DETERMINE SUBSURFACE STRUCTURE FOR DEVELOPMENT PLANNING DEPARTMENT OF

GEOPHYSICAL ENGINEERING

Dian Redita and Fanda Fitrianditha Program Studi Teknik Geofisika, Universitas Pembangunan Nasional “Veteran”

YogyakartaJalan SWK 104 Condongcatur Yogyakarta

fandaft@gmail.com

ABSTRACT

The research of subsurface structure for building development planning department of geophysical engineering with seismic refraction method has been done. The research was held on 25-26 April 2015 at Pembangunan Nasional University " Veteran " Yogyakarta. OYO McSeis seismic refraction instrument with 7 line. Each line has 32 meter length and the distance of each geophones is 2 meter. Interpretation of data using Intercept Time ( ITM ) and Plus Minus method, both of these methods have different concepts and data processing that will be correlated .

From the result of data the calculated with Intercept Time method the wave velocity of the first layer is 42,09 m/s – 114,05 m/s and the average thickness is 0,26 m that can be interpreted weathered layer or soil. Meanwhile the velocity of the second layer is 336,37 m/s – 1078,35 m/s and the depth of the top of the second layer is 0,6 m tha can be interpretated with sand. And the data processing using Plus Minus methods, wave velocity values obtained the first layer of 93.8 m / s - 114.05 m /s with a average thickness of 0.4 meters is interpreted as a layer of weathered material or soil. Meanwhile, the speed of the wave in the second layer 747.86 to 1217,925 m / s that can be interpreted as sandstone with undulated structure.

Key words : Soil Strsucture, Seismik Refraction, ITM and Plus Minus

1. INTRODUCTION Pembanguanan Nasional "Veteran" Yogyakarta University is planning to be built lecture building Geophysical Engineering study program requires a land where it will be investigated the circumstances and subsurface structures as geotechnical data support in its development. Geotechnical data instrumental to consider and take into account the state of the resilience and structure of carrying capacity factors such as land and environment. The development process begins from where the foundation work on this process takes a layer that can bear the weight of the building so it takes a layer of thick, tough and relatively flat. Kedaan subsurface construction will be done greatly affect the robustness of the building, so that when a coating found weathering and with a big layer slope at

locations it is said to be less feasible development. Sedangakn for areas with a layer of material which has a thick, hard and do not undergo further weathering processes and has a slope or the slope of a small layer, it can be said is feasible and safe area to do the construction in accordance with the standards of development that has been suggested. Seismic refraction method take advantage of the wave velocity in response to the boundary of bedding so it can be used to determine slope and identify subsurface lithology. Objectives and problem definition of research for menegtahui subsurface state that serves to identify subsurface structures, layer thickness and type of lithology contained as a reference and consideration of development planning in the targeted area.

1

2.THEORYTICAL BASIC2.1 SEISMIK REFRAKSI

The seismic method is one method to study geophysical subsurface utilizing the propagation of the wave generated by the source. Seismic refraction has some assumptions. The assumptions are:a) Medium Earth is considered layered and each layer menjalarkan seismic waves at different speeds.b) The more the increasing depth of the earth's rock layers more compact.c) The length of the seismic waves are smaller than the thickness of the layer of earth. This allows each layer of earth that qualify will be detected.d) At the boundary between the layers, seismic wave propagates at the speed of the wave in the layer below it.e) wave velocity increases with depth.

2.2. Methods T-XTX method is the simplest method and the results are relatively quite rough, the depth of the layer is obtained only at certain points only, however for bedding system that is fairly homogeneous and relatively capable of delivering sufficient results (with a relatively small error). But on the condition that the complex should use another method more accurate interpretation. This method consists of two kinds, namely Intercept Time Method (ITM) and Critical Distance Method (CDM).

2.2.1 Intercept Methods TimeIntercept Intercept method Time or Time Method (ITM) is the simplest method, the result is quite rough and is the most basic method in seismic data processing.Assumptions used this method are:a) homogeneous layer (layer of relatively uniform velocity)b) The field boundary layer should be flat (without undulations)Intercept time means time seismic wave propagation from source to geophone vertically (zero offset)Refraction seismic data processing using ITM method consists of two kinds:a) One layer Flat (Horizontal Single Layer)b) Many Layers Flat (Horizontal Multi Layer)

2.2.2 Method For Coating Leaning Intercept TimeWhen the reflector has a dip, then:a) speed on a curve TX not the actual speed (true velocity), but the apparent velocity (apparent velocity)b) Requires two types of shooting, namely: Forward and Reverse Shootc) Intercept different time in the second shooting, the refractor is also different thicknessesApparent Velocity is the speed that propagate along the stretch of geophones.

