Effect of autocorrelation function model on spatial prediction of geological interfaces

This study evaluated the performances of various autocorrelation function (ACF) models in predicting the geological interface using a well-known conditional random field method. Prediction accuracies and uncertainties were compared between a flexible Matérn model and two classical ACF models: the Gaussian model and the single exponential model. The rockhead data of Bukit Timah granite from boreholes at two sites in Singapore as well as simulated data were used for the comparisons. The results showed that the classical models produce a reasonable prediction uncertainty only when its smoothness coefficient is consistent with that of the geological data. Otherwise, the classical models may produce prediction errors much larger than that of the Matérn model. On the other hand, the prediction accuracy of the Matérn model is affected by the spacing of the boreholes. When the borehole spacing is relatively small (< 0.4 × scale of fluctuation), the Matérn model can reasonably quantify the prediction uncertainty. However, when the borehole spacing is large, the prediction by the Matérn model becomes less accurate as compared with the prediction using the classical models with the right value of smoothness coefficient due to the large estimation error of the smoothness coefficient

Deep to shallow-marine sedimentology and impact of volcanism within the Middle Triassic Palaeo-Tethyan Semantan Basin, Singapore

The Middle Triassic Pulau Ayer Chawan Formation is a predominantly deep-marine, occasionally shallow-marine sedimentary succession, deposited in the Singapore sector of the Palaeo-Tethyan Semantan Basin. The formation provides an important record of the dynamic interplay between a siliciclastic sedimentary system and the products of an adjacent active volcanic arc. It is characterised by six sub-environments, including: deep-marine turbidite fan, deep-marine background sedimentation, subaqueous debris cone, shallow-marine, volcanically-sourced turbidite fan, and hyaloclastite mound or ridge. Turbidite fan deposits preserve the input of both siliciclastic and volcaniclastic sediments from the shelf, transported into the deep-marine environment by a suite of subaqueous sediment gravity flow processes, including: turbidity currents; mixed flow types (hybrid event beds); concentrated and hyper concentrated sediment gravity flows, and debris flows. Thick heterolithic successions of debrites were likely sourced through regular collapse of an unstable shelf. The presence of hybrid event beds, encountered within the deep-marine turbidite fans, supports a slope that was out-of-grade, and may have been actively retreating towards the hinterland. Together, these factors suggest regional-scale uplift of the eastern margins of the Semantan Basin during Triassic times, most likely facilitated through volcanic activity in the adjacent Palaeo-Tethys Sukhothai Arc. Evidence for contemporaneous, arc-related magmatism includes ubiquitous volcaniclastic sedimentary rocks within formation, including pyroclastic density current deposits and perhaps more-strikingly through the hyaloclastites of the Nanyang Member. The hyaloclastites formed through quenching of magmas delivered into the deep-marine setting from a series of sub-sea vents or mounds

Paleozoic to Cenozoic sedimentary bedrock geology and lithostratigraphy of Singapore

A new lithostratigraphical framework for Singapore is proposed, based on the analysis of c. 20,000 m of core recovered from 121 c. 205 m deep boreholes and augmented with 218 field localities from across Singapore. The new framework describes a succession dating from the Carboniferous to the Quaternary. New U-Pb detrital zircon dates and fossil analysis were used to constrain the ages of key sedimentary units. The oldest known sedimentary rocks in Singapore are found to be the deformed Carboniferous (Mississippian) Sajahat Formation. These are succeeded by the newly erected, Middle and Upper Triassic, marine to continental Jurong Group and Sentosa Group successions that accumulated in the southern part of the Semantan Basin. The Jurong Group comprises four formations: the Tuas Formation, the Pulau Ayer Chawan Formation, the Pandan Formation and the Boon Lay Formation. The Sentosa Group contains two formations: the Tanjong Rimau Formation and the Fort Siloso Formation. In Singapore, the depositional record during this time is related to late Permian to Triassic arc magmatism in the southern part of the forearc basin to the Sukhothai Arc. The Jurong and Sentosa groups were deformed and weakly metamorphosed during the final stages of the Late Triassic to Early Jurassic orogenic event, deformation that led to the formation of the syn-orogenic conglomerates of the Buona Vista Formation. Following this, two distinct Lower Cretaceous sedimentary successions overstepped the Jurong and Sentosa group strata, including the Kusu Formation and the Bukit Batok Formation, both deposited in the southern part of the Tembeling Basin. A series of Neogene to Quaternary formations overly the Mesozoic and Palaeozoic stratigraphy, including the Fort Canning Formation, Bedok Formation and the Kallang Group

Two-dimensional prediction of the interface of geological formations: A comparative study

The location of the interface of geological formations is an important piece of information for tunneling construction. As site investigation data are usually limited, the uncertainties in locating geological interfaces for the sections between boreholes can be large and challenging to estimate. A suitable geostatistical method is thus needed for spatial prediction of the geological interfaces. In this paper, the performance of three commonly used spatial prediction methods, namely the multivariate adaptive spline regression (MARS), conditional random field (CRF) method, and thin-plate spline interpolation (TPSI) methods, are evaluated for two-dimensional cases using the boreholes data from three sites in Singapore. The prediction accuracies, patterns of the predicted surfaces, and prediction uncertainties obtained from the three methods are compared. A zonation is also proposed to improve the prediction accuracy of the MARS method. The results indicate that the MARS method can show the spatial trend of the geological interface more clearly than the other two methods. The TPSI method produces undesirable oscillations of the surface of geological interfaces and the CRF method may underestimate the extreme values of the geological interface elevations. In general, the prediction accuracy of the MARS method is similar to that of the CRF method, but higher than that of the TPSI method. For cases with very limited data in geologically complex areas, the MARS may have larger errors than the CