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

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Carboniferous formations have an extensive subcrop beneath the North Sea. The sequence is up to 9000 m thick, with rapidly changing facies and pronounced local changes in thickness (Besly 1998). It is economically important for two main reasons:* The coals and carbonaceous shales are source rocks, particularly for gas, in the overlying Permian and Triassic reservoirs of the southern North Sea (SNS).
 
Carboniferous formations have an extensive subcrop beneath the North Sea. The sequence is up to 9000 m thick, with rapidly changing facies and pronounced local changes in thickness (Besly 1998). It is economically important for two main reasons:* The coals and carbonaceous shales are source rocks, particularly for gas, in the overlying Permian and Triassic reservoirs of the southern North Sea (SNS).
 
* The Carboniferous sequence itself contains important sandstone bodies that act as gas reservoirs, notably within the Westphalian of the northern part of the SNS.
 
* The Carboniferous sequence itself contains important sandstone bodies that act as gas reservoirs, notably within the Westphalian of the northern part of the SNS.
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Between 1983 and 1993 some 20 gas discoveries were made within the Carboniferous of this region, including several major finds, and commercial development of the Murdoch, Caister and Ketch fields began. However, the Westphalian reservoirs are difficult to locate, because, although the ratio of net reservoir sandstones to gross interval (net-to-gross) can be relatively high, the seismic resolution is low. Westphalian strata lie beneath up to 4000 m of overburden, including Zechstein salts, which rapidly attenuates the seismic signal. Within the Westphalian itself, the highly reflective coals dominate the seismic response. The sandstone bodies forming the gas reservoirs are the coarse-grained fluvial channel features, which are inherently complex in both their geometry and internal architecture. They are generally less than 100 m thick and close to or below seismic resolution. Additionally, late Carboniferous uplift and erosion has reduced the area within which the Carboniferous strata are preserved, and subsequent tectonic activity has broken them into a series of northwest-trending fold and fault blocks, all of which makes their identification and correlation on seismic data particularly difficult.
 
Between 1983 and 1993 some 20 gas discoveries were made within the Carboniferous of this region, including several major finds, and commercial development of the Murdoch, Caister and Ketch fields began. However, the Westphalian reservoirs are difficult to locate, because, although the ratio of net reservoir sandstones to gross interval (net-to-gross) can be relatively high, the seismic resolution is low. Westphalian strata lie beneath up to 4000 m of overburden, including Zechstein salts, which rapidly attenuates the seismic signal. Within the Westphalian itself, the highly reflective coals dominate the seismic response. The sandstone bodies forming the gas reservoirs are the coarse-grained fluvial channel features, which are inherently complex in both their geometry and internal architecture. They are generally less than 100 m thick and close to or below seismic resolution. Additionally, late Carboniferous uplift and erosion has reduced the area within which the Carboniferous strata are preserved, and subsequent tectonic activity has broken them into a series of northwest-trending fold and fault blocks, all of which makes their identification and correlation on seismic data particularly difficult.
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=== 2.1 Previous work ===
 
=== 2.1 Previous work ===
  
The encoding of variations in seam thickness within the amplitude of the reflection follows the Widess (1973) “thin bed” theory. Widess considered a layer embedded in an infinite homogeneous medium and defined a thin bed as one that produces a composite reflection because of the interference of the reflections from the top and bottom of the layer. The point at which the composite reflection no longer changed with the dimensions of the bed was the maximum thickness of the “thin bed”. In practical terms he concluded that a bed is defined as seismically thin where the layer thickness is less than one eighth of the dominant wavelength. He also demonstrated that the amplitude of the thin-bed reflection is approximately proportional to the thickness of the bed and inversely proportional to the dominant wavelength. Widess assumed that the wavelet complex reflected by the layer is obtained by addition of the wavelet reflected from the top of the layer and the one reflected from the bottom, the latter being phase inverted and time shifted relative to the former. The tuning thickness is the bed thickness at which constructive interference from the top and bottom of the bed produces the strongest reflection. Tuning occurs at a thickness of one quarter of the wavelength and is therefore a function of the dominant temporal frequency and interval velocity. Widess’s theory assumes transmission loss and internal multiple reflections to be negligible; both are untrue in cyclic coal sequences.
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The encoding of variations in seam thickness within the amplitude of the reflection follows the Widess (1973) “thin bed” theory. Widess considered a layer embedded in an infinite homogeneous medium and defined a thin bed as one that produces a composite reflection because of the interference of the reflections from the top and bottom of the layer. The point at which the composite reflection no longer changed with the dimensions of the bed was the maximum thickness of the “thin
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bed”. In practical terms he concluded that a bed is defined as seismically thin where the layer thickness is less than one eighth of the dominant wavelength. He also demonstrated that the amplitude of the thin-bed reflection is approximately proportional to the thickness of the bed and inversely proportional to the dominant wavelength. Widess assumed that the wavelet complex reflected by the layer is obtained by addition of the wavelet reflected from the top of the layer and the one reflected from the bottom, the latter being phase inverted and time shifted relative to the former. The tuning thickness is the bed thickness at which constructive interference from the top and bottom of the bed produces the strongest reflection. Tuning occurs at a thickness of one quarter of the wavelength and is therefore a function of the dominant temporal frequency and interval velocity. Widess’s theory assumes transmission loss and internal multiple reflections to be negligible; both are untrue in cyclic coal sequences.
  
