An intelligent multi-fidelity surrogate-assisted multi-objective reservoir production optimization method based on transfer stacking

Computational Geosciences - Recently, many researchers have focused on reservoir production optimization because it is one of the most essential processes in closed-loop reservoir management....     

Moment tensor inversion of very long period seismic signals from Strombolian eruptions of Erebus Volcano

Strombolian eruptions from the long-lived lava lake of Erebus volcano, Ross Island, Antarctica, generate repeating Very Long Period (VLP) signals, containing energy between approximately 30 and 5 s, that persist for several minutes and through the post-eruptive refilling of the lava lake. The initial approximately 10 s of this signal is moderately variable, particularly with respect to its initial polarity, while the following VLP coda has been observed to be stable since the earliest VLP observations were made (1996). To estimate forces and force couples consistent with the Erebus VLP signature, we perform moment tensor inversions for point sources using high signal-to-noise data stacks from the six-station, 18-component broadband seismographic network and Green's function forward calculations that incorporate topography. We infer a shallow (approximate depth of less than 400 m below the lava lake surface) source centroid that underlies the center to the northwestern rim of the main crater, east and north of the lava lake. Integrated M_ii functions over the predominant (180 s) signal duration of VLP events show that the net scalar moments for these events are on the order of 4 × 10¹³ N m (corresponding to a moment magnitude mw ≈ 3) for typical sized VLP events. Moment rate tensors which characterize force couple components are dominated (85–97% of variance) by dilatational components. Approximately 25% of the data variance is attributable to single forces that are attributable to oscillatory reaction forces caused by fluid transport, however, the relative contributions of vertical forces and couples with this sparse network is poorly resolved for these shallow sources. The generally high degree of repeatability in the VLP signal across thousands of eruptions over the past decade indicates that the response of the conduit system to gas slug ascent and subsequent gravitational disequilibrium is stable, consistent with the generally unchanging surface manifestation of the convecting lava lake system, and arguing for a thermally and dynamically stable conduit system beneath the lava lake.     

Systematic change of foraminiferal Mg/Ca ratios across a strong salinity gradient

The Mg/Ca ratio of foraminiferal calcite is an important proxy for estimating past ocean temperatures. Used in conjunction with δ¹⁸O of foraminiferal calcite it allows deconvolution of temperature and ice-volume signals to infer past ocean temperatures and salinities (assuming the relationship between seawater δ¹⁸O and salinity is known). Such work assumes that temperature is the only, or at least the dominant, environmental controller of foraminiferal Mg/Ca. The semi-enclosed Mediterranean Sea, where salinity varies from 36 to 40 psu over a seasonal temperature range of between only 5 °C to 8 °C, provides a relevant setting to test this assumption outside the laboratory. In this study, planktonic foraminifera (O. universa, G. siphonifera, G. bulloides and G. ruber (white) and (pink)) were picked from 11 box core tops spanning the Mediterranean salinity gradient and analysed for their trace-element concentrations. Mg/Ca ratios are higher, for the associated calcification temperatures, than in other regions where calibrations have been conducted and correlate poorly with calcification temperature. Mg/Ca ratios are particularly high for samples from the Eastern Mediterranean where salinity is unusually high. Correlations of Mg/Ca with the calcification salinity are statistically significant with Mg/Ca changing by 15–59% per psu, suggesting that salinity may act as a control on Mg/Ca ratios in addition to the dominant temperature control. We show that contamination by non-carbonate material and diagenetic high-Mg carbonate overgrowths cannot account for the observed trend of increasing Mg/Ca with salinity. A relationship between Mg/Ca and salinity is also suggested by re-analysis of calibrations from open-ocean settings. These new Mediterranean results are from a region with unusually high salinity but suggest that the effects of salinity on the Mg/Ca palaeothermometer should be considered even in open-ocean settings, particularly where large salinity changes occurred in the past.     

Grazing impacts on gully dynamics indicate approaches for gully erosion control in northeast Australia

Drainage network extension in semi‐arid rangelands has contributed to a large increase in the amount of fine sediment delivered to the coastal lagoon of the Great Barrier Reef, but gully erosion rates and dynamics are poorly understood. This study monitored annual erosion, deposition and vegetation cover in six gullies for 13 years, in granite‐derived soils of the tropical Burdekin River basin. We also monitored a further 11 gullies in three nearby catchments for 4 years to investigate the effects of grazing intensity. Under livestock grazing, the long‐term fine sediment yield from the planform area of gullies was 6.1 t ha^‐1 yr^‐1. This was 7.3 times the catchment sediment yield, indicating that gullies were erosion hotspots within the catchment. It was estimated that gully erosion supplied between 29 and 44% of catchment sediment yield from 4.5% of catchment area, of which 85% was derived from gully wall erosion. Under long‐term livestock exclusion gully sediment yields were 77% lower than those of grazed gullies due to smaller gully extent, and lower erosion rates especially on gully walls. Gully wall erosion will continue to be a major landscape sediment source that is sensitive to grazing pressure, long after gully length and depth have stabilised. Wall erosion was generally lower at higher levels of wall vegetation cover, suggesting that yield could be reduced by increasing cover. Annual variations in gully head erosion and net sediment yield were strongly dependent on annual rainfall and runoff, suggesting that sediment yield would also be reduced if surface runoff could be reduced. Deposition occurred in the downstream valley segments of most gullies. This study concludes that reducing livestock grazing pressure within and around gullies in hillslope drainage lines is a primary method of gully erosion control, which could deliver substantial reductions in sediment yield. Copyright © 2018 John Wiley & Sons, Ltd.     

