Editing Imaging coals with seismic reflection data for improved detection of sandstone bodies

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The geological succession in the study area consists of Triassic mudstones with dolomitic sandstones and waterstones and irregular sandstone beds, overlying a Permian sequence of marls and limestones. The Upper Magnesian and the Lower Magnesian Limestones are important reflectors, generating distinctive seismic reflections in the upper part of the seismic data. The Basal Permian Sand is difficult to distinguish on seismic sections, but in this area lies just below the Lower Magnesian Limestone. The base of Permian overlies the Westphalian C strata with a slight angular unconformity. The Cambriense Marine Band (Bolsovian–Westphalian C) lies near the top of the Westphalian succession and there are approximately 145 m of Bolsovian strata consisting of a sequence of mudstones with frequent seat earths and thin coal seams, and occasional sandstones. The main coal development lies in approximately 240 m of Duckmantian strata with the Main Bright, Kent’s Thick, Top Hard and Dunsil seams being the thickest of many coal seams. Below the Vanderbeckei Marine Band, the top part of the Westphalian A (Langsettian) succession is similar to the Duckmantian, with the Deep Soft, Parkgate and Blackshale coal seams forming significant reflectors above increasingly sand-rich lower Westphalian A and Namurian sequences. The top Dinantian limestone forms the next widespread and correlatable reflector. The Westphalian strata lie in a shallow syncline plunging gently to the northwest. The surface elevations range from 29 m to 90 m above ordnance datum.
 
The geological succession in the study area consists of Triassic mudstones with dolomitic sandstones and waterstones and irregular sandstone beds, overlying a Permian sequence of marls and limestones. The Upper Magnesian and the Lower Magnesian Limestones are important reflectors, generating distinctive seismic reflections in the upper part of the seismic data. The Basal Permian Sand is difficult to distinguish on seismic sections, but in this area lies just below the Lower Magnesian Limestone. The base of Permian overlies the Westphalian C strata with a slight angular unconformity. The Cambriense Marine Band (Bolsovian–Westphalian C) lies near the top of the Westphalian succession and there are approximately 145 m of Bolsovian strata consisting of a sequence of mudstones with frequent seat earths and thin coal seams, and occasional sandstones. The main coal development lies in approximately 240 m of Duckmantian strata with the Main Bright, Kent’s Thick, Top Hard and Dunsil seams being the thickest of many coal seams. Below the Vanderbeckei Marine Band, the top part of the Westphalian A (Langsettian) succession is similar to the Duckmantian, with the Deep Soft, Parkgate and Blackshale coal seams forming significant reflectors above increasingly sand-rich lower Westphalian A and Namurian sequences. The top Dinantian limestone forms the next widespread and correlatable reflector. The Westphalian strata lie in a shallow syncline plunging gently to the northwest. The surface elevations range from 29 m to 90 m above ordnance datum.
  
The 3-D seismic dataset was acquired using 1lb (454g) dynamite charges in 40ft (12.2 m) shot-holes. The receiver group interval was 12 m and the receiver lines were placed 120 m apart. The shot lines were approximately 168 m apart and were at right angles to the receiver lines. The shot interval was 12 m. Each 40 ft shot hole was drilled twice and shot into a 144-channel patch. The total area of common mid-point coverage was 1.9km<sup>2</sup> and the nominal fold of cover was 6. The common mid-point (CMP) bins were 6x6 m. The acquisition parameters are summarized in [[:File:YGS_CHR_08_IMAG_TAB_01.jpg|Table 1]].
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The 3-D seismic dataset was acquired using 1lb (454g) dynamite charges in 40ft (12.2 m) shot-holes. The receiver group interval was 12 m and the receiver lines were placed 120 m apart. The shot lines were approximately 168 m apart and were at right angles to the receiver lines. The shot interval was 12 m. Each 40 ft shot hole was drilled twice and shot into a 144-channel patch. The total area of common mid-point coverage was 1.9km<sup>2</sup> and the nominal fold of cover was 6. The common mid-point (CMP) bins were 6x6 m. The acquisition parameters are summarized in Table 1.
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'''Table 1 Acquisition parameters.'''
  
'''[[:File:YGS_CHR_08_IMAG_TAB_01.jpg|Table 1]] Acquisition parameters'''
 
  
 
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* The short offsets limit the usefulness of some seismic attributes such as AVO.
 
* The short offsets limit the usefulness of some seismic attributes such as AVO.
  
A suite of seismic attributes was calculated on the 3-D volume. Of these, the most useful were expected to be reflection strength or amplitude envelope, instantaneous phase and instantaneous frequency. Reflection strength highlights changes in acoustic impedance and is used to identify anomalous impedance horizons such as gas-charged sandstones. As coal seams have a very low impedance compared with the surrounding strata, the reflection-strength plots will show detail in the coal seam that can lead to the recognition of small faults and variations in seam thickness . Instantaneous phase emphasizes the linearity of events and thus highlights breaks such as pinch-outs at unconformities and faults; it is not a useful attribute for detecting variations in seam thickness. The cosine of the instantaneous phase is particularly useful for enhancing sedimentary features ([[:File:YGS_CHR_08_IMAG_FIG_07.jpg|Figure 7]]a). Instantaneous frequency measures the temporal change in instantaneous phase and can be used to identify features that alter the frequency content of the data, such as gas accumulations that attenuate the high frequencies. Theoretically, instantaneous frequency can be used to detect variations in seam thickness, but it is susceptible to noise contamination and can therefore be of limited use when the anomalies are very small. It was not found to be very useful in this study. The absolute trace amplitude plot requires careful processing, but makes the identification of small faults and other low-amplitude zones easier ([[:File:YGS_CHR_08_IMAG_FIG_07.jpg|Figure 7]]b).
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A suite of seismic attributes was calculated on the 3-D volume. Of these, the most useful were expected to be reflection strength or amplitude envelope, instantaneous phase and instantaneous frequency. Reflection strength highlights changes in acoustic impedance and is used to identify anomalous impedance horizons such as gas-charged sandstones. As coal seams have a very low impedance compared with the surrounding strata, the reflection-strength plots will show detail in the coal seam that can lead to the recognition of small faults and variations in seam thickness . Instantaneous phase emphasizes the linearity of events and thus highlights breaks such as pinch-outs at unconformities and faults; it is not a useful attribute for detecting variations in seam thickness. The cosine of the instantaneous phase is particularly useful for enhancing sedimentary features ([[:File:YGS_CHR_08_IMAG_FIG_07.jpg|Figure 7]]a). Instantaneous frequency measures the temporal change in instantaneous phase and can be used to identify features that alter the frequency content of the data, such as gas accumulations that attenuate the high frequencies. Theoretically, instantaneous frequency can be used to detect variations in seam thickness, but it is susceptible to noise contamination and can therefore be of limited use when the anomalies are very small. It was not found to be very useful in this study. The absolute trace amplitude plot requires careful processing, but makes the identification of small faults and other low-amplitude zones easier (Figure 7b).
  
'''[[:File:YGS_CHR_08_IMAG_TAB_02.jpg|Table 2]] Processing sequence'''
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'''Table 2 Processing sequence.'''
  
 
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