 
An important factor in a multi-seam environment is that not only do the large reflection coefficients cause high amplitude primary reflections, but strong surface multiples and interbed multiples are also set up within and between seams (Greenhalgh et al. 1986). The generation of a train of short-path multiples broadens the transmitted seismic pulse with a consequent lowering of its frequency content. Ri ter & Schepers (1978) calculated synthetic seismograms from the Coal Measures of the Ruhr and concluded that a single seam can give a distinct reflection even at a thickness of one fiftieth of the wavelength. However, in a sequence of layers containing many coal seams, they noted that individual reflections may become obscured by constructive interference from short-lag multiples.
 
An important factor in a multi-seam environment is that not only do the large reflection coefficients cause high amplitude primary reflections, but strong surface multiples and interbed multiples are also set up within and between seams (Greenhalgh et al. 1986). The generation of a train of short-path multiples broadens the transmitted seismic pulse with a consequent lowering of its frequency content. Ri ter & Schepers (1978) calculated synthetic seismograms from the Coal Measures of the Ruhr and concluded that a single seam can give a distinct reflection even at a thickness of one fiftieth of the wavelength. However, in a sequence of layers containing many coal seams, they noted that individual reflections may become obscured by constructive interference from short-lag multiples.
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'''[[:File:YGS_CHR_08_IMAG_TAB_02.jpg|Table 2]] Processing sequence'''
 
'''[[:File:YGS_CHR_08_IMAG_TAB_02.jpg|Table 2]] Processing sequence'''
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{| class="wikitable"
 
{| class="wikitable"
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A significant sandbody is known to lie between the Dunsil and Top Hard seams (mid-Westphalian B), these seams forming the effective lower and upper limits, respectively, of the overall channel system containing the sandbody. The sandbody is up to 4km wide, possibly including more than one contemporary channel belt, and crosses the study area from northwest to southeast. The maximum sandstone thickness is probably about 15 m, the overall feature being essentially single storey. The Top Hard is typically a multi-coalbed seam up to 2 m thick, and is locally eroded in the wider area (the result of later Coal Measures fluvial systems). The Dunsil Seam is typically less than 1 m thick itself, but is locally united with the underlying First Waterloo Seam, to give a composite seam of more than 2 m (as at Nickerbush). The Dunsil Seam is locally eroded within the channel belt area.
 
A significant sandbody is known to lie between the Dunsil and Top Hard seams (mid-Westphalian B), these seams forming the effective lower and upper limits, respectively, of the overall channel system containing the sandbody. The sandbody is up to 4km wide, possibly including more than one contemporary channel belt, and crosses the study area from northwest to southeast. The maximum sandstone thickness is probably about 15 m, the overall feature being essentially single storey. The Top Hard is typically a multi-coalbed seam up to 2 m thick, and is locally eroded in the wider area (the result of later Coal Measures fluvial systems). The Dunsil Seam is typically less than 1 m thick itself, but is locally united with the underlying First Waterloo Seam, to give a composite seam of more than 2 m (as at Nickerbush). The Dunsil Seam is locally eroded within the channel belt area.
  
'''[[:File:YGS_CHR_08_IMAG_TAB_03.jpg|Table 3]] Sand bodies, mapped from borehole and mining records, across or adjacent to 3D study area (in descending stratigraphical order)'''
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'''[[:File:YGS_CHR_08_IMAG_TAB_03.jpg|Table 3]] Sand bodies, mapped from borehole and mining records, across or adjacent to 3D study area (in descending stratigraphical order).'''
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==== 3.2.3 Selected sandbodies: above the Parkgate Seam ====
 
==== 3.2.3 Selected sandbodies: above the Parkgate Seam ====

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