Investigation of Weathering Layer using Reflection and Refraction.pdf

8/9/2019 Investigation of Weathering Layer using Reflection and Refraction.pdf

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Investigation of Weathering Layer Using Seismic Refraction and

High Resolut ion Reflection Methods, Northeastern Riyadh city

Oil and Gas Research InstituteSeismic Analysis Center

Ghunaim T. Al-Anezi (KACST) , Abdullah M. Al-Amri (KSU) and Haider Zaman (KSU)

December 2011


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Objectives

Until present there is no subsurface ground model available about

the velocity distribution and variation of the weathering layer in the

study area. So, this study deals with the investigation of the

weathering layer using both the seismic refraction and high

resolution seismic reflection (HRSR) techniques. This integration

allows, to great extent, in exploiting the advantages andovercoming the individual limitations of both the refraction and

HRSR methods.


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Figure: Location map of seismic profiles

Refraction profile HRSR profile

Location


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The weathering layer The weathered layer lies just below the ground surface and

presents a good example of irregularity. It generally consists of

unconsolidated sediments or soil materials overlaying the waterbearing rocks. It is heterogeneous in composition with wide range

of velocities, which causes variable delay in travel time of the

seismic waves.

Figure shows the weathering layer

(Cox, 1999)


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Geologic setting

Figure shows Surface geological map of the area surrounding Riyadh

(Al-mahmoud et al., 2009)


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Seismic methods

Seismic Refraction High Resolution Seismic Reflection (HRSR)

Figure shows Seismic Refraction and Reflection Geometry


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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 ofattenuation 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

Five seismic profiles for both refraction and high resolution reflection

are conducted in the study area.

Figure: Seismic data acquisition system


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Figure: Field Pictures


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Data formatting SEG2

Geometry In-line-end-on offset

Number of receivers 32

Receiver spacing 3 m

Receiver type Model, GS – 20 DH, Response, 365 ohm , 40 Hz , 0.70 Damping

Shot spacing -5,0, 46.5, 93, 98 m

Source type Hammer (6 Kg)

Minimum offset 1 m

Maximum Offset 98 m

Number of stacking 10

Sampling interval 0. 25 ms

Record length 1 s

Filter type Out

Gain Out

Table : Acquisition parameters for seismic refraction profiles


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Data formatting SEGD

Geometry Split Spread

Number of receivers 48

Receiver spacing 1 m

Receiver type Model, GS – 20 DH, Response, 365 ohm , 40 Hz , 0.70 Damping

Shot spacing 1 m

Source type Hammer (6 Kg)

Minimum offset 0.5 m

Maximum Offset 24.5 m

Number of stacking 5

Sampling interval 0.125 ms

Record length 1 s

Filter type Out

Gain Out

Table : Acquisition parameters for HRSR profiles


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Refraction method is widely used in determining the thicknesses

and velocities for the near-surface layer. It requires an accurate

picking for the first arrival times. Using SeisImager SoftwarePackage (Geometrics Inc., 2005), the first break- picking was

made for the digitized seismic waveforms from all channels along

the surveyed profiles.

Figure: Record for midpoint shot at profile no. 1.

Data analysis and results


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After picking the first break from all profiles, traveltime- distance

(T- D) curves were established for each of them.

Figure: Traveltime- Distance curve for P- wave profile no.

1.


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Landmark’s ProMax Software Package was used for the processing

of HRSR data.

Figure: An example for the field shot record


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Processing sequence

Geometry assignment

Trace edits

Automatic gain control (AGC)

Ampl itude compensation

Band pass filter

Common midpoint (CMP) sorting

Velocity analysis

Normal moveout (NMO) and stacking


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Figure: An example for velocity analysis


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12 m14 m

Refraction HRSR

Site 1

Figure: Ground model and Brute stack for site no. 1


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25 m28 m

Refraction HRSR

Site 2

Figure: Ground model, well information and Brute stack for site no. 2


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17 m 20 m

Refraction HRSR

Site 3



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12 m

13 m

Refraction HRSR

Site 4



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16 m 18 m

Refraction HRSR

Site 5



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Conclusions and Recommendations

Main purpose of the present study is to determine thickness of the

weathered sedimentary layer using 5 seismic refraction and 5 HRSR

profiles. According to seismic parameters obtained from these profiles

and their comparison with the available water-well data, the following

conclusions and recommendations can be made.

1- Using seismic refraction data from 5 profiles, it has been noticed that just beneath the ground surface a thin layer of lose sediments overlies

the main layer of the weathered materials. In order to count them both as

a single unit, the thin layer of sediments is added to overall thickness of

the main weathering layer. The depth of this weathering layer at sites 1-5

is estimated at 12, 25, 17, 12, and 16 meters, respectively. Lithology of

this targeted layer consists of sediments and gravel, which make a

distinguishable contact with the underlying bedrock layer of limestone anddolomite.


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2- Similar to seismic refraction profiles, 5 HRSR profiles were

acquired as a part of this study. Based on the processed data, depthsof the weathering layer at sites 1-5 are estimated at 14, 28, 20, 13,

and 18 meters, respectively.

3- As an additional support, seismic refraction results from site no. 2

got verification from lithological information available from the

adjacent water-well as well as by the HRSR data.

4- At site no. 3, penetration related problems have been encountered

with hammer method during the field operation. It is, therefore,

recommended that an alternative seismic source (including weight-

drop or vibroseis) should be used in any future seismic survey in the

study area.


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5- In order to obtain an improved image of subsurface features in thestudy area, the use of 3D - high resolution seismic reflection method is

strongly recommended.

6- 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 high-ups of the King Abdul Aziz City for Science and

Technology (KACST) for the accomplishment of this project. We are grateful

to Meteb Alshammari (Ministry of Water and Electricity) for providing water-

well information from one of the studied site. 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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Investigation of Weathering Layer using Reflection and Refraction.pdf

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