, SI = 0 for a fault, and SI = –1 for a contact [18].

The Euler deconvolution method is applied to the complete Bouguer gravity data in order to estimate the depth and location of the gravity anomaly sources. Accordingly, the Euler deconvolution gravity map (Figure 7) of the study area is produced for a structural Index (SI = 0.5). The structural index (SI = 0.5) is chosen based on the objective of the survey, which is mapping the lateral and depth wise variation of linear structures (faults and fractures) found in the study area. Euler solutions obtained from the complete Bouguer gravity data show well clustering over the area of Ziway Langano Corridor. These clustering confirms the minor faults in the geologic map and infer new faults in different directions at different depths. Combining these results in one tectonic map could be giving a clear image of the subsurface structure of the area of Ziway-Langano corridor. The Euler gravity map (Figure 7) show that high clustering of the depth solutions for SI = 0.5 are values associated with regions of intense faulting, fracturing and extensional activities.

Figure 7. Standard Euler deconvoltiongravity depthmap for SI = 0.5. Where (LZ) Lake Ziway, (AT) Adami Tulu, (AL) Aluto Langano, (LL) Lake Langano.

Figure 7. Standard Euler deconvoltiongravity depthmap for SI = 0.5. Where (LZ) Lake Ziway, (AT) Adami Tulu, (AL) Aluto Langano, (LL) Lake Langano.

5. Conclussions

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