Oil and Gas Research Institute Seismic Analysis Center

Ghunaim T. Al-Anezi (KACST)

March 2013

Faults Detection Using High-Resolution Seismic Reflection Techniques

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Objectives

The objective of the survey was to locate faults and to map the bedrock up to the depth of 20-25 m. The survey covered 3 lines of high-resolution seismic reflection with a total length of 282 m in Salboukh, about 50 km towards north from Riyadh city.

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Location

Figure 1: diagram of geophone array development and shot locations used for the acquisition of seismic CMP line. The geophone spread consists of 48 receivers spaced 2 m apart. 2 receiver lines spread and three source lines shoot. Source lines 1 and 2 shoot at 4 m offset from receiver line 1 either side and source line 3 shoots at 4 m offset from receiver line 2.

The study area is located about 50 km north of Riyadh city.

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The seismic method represents one of the most important geophysical techniques for oil and gas exploration due to its high accuracy, high resolution, and deep penetration. On relatively smaller scale, this method can also be applied to groundwater searches, environmental and civil engineering investigations and to some extent in mineral exploration.

During the past 30 years, the growing interest in engineering and environmental problems has increased the application of seismic reflection surveys to study shallow targets of hydrogeological, engineering, environmental, archaeological, and geotechnical aspects.

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The main challenge in using high resolution shallow seismic data for estimating the near surface features is the maintenance of the high frequencies reflections from shallow interfaces in the face of attenuation and possible aliasing. In order to acquire high resolution seismic data for shallow subsurface investigation, spacing between source and receiver must be perfect enough to ensure un-aliasing of the data. Frequencies for high resolution acquisition can reach up to 500 Hz.

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Seismic data acquisition Three seismic profiles for high resolution reflection are conducted in the study area.

Figure 2 : Seismic data acquisition system 6

Figure 3: field pictures 7

Table 1 : Acquisition parameters for high resolution seismic reflection profiles 8

SPREAD

Type Continuous Shooting

Number of trace 48

Receiver interval 2 m

Source interval 2 m

Offset 4 m

Near Offset 4 m

Max. Offset 96 m

Nominal CDP Fold 48 Fold

SOURCE

Type Weight Drop

Number of drops at one shot location 6

RECEIVER

Type Geophone Flat base

Model GS - 20 DH

Response 365 ohm , 40 Hz , 0.70 Damping

INSTRUMENTS

Type Geometrics , Strata Visor NZ

Sampling interval 1ms

Record length 2000 ms

Filter out

Figure 4 : raw shot gather of reflection line 1

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Data analysis and results Landmark’s ProMax Software Package was used for the processing of seismic data.

Processing sequence Geometry assignment

Trace edits

Automatic gain control (AGC)

Amplitude compensation

Band pass filter

Common midpoint (CMP) sorting

Velocity analysis

Normal moveout (NMO) and stacking

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Figure 5 : shot gather after chosen TAR 2db/s of reflection line 1

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Figure 6 : shot gather after SWNA (surface wave noise attenuation) application of reflection line 1

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Figure 7 : spectral analysis before the application of deconvolution of reflection line 1

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Figure 8 : spectral analysis after the application of deconvolution of reflection line 1

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Figure 9 : velocity analysis after DMO of reflection line 1 15

Reflection line 1 length is 94 m; its direction is SW-NE. In this line, the bedrock depth is ranging from 18 - 25 m (Figure 11). A big fault noticed on the right portion where the portion of bedrock is uplifted to be at 18 m depth. The frequency of the data is a bit on the lower side that’s maybe due to the energy penetration problem or attenuation in the complexity or heterogeneity of the near surface. reflection line 3 cross reflection line 1 and the time of the bedrock layer matched which gives enough confidence in saying that this is the true image of bedrock.

Line 1

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Figure 10 : final stack of reflection line 1 in time scale

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Figure 11 : final stack of reflection line 1 in depth (time to depth converted with final velocity model)

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Line 2

Reflection line 2 length is 94 m; and its direction is NE-SW and is running parallel to reflection line 1. This line is just 8 m away from Reflection line 1 so it will follow almost same subsurface structure. In this line, the bedrock depth is ranging from 18 - 25 m (Figure 13). A big fault noticed on the right portion where the portion of bedrock is uplifted to be at 18 m depth. The frequency of the data is a bit on the lower side that’s maybe due to the energy penetration problem or attenuation in the complexity or heterogeneity of the near surface. The signal to noise ratio is not that good here as it was in reflection line 1 but it is conforming the same structure. Reflection line 2 was also crossed by reflection line 3 shows the same time of bedrock at crossing.

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Figure 12 : final stack of reflection line 2 in time scale

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Figure 13 : final stack of reflection line 2 in depth (time to depth converted with final velocity model)

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Line 3

Reflection line 3 also shows the same depth range of bedrock, that is 18 to 25 m (Figure 15). Reflection line 3 length is also 94 m; its direction is NW-SE and is crossed by reflection line 1 and Reflection Line 2. The Fault is not obvious in this line but it is following a synform shape. Again the frequency content looks lower but as long as it is mapping the bedrock, the worries remain away. The maximum offset was very small, so we cannot expect anything else at deeper depth. The high frequencies attenuate very fast in near surface due to its heterogeneity. The signal to noise ratio is good up to the desired objective.

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Figure 14 : final stack of reflection line 3 in time scale

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Figure 15 : final stack of reflection line 3 in depth (time to depth converted with final velocity model)

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Conclusions and Recommendations

The high-resolution seismic reflection technique has been shown to be useful for mapping the bedrock. This study was initiated with acquiring 2D high-resolution seismic reflection data over about 282 m line length consisting of 3 seismic lines. The results show that the bedrock lies at about 18-25 m depth and some faulting observed in seismic reflection data. It seems that imaging depths shallower than 20 m is still a great challenge.

1- In order to obtain an improved image of subsurface features in the study area, the use of 3D - high resolution seismic reflection method is strongly recommended.

2- Additional information about the local up-hole lithology from the oil companies can significantly improve the level of interpretation in any future geophysical endures.

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Acknowledgment

We are thankful to King Abdul Aziz City for Science and Technology (KACST) for the accomplishment of this project. We are also thankful to staff members of the Seismic Analysis Center (KACST) for their support in data processing.

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THANK YOU FOR YOUR ATTENTION!

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