Investigating of soil features and landslide risk in Western-Atakent (İstanbul) using resistivity, MASW, Microtremor and boreholes methods

3.1 Dipole-Dipole Multi-Electrode Array, ERT and Applications

In the dipole-dipole system electrode array, the potential electrodes are placed outside of the current electrodes. Here, the distance between the current and the potential electrodes is opened at equal distances. In this system, the potential electrodes are very sensitive to lateral changes as they are outside the current electrodes. Unlike the Wenner and Schlumberger systems electrode arrays, the depth and location of measured values in the dipole-dipole system electrode array is the point of cut of two straight lines drawn at a 45-degree angle to the midpoint of current and potential electrodes. In this method, as the distance between the current and potential electrode pairs is opened, the resistivity values of depth levels such as n = 1, n = 2, n = 3 and etc. are measured respectively (Figure 3a and 3b). The dipole-dipole resistivity technique is the electrode array with the greatest penetration depth. It is also very sensitive to the determination of vertical discontinuities. This electrode array gives good results in the mapping of vertical structures such as dykes and voids (Figure 3c).

Figure 3

a) Dipole-Dipole electrode arrangements used in DC resistivity surveying and their geometric factor (modified from [40], b) Construction of a pseudosection for dipole-dipole resistivity data [41], c) Visual plan of multi electrode array showing the set-up and a possible ERT model obtained after inversion [42].

In recent years, improvements in electronics and computer technology have enabled the implementation of a multi-electrode resistivity measurement system, which can automatically change drilling-profile dimensions in one direction. The apparent resistivity values measured in this manner are calculated from the resistivity-depth values of the profile measured using inversion algorithms. With this measurement technique, information can be collected about the resistivity changes in both the horizontal and vertical directions. Since the multi-electrode system is developed in such a way that it can be taken in order of many measurements, it reduces the time required for measurement of the ground very much. Thanks to these practicalities, this system presents 2-D and 3-D electrical resistivity tomography (ERT) imaging to reveal subsurface geological structures. Although many different electrode arrays can be used in the ERT method, choosing the most suitable array for the study increases the success of the result. ERT is one of the most used geophysical methods due to its solution powerful, cheaper than other methods, comfortable and present visual result of application and it is generally preferred in archaeogeophysics, groundwater exploration, landslide risk analysis and mining research etc.

In this study, resistivity measurements were taken on two profiles to model the electrical resistivity changes of the study area (Figure 2). A dipole-dipole array technique was applied along multiple electrode profiles with the aid of a 60-electrode device to obtain detailed information during measurements. ABEM SAS 1000 model resistivity meter and electrode separator ABEM Lund separator were used for the measurement [21]. As the electrode feature, 40 cm long and 12 mm diameter special copper alloy steel electrodes were used. For the operation of the device, 12 volt, 45 amperes dry type battery, 5.00 m electrode lead wire, each of which has 21 stations with 4 × 100 m length cables are used. The electrode spacing was chosen as 2.5 m and measurements were made on 663 points on two lines with a length of 147.5 m. The first of the resistivity lines is named R-1 and the second is named R-2. The apparent resistivity values obtained in the R-1 profile range from 0.84 to 862.52 Ω·m. In R-2 profile these values are between 0.40-764.25 Ω·m. The apparent resistivity data and the topographic level values of the electrodes were analyzed with RES2DINV [22] software and final sections were created (Figure 4). Resistivity values in the R-1 line have been found to be between 1.02 and −311 Ω·m and in the R-2 line between 0.5 and 252 Ω·m. The iterative process has been continued until the 5th in order not to move away from the actual data in the inverse solution process.

Figure 4

2-D ERT with dipole–dipole array of profiles; a) Final section of R-1 profile and b) Final section of R-2 profile.

Considering examine the final section of R-1 line, the area from the surface to the 20-30 m depth and between 10-30 Ω·m resistivity values shows a high filling material with dark green-light green colored clay ratio (Figure 4a).

The yellow-brown-orange regions appearing with high inclusions after about 110th meters of the horizontal axis gave high resistivity values due to the voided structure. The blue colored low resistivity region, which is also below the 120th metric in the horizontal axis, is the traces of water traced from hollow structure due to seasonal precipitation.