Frequency–magnitude statistics and spatial correlation dimensions of earthquakes at Long Valley caldera, California

Intense earthquake swarms at Long Valley caldera in late 1997 and early 1998 occurred on two contrasting structures. The first is defined by the intersection of a north-northwesterly array of faults with the southern margin of the resurgent dome, and is a zone of hydrothermal upwelling. Seismic activity there was characterized by high b -values and relatively low values of D , the spatial fractal dimension of hypocentres. The second structure is the pre-existing South Moat fault, which has generated large-magnitude seismic activity in the past. Seismicity on this structure was characterized by low b -values and relatively high D . These observations are consistent with low-magnitude, clustered earthquakes on the first structure, and higher-magnitude, diffuse earthquakes on the second structure. The first structure is probably an immature fault zone, fractured on a small scale and lacking a well-developed fault plane. The second zone represents a mature fault with an extensive, coherent fault plane.     

Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)

 The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (T _cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the type ΔG = −hQ + gTQ +  (T − T _c)Q ² +  Q ⁴, with a/h = 0.00406 K⁻¹, b/h = 2.3, T _c = 572 K and g/h = 0.00106 K⁻¹. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions become less ideal as temperature increases. Received: 12 August 1999 / Accepted: 25 April 2000     

Rare earth element partition between sphene,apatite and other coexisting minerals of the Kangerdlugssuaq intrusion,E. Greenland

Cumulus apatite, sphene, feldspar, amphibole and biotite from the pulaskite of the Kangerdlugssuaq alkaline intrusion have been analysed for rare earth elements (REE) by instrumental neutron activation analysis. The apatite is particularly rich in REE, contains 3.6% Ce and shows a steep, light REE-enriched, chondrite-normalised pattern. The other minerals have light REE enrichment but with sphene showing a peak at Ce on a chondrite-normalised plot. REE partition coefficient values show that the light REE are preferentially accommodated by apatite relative to sphene. The differences in these coefficients result from differences in the co-ordination of the REE in the two minerals.     

Inversions in sub-potassic nephelines

Sub-potassic nephelines in the system NaAlSiO₄(Ne)-KAlSiO₄(Ks) were synthesized under a variety of conditions and studied at room temperature and up to 1000 °C using an X-ray powder diffractometer. At low temperatures they do not have the hexagonal structure determined by Hahn and Buerger (1955) for natural nepheline. Samples with 0.7 to 2.5 mole % Ks have an orthorhombic supercell with parameters equivalent to a, 3a, 3c where a and c are Hahn and Buerger structure cell parameters. Nephelines with 0 to 0.7% Ks consist of two phases with different c axes; one of these phases has the orthorhombic supercell.Pure-Na nephelines (NaAlSiO₄) invert to a hexagonal phase with the Hahn and Buerger structure at 190 °±10 °C; this inversion temperature decreases with increasing Ks and a sample with 0.5% Ks inverts at 170 °±5 °C. The inversion is reversible and is displacive. Another reversible inversion begins at 875 °±10 °C in pure-Na nepheline; this inversion increases in temperature with increasing Ks and a sample with 1.8% Ks begins to invert at 960 °±10 °C.Superstructures with anomalous low-temperature cell parameters in sub-potassic nephelines are attributed to reversible collapse of the framework about the larger cation sites which must be occupied by small Na in subpotassic nephelines. Superstructures in natural nephelines are also related to framework collapse at a displacive inversion.     

Variation in fold geometry in the Yuso basin, northern Spain: implications for the deformation regime

First generation structures in greywackes of the Yuso Group from the Cantabrian Mountains of northern Spain show a distinct variation in geometry with depth in a regional synclinal structure (Curavacas and Lechada synclines); they are easily distinguished from other deformation events. In the structurally uppermost level we find ‘flap folds’. Flap folds are recumbent structures with the inverted limb preserved. Below this level ‘cascade folds’ are found. These structures have a vergence opposite to that of parasitic folds. The nomenclature adopted is from Harrison and Falcon. Characteristically, these structures have shallowly dipping axial surfaces, in agreement with the shallow dip of the axial plane (regional) cleavage. In the lowermost structural level, upright parasitic folds with a steep cleavage are present. The variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth. In the absence of overprinting relationships the F₁ fold geometries are included in a single deformation event.The steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub-vertical in the upper part of the syncline to sub-horizontal in the lower part. With increasing depth the deformation regime during F₁ changed from bending to buckling. The deformation regime on the regional scale, however, is associated with basement subsidence and passive formation of the regional synclinal structure. Furthermore, the absence of a distinct microfabric for the different F₁ folds indicates that on a small scale a similar deformation regime was present. We conclude, therefore, that the scale at which we study a structure only reflects the deformation regime at that particular scale. Consequently, the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone.