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OUT LINES:Challenge Of Basalt In Seismic Resolution.

Reflection and Transmission in Basalt.

Review of Rawat Old 2D Data.

Technique used for better imaging sub –basalt.

Proposed Parameters Test for New 2D Project.

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Challenge Of Basalt In Seismic Resolution

Objective: The objectives of this presentation is to

reflect the influences of the basalt in the seismic data .

Introduction: Basalt layers usually have a complex

structure with thin layers and large velocity and density differences. and scatter the seismic energy of conventional seismic reflection system so that becomes difficult to obtain information on deeper Reflectors.

High frequencies are scattered more than low frequencies. and may display a positive velocity gradient. In general, it is not difficult to map the top of a basalt layer because it is a strong reflector. But multiples from the bottom and top of the basalt generally mask sub-basalt reflected waves.

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Volcanic Geometries

• Advantage Of Basalt In seismic Data:• Volcanic rocks may form lateral seal

or migration barriers providing positive impact on the petroleum system, Non preamble volcanic can seal the top of reservoir or they can build migration barrier for fluids on its way from the source rock into the trap.

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Line SD01-062

Line SD01-047

• The high impedance contrast between sedimentary and basaltic rocks directly influences the reflection and transmission of waves. If a wave encounters an interface between a sedimentary layer and a basalt, most of the energy will be reflected back and less energy will be transmitted through the interface. The top of a basalt sequence will have a high reflectivity and therefore will transmit less energy through it. An incident P-wave will produce reflected P- and S- modes also beyond the critical angle, but the transmission of a P-wave down into the high-velocity layer will stop at this point and only S-wave energy will travel further down.

Reflection and Transmission in Basalt

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Basalts have velocities and densities much higher than the surrounding and interbedded sediments. Forming strong seismic reflectors.In general , interflow materials possess very low densities (1.5 – 2.5 g/cm³) and P- wave velocities (1 – 2km/s) (Bergman 1997).The lava flows themselves have internal structure which can be detected on most well logs. Each flow consists of an inner core . And a top and a bottom margin. Where approximately 25% of the flow thickness is defined as top margin and 5% as bottom margin. These margins build during the cooling and degassing of the Lava flow and show an increased velocity gradient to – wards the flow core. Cerney and Carlson (1999) studied the basalts drilled at south east Greeland margin and concluded that P- wave velocity controlled by the porosity of the internal basalt zones. From the flow top the velocities increase from about 2-3 km/s to around 5 – 6.5 km/s in the interior and degrease rapidly again towards the basalt base (Planke.1994:Shipboard Scientific Party.1994).

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• FMI result is reflect there is high resistivity, low ROP zones were interpreted as igneous rocks and they covered part of the Northern Sub-basin.

• The effect of these igneous bodies on the petroleum system of the area is poorly known.

Volcanic Interpretation for the Drilled Well

Top VolcanicBottom

Volcanic

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Review of Rawat Old 2D Data RPOC was re-processed two 2D lines

from the northern Basin to improve target image within the volcanic Rocks (Basalt).

There are some improvement at top of volcanic compare to the bottom due to acquisition parameters limitation such as fold and dipping angle (far offset).

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BA

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Re-Processed data has slightly improvement beneath the basalt

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Re-processed (2016)

Processed (2001)

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S.Int

R.Int

Charge Size

T. Station

Max.Offset

Fold Coverage

50 m

25 m

2 Kg 280 3487.5 70

S.Int R.Int

Charge Size

T. Station

Max.Offset

Fold Coverage

125 m

25 m

12 Kg 248 2312.5 25

2001 2009Dip Line

Probability Of Improving Data Quality with dynamite crew

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S.Int R.Int

Charge Size

T. Station

Max.Offset

Fold Coverage

125 m

25 m

12 Kg 248 2312.5 25

S.Int

R.Int

Charge Size

T. Station

Max.Offset

Fold Coverage

50 m

25 m

2 Kg 280 3487.5 70

Probability Of Improving Data Quality with dynamite crew

Strike Line

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Technique used for better imaging sub –basaltWide Angle

Technique used for better imaging sub –basalt:

Wide –angle (long offset) seismic surveys to avoid scattering problem to some extent , usually give results related only to large scale interval velocity of the basalt.

The Use of Broad Band contain low frequency seismic waves may avoid the scattering problem of thin layering and lateral heterogeneous and may provide the basis for reflection energy to penetrate through the basalt (Gatliff et al.1984

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Long offset to improve S/N and NMO resolution and to allow processing of most critical reflections.

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Low FrequencyHigh FrequencyImpedance

Sediments

Volcanic

Technique used for better imaging sub –basaltLow Frequency

The basalt lead to filter and scattered most of the High Frequency and reflected as multiples layers.

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35 HZ 10 HZ

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10-20 Hz

Frequency Analysis

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Band pass 20-40Hz

Frequency Analysis

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Band pass 30-60Hz

Frequency Analysis

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Ground Equipment has used for Low frequency Micro Electro Mechanical System (MEMS) is

advanced geophone use for low frequency and it is accelerometers that work below their resonant frequency ,while coiled geophone are velocimeters that work above their resonant frequency .This different provides the two types of sensor with quite different dimensions and specification.

Ground motion can be measured as displacement, velocity and acceleration. A mass/ spring assembly used for these measurement.

The essential benefit of MEMS accelerometers is broadband linear amplitude and phase response that may extend from 0 DC to 800Hz within ±1% in amplitude and ±20 µs in time. MEMS resonant frequency is far above the seismic band (1 kHz).

This makes it possible to record frequencies below 10 Hz without attenuation, including the direct current related to gravity acceleration.

The gravity vector provides useful reference for sensitivity calibration and tilt measurement (3C sensor).

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Comparison Between Analog and Digital Geophone

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Linear phase and amplitude response compared between MEMS and 10HZ Geophone.

Comparison in the velocity domain of electric noise of geophone with one of digital sensor unit with MEMS.

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No.

No.Holes

Hole Depth

Holes Patterns

Charge Size

T. Charge Size

1 1 12 m 12.5m 4 Kg 4 Kg

2 2 12 m 8 - 12.5 – 8 m

3 kg 6 Kg

2 2 9 m 8 - 12.5 – 8 m

3 kg 6 Kg

4 3 9 m 8 – 8 – 8 m 2 kg 6 Kg

5 4 6 m 4 – 4 – 4 – 4 m

2 kg 8 Kg

Proposed Source Parameters Test

As far as we are looking for broad band frequency rather than strong energy which could be lead to increase source generate Noise and ground Roll amount. Deep hole with reasonable charge size is recommended.

2D Geometry:

Total Live Channels:420

Receiver Interval: 25

Source Interval: 50

Fold Coverage:105Max Offset: 5250 m

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Propose test will carry out with using double geophones string for better sampling and n ambient noise attenuation.

Proposed 2D Lines (300 Km)

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Fold Plot shows the effects of mute function on CMP across the survey

The Diagram shows the strike line azimuth and Bin or CMP distribution

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