High-resistivity structures of yellow-brown-orange-red color are observed in the horizontal axis at approximately 80 and 120 meters. These structures are thought to be limestones belonging to Upper Eocene-Lower/Middle Miocene Ceylan Formation. The middle region between these two units consists of claystones belonging to Gürpınar Formation of Middle Oligocene-Early Miocene Danişmen Formation and Güngören Member of Upper Miocene Çekmece Formation.

In the final section of the R-2 line, the part from the surface to the dark green region is a filling material with a high clay content (Figure 4b). The filling properties of line R-2 are quite similar to those of line R-1. Although the limestone levels of the Ceylan Formation are not as pronounced, they can be distinguished by yellow-brown-orange colors and also high resistivity values at the lowest levels between 40m- 80 m and 80 m- 120 m in the horizontal axis. The area between these two units is the light green-dark green color level in the section and it represents the claystones of Gürpınar Formation and Güngören Member.

When R-1 and R-2 cross-sections have been evaluated jointly, the resistivity values of the ground and filler levels are increased. The high clay content in the filler has been caused a difference in the resistivity values of this unit. The low resistivity values observed with small structures just below the surface are traces of water accumulation on the impermeable clayey layers.

3.2 MASW Applications

The technique to determine the shear wave velocities is applied using the surface waves. This is the most common type of MASW survey [23]. The maximum research depth that can be achieved is usually in the range of 20-30 m, but this depth may vary with the strength of the active source and the structure of the lithology and soil types. MASW has been a surface wave analysis method in which various artificial and active sources such as sledgehammer or weight throwing have been used. The MASW method is based on the net dominance factor of the S-wave velocity, which propagates in the layered medium of the Rayleigh wave, from which the S-wave velocity profiles of the phase velocities are obtained. MASW data measurement system, seismic source, multiple geophones, multi-channel seismograph, portable computer and a record are shown Figure 5. In this method, the source and receivers have been arranged in the linear direction and multi-channel records have obtained.

Figure 5

Typical multichannel seismic surface-wave experiment setup (modified from [23]).

The MASW data processing consists of three steps (Figure 6): the first step; preparation of land records for data processing phase (converting format), applying area geometry based on source and geophone locations. The second step is to obtain one dispersion curve for each record from the prepared data. Generating a good dispersion curve is important step, because the final result of a MASW survey depends on the frequency-phase velocity relation analyzed.

Figure 6

MASW data processing procedure (modified from [23]).

This process is achieved by first converting the data from the time-space domain to frequency-phase velocity domain by using a suitable mathematical transformation process like the pi-omega transform [24] or the phase-shift method [8]. This type of 2-D (i.e., time and space) transformation generates image of dispersion patterns in both fundamental and higher modes through successive energy accumulations, instead of calculating individual phase velocities one by one. Among multiple modes of dispersion possibly recorded has been particularly interested in the fundamental mode dispersion curve, the one that is at the lowest velocity range. The last step of the MASW data processing involves inverting the surface wave dispersion data in order to obtain a 1-D seismic shear wave velocity profile. The inversion process involves back-calculating shear wave velocity variation with depth that yields the best fit between theoretical and measured dispersion curves [25].

In this study, MASW measurements have been made at 25 points on 5 seismic profiles in the field. Seismic measurement profiles have been shown in Figure 2. In this method, active energy source having been preferred and 10 kg sledgehammer having been used. In the survey measures, Seismic Source-DAQ Link-III model 24-channel engineering seismograph, the sampling interval is 0.25 ms and the recording length is 2 seconds, 3.00 m group distance and 3.00 m offset distance were used. In study 24 channels system, 69.00 m group and 72.00 m system distances have been selected. In addition to making straight and reverse shots from the offset distance of 3 m in the measure system, the shootings were also made among the geophones 6-7, 12-13 and 18-19 (Figure 7).

Figure 7

Measurement geometry of seismic lines.

Dispersion curves were obtained using the PickWin-Surface waves Analysis program (SeisImager). Dispersion curves express the change of the phase velocity depending on the frequency. This has helped me to calculate depths in one dimension for every point. It is aimed to model the S wave velocities of seismic environments from the inversion of the dispersion curves obtained in the MASW method. For this purpose, inverse solution of dispersion curves was applied and S wave models were obtained (Figure 8).