Figure 2.1 Scheme oblique wave propagation in the layer and its relationship with the curve TX on layers using the forward and reverse oblique shoot.

A method previously only used shooting forward, while for the application layer using a forward tilted shooting and reverse shooting. In figure 4, point A = B = source and geophone (shooting forward), while point B = A = source and geophone (reverse shooting). A point source of energy in generating down-going wave refraction (raypath AMPB), and other sources of energy at point B generates up-going wave refraction (ray path BPMA).

2.3 Method of Delay TimeOne of the seismic refraction method is a method of delay time. Delay time method is a method that utilizes the wave's propagation

2

delay time determines the depth of the layer.

2.3.1 Methods Plus Minusplus-minus method is a method for the analysis of the depth and speed. Pussy-time analysis to determine the depth. While analysis of minus time for analysis speed. The method used to determine the thickness of the first layer is metodeplus-minus covering PlusTime analysis for depth analysis and analysis Minus Time for determination speed (Van Ovemeeran, 1987). This method assumes that each individual layer is homogeneous. Plus timing analysis using timing analysis tundayang introduced by Gardner (1939.1967) and further developed by Hawkins (1961) and Barry (1967). Analysis Plus Time consists of a combination of the two analyzes the time delay in the opposite direction. Time Plus Untukmenggunakan analysis, the data is spread reciprocal important that the widespread deployment of next least reverse the position of the source (Sr) and sebaliknyamenyebar to the forward position source (Sf). Then time analysis plus-minus method defined by two crossover points (forward scatter (Xf) and reverse spread (Xr)), which determines the boundary between the first layer and the second layer.Plus-minus method is derived from the method of delay time for more complex cases such as:

Boundary layer uneven To Found thick layer of weathered For static correction on reflection

seismic data on the plus-minus method is also using several assumptions for the configuration of the earth's subsurface, is as follows:

boundary layer C-F is straight Careen of the refractor is not too big

or <100

Figure 2.2 Analysis time minus the plus-

minus method

Definition minus time at receiver (TD) is the reduction of travel time in the receiver of the reverse source (THD) of travel time at receiver from the source forward (TAD) reduced the travel time between the two sources (TAH).The second layer speed (V2) equal to twice the inverse slope fit through variation minus time (DT-D) are calculated for each recipient in the analysis time on Plus-Minus method.

3. METODOLOGY The research was conducted on 25 to 26 April 2015 in the softball field UPN "Veteran" Yogykarta is located at Jl. SWK 104 Condongcatur, Depok, Sleman, Yogyakarta. This activity takes place from 06.00 - 16.00 pm using a set of tools OYO MCSEIS 3 chanels. This tool is used for the acquisition of seismic refraction data that will display data from the reception wave of geophone recordings. Support equipment in data acquisition, these include the hammer as a wave source, tabular data, the GPS function to determine the coordinates of the position or trajectory, the meter to measure the length of the line and the local geological maps Sleman Yogykarta as controlling subsurface structure of the study area.

3

Figure 3.1 Refraction seismic acquisition equipment

Collecting data in research conducted by way of spread 7 lines with each 32 meters long and 5 meters distance between the lines. Geophones attached to the line in a straight line with a 2 meter distance between geophones.

Figure 3.2 design survey of the research area

4. RESULT AND DISCUSION4.1 Intercepet Time Method From the result, there are two waves consist of forward waves and reverse waves. The velocity of forward direct wave are 46,296m / s - 112.36 m / s. Then for refraction wave, has the velocity are 403.65 m / s - 1234 m/s. As for the reverse wave has a speed of 37.87 m / s - 120.19 m / s and wave refraction has a speed reverse 269,097m / s - 1126.45 m / s. Critical angle be obtained from the processing of this method is is 9.590 to 12.771135930.

The velocity of the wave can determining the physical properties of rocks in the layer. The physical properties that can be determined is the porosity of the rock. As already mentioned before, that the smaller the porosity value then the rock more compact and rapid

propagation of the wave will be more rapid or large. Of the average speed of the first, it can be determined that the material beneath the surface. With the speed value, then the existing material is estimated soil. Then, with the average speed of the two can be determined lithology second layer. With an average speed values mentioned above can be expected lithological constituent layers is sand. Then on the subsurface profile, it appears that the subsurface layer is assumed to be skewed. The first layer thickness ranges between 0.28 m - 0.59 m. The thickness of the first layer is also the depth of the top of the second layer.

4.2 Plus Minus MethodThe data showed the acquisition of 12

line with the processing method obtained on line plus minus 1 wave velocity measurements of the forward first layer (V1 forward) 100.80 m / s, the first layer wave velocity measurement of reverse (reverse V1) of 86.8 m / s, the average speed of the second layer (V2 average) 747.86 m / s and a depth of at least 0074 m layers and layers of a maximum depth of 0.38 m / s. On the second line the first layer wave velocity measurements of the forward (forward V1) 91.91 m / s, the first layer wave velocity measurement of reverse (reverse V1) 120.12 m / s, the average speed of the second layer (V2 average) 791 , 58 m / s and a depth of at least 0024 m layers and layers of a maximum depth of 0.86 m / s. Line 3 wave velocity measurements of the forward first layer (V1 forward) 104.16 m / s, the first layer wave velocity measurement of reverse (reverse V1) 97.65 m / s, the average speed of the second layer (V2 average) 821, 28 m / s and the depth of the layer of at least 0:13 m and a maximum layer depth of 0.44 m / s. Line 4 first layer wave velocity measurements of the forward (forward V1) 116.27 m / s, the first layer wave velocity measurement of reverse (reverse V1) 94.33 m / s, the average speed of the second layer (V2 average) 899, 01 m / s and the depth of the layer of at least 0:01 m and a maximum layer depth of 0.68 m / s. On line 5 wave velocity measurements of the forward first layer (V1 forward) 107.75 m / s, the first layer wave velocity measurement of reverse (reverse V1) 104.2 m / s, the average speed of the second layer (V2 average) 788 , 5 m / s and a depth of at least 0084 m layers and

4

layers of a maximum depth of 0.69 m / s. On line 6 the first layer wave velocity measurements of the forward (forward V1) 108.69 m / s, the first layer wave velocity measurement of reverse (reverse V1) 115.74 m / s, the average speed of the second layer (V2 average) 1168 , 08 m / s and a depth of at least 0078 m layers and layers of a maximum depth of 0.86 m / s. Last measurements at line 7 the first layer wave velocity measurements of the forward (forward V1) 112.35 m / s, the first layer wave velocity measurement of reverse (reverse V1) 115.74 m / s, the average speed of the second layer (V2 average ) 1217.92 m / s and the depth of the layer of at least 0:12 m and a maximum layer depth of 0.53 m / s.

Figure 4.1 Depth Map of Plus Minus Method

Map above is a picture of the subsurface depth of the research area with the data that is processed using the method of Plus Minus. Map above can be explained that the purple color on the map indicates the depth of the deepest layers. This map is divided into 3 depth, ie the depth of low, medium and greatest depth. For lower depth on the map are characterized by intervals of color from red to orange ( 0- 0,25 meters), this depth is very low or close to the surface. Relatively deep depth interval characterized by color from yellow to light green (0,3 – 0,55 meters) and to the greatest depth lies in the interval dark green to purple (0,55 – 0,85 meters).

Figure 4.2 Second Layer Velociity Map of Plus Minus Method

Map above is a map that is based on the results of data processing plus minus method that generates the wave velocity in the subsurface. Areas with depths deeper it will be growing wave velocity and wavelength is also increasing due to the condition that the deeper rocks will be more compact, thus simplifying the wave propagates. To scale readings, the area with the color orange to red (850-1100 m / s) has the highest wave speed, an area with a light green color to yellow (600-800 m / s) and for areas that light blue to dark green (300-550 m / s) have the lowest wave speed. So in the above map areas that have the greatest wave velocity is at the north map or northern area of research.

5