Figure 8

An example of the processed steps of MASW data (from S-1); a) Seismic record, b) Frequency-Phase velocity conversion, c) Iteration applied dispersion curve and d) Inverse solution result obtained depth-S wave velocity model.

The 1D S-wave velocity structure of each shot in each shot was combined with GPS and topographical values to create 2D depth-velocity sections (Figure 9).

Figure 9

Depth-S wave velocity sections of the seismic profiles; a) S-1 line, b) S-2 line, c) S-3 line, d) S-4 line and e) S-5 line.

The 3D image obtained from the MASW method of the study area is shown in Figure 10.

Figure 10

a) 3-D image obtained from MASW method of the study area, b) Level maps with 10 m intervals.

Seismic velocities near the surface are measured very low because the plates contain plenty of space and water. Below these levels, the presence of very solid-hard clay and claystone layers causes a noticeable increase in seismic speeds and makes a difference from the strata above. The limestones at the bottom are striking at very high seismic velocities due to their geological characteristics.

As a result, that, the high frequency parts of the MASW dispersion curves, whichever correspond to shallow depths < 5 m, while the low frequency parts of the dispersion curves, so that correspond to areas deeper than average 25 m. When all seismic velocity profiles are investigated, it is seen that the result is the S wave velocity is less than 200 m/s near the surface. At the bottom, limestone levels imply that the S wave velocity is at very high speeds such as 850-1000 m/s. Among these two distinct seismic environments, there is another layer which is observed at 500-800 m/s (Figure 9 and Figure 10).

As a result, clay with low seismic velocities at the top, silty-sandy-gravel clay and marls with moderate seismic velocities immediately below, and limestones with high seismic velocities at the bottom can be interpreted. The differences observed at seismic velocities in the marl and limestone levels are thought to be due to the effect of the potential groundwater on the areas that do not show uniform propagation.

3.3 MSM Applications

According to Nogoshi and Igarashi [26], involves recording a few minutes of seismic background noise to provide a reliable estimate of the soil resonance frequency, in here based on the essentially Rayleigh-wave nature of the MSM used. The researchers compute the spectral ratio of the horizontal and the vertical components of an MSM measurement for a single station. The resulting curves pinpoint a frequency that is supposed to fit remarkably with the S-wave resonance frequency of the studied site. This technique was later revised by [11], who claimed more largely that this HVSR is a reliable assessment of the site transfer function for S-waves with respect to bedrock, because of the mainly body wave nature of the noise. In many research carried out over the recent years, showed that this H/ V ratio on MSM is much more stable than the raw noise spectra, and that in case of high impedance contrast between surface and deep materials, the MSM exhibit a clear peak which is well correlated with the fundamental resonance frequency [12, 27, 28]. The recent numerical simulations of background noise [12, 17, 29, 30] confirmed Nogoshi and Igarashi’s [26] first explanations as concerns the surface-wave nature of MSM: synthetic calculations showed the direct link between three values in case of a single soft surface layer over bedrock: the ellipticity curves of Rayleigh waves, the frequency pinpointed by the HVSR on MSM and the S-wave resonance frequency of soft soil under the studied site. But these results also predicted no correlation between the amplitude of this H/V peak and the actual amplification value under S-wave incidence (except for [29]). Yet, several authors claim a satisfactory empirical agreement between the level of the two types of curve [31, 32]. Recent investigations concerning this technique are reviewed in [33, 34, 35, 36]. The computation of the HVSR follows different steps is shown Figure 11.

Figure 11

Description of the computation of HVSR [37].

MSM measurements have been made to determine the fundamental period for the study area and its immediate surroundings. In particular, 5-pieces measuring points are selected in locations that allow us to measure all the characteristics of the study area (Figure 2). Measurements in the area were taken using PE-6/B, 3-component geo-phone. This geophone contains the SM-6/U-B 4.5Hz vertical geophone and two SM-6/H-B 4.5Hz horizontal geo-phones. Seismic Source-DAQ Link III model 24-channel engineering seismograph was used as data collector. MSM records were primarily filtered with a bandpass filter at 0.1-20 Hz for environmental noise. Fourier spectra were calculated using window length of 100 seconds after the filtering process was completed. During the calculation of the Fourier spectra, the value of bwas applied to the measured values by assuming a value of 40 for the rounding. The MSM records of the MSM points in the area and the selected windows on the records are shown in Figure 12. HVSR in solution phase and MSM results of the measured data is using solved the GEOPSY software [37], which works with the Nakamura method [11].

Figure 12

MSM records of MSM stations and selected windows; a) MSM-1, b) MSM-2, c) MSM-3, d) MSM-4 and e) MSM-5.

The results obtained and the number and length of windows used for evaluation are shown in Table 1. As a result of interpretation of H/V curves; it is seen that the same frequency values are obtained at the MSM-1 and MSM-3 points. When the H/V curves at MSM-4 and MSM-5 points are examined, a single frequency peak is observed. Considering at the H/V curves obtained from other stations, it is seen that the peaks do not collect a Single frequency and give peaks at different frequencies. As a result, soil Layer thickness was calculated for each station using MSM data (Figure 12). Soil layer thickness of about 9-24 m was found in the area (Table 1). The measurement points of MSM-1, MSM-3 and MSM-5 are located on the line of B-B’ geological section with S-3 MASW section. Period values of these 3 points in the range of 0.24 sec to 0.27 sec indicate the landslide area along this profile. MSM-2 and MSM-4 points are consistent with the geological section A-A’. When the A-A’ geological section was examined, the period values of these points were low (0.13 sec - 0.19 sec) due to the upward intrusion of limestone (Ceylan Formation).

3.4 Drillings and Lithological Sections

In this study, ERT, 3D MASW, MSM and boreholes methods have been applied in order to investigate the engineering base rock topography, underground layer models, landslides and faults of the land located in Western-Atakent (İstanbul). In order to determine the lithological structure of the study area and to obtain sections, 10 wells with a depth of 20 m have been drilled. Using all the results obtained, 4 lithological sections have been created in the directions shown in Figure 2.

From the lithological sections A-A’, B-B’, C-C’ and D-D’ obtained respectively; Starting from the surface, silt-gravel within the range of 7-7.5 m is generally more clay, artificial filling material has been observed. Widespread fillings in the region have been intertwined with natural grounds due to the long-term effect and irregular structure. The environment in which A-A’ section is taken is inclined from the topographical aspect. When the A-A’ section is examined from the surface to the deep, the first 8 m alluviums are followed by the 2-8 m Çekmece Formation and the lowest layer is followed by Ceylan Formation. Here the sedimentary layer is very thick and the Çekmece Formation is a very thin layer. The Ceylan Formation,which was observed to be very thick in the drills of D-7 and D-10, could not be observed in drilling of D-6. Only the alluvial layer and the Çekmece Formation were observed in the drilling D-6 (Figure 13a).

Figure 13

Lithological section lines used in the study area; a) A-A’, b) B-B’, c) C-C’ and d) D-D’.

It is understood that Çekmece and Danişmen formations are found as the second layer in D-9 sounding also Ceylan formation layer are found in D-4,D-3 and D-5 soundings. In the drilling of D-9, Çekmece Formation is located as the second layer, while in the drillings of D-4, D-3 and D-5, Danişmen Formation layer is located. In all the boreholes in this section, the lowest layer is Ceylan Formation (Figure 13b). In the C-C’ section, the drilling of D-7 passes through the alluvial stratum, then Çekmece Formation and then Ceylan Formation. In the drilling of the D-7, Çekmece Formation is a very thin layer. Alluvium, Danişmen Formation and Ceylan Formations were observed in D-4 drilling, respectively. The layers under this alluvium are curved. In the drilling of D-1, 10m thick alluvium, 3m thick Çekmece Formation and then Danişmen Formation layers were passed (Figure 13c). In drilling D-8 located at section D-D’, alluvium is 6 m thick, Çekmece Formation is 1 m thick and Ceylan Formation is 12 m thick. In drilling D-5, there are alluvium of 9 m, Danişmen Formation of 4 m and Ceylan Formation of 7 m. almost the same values are observed in the drilling D-2 (Figure 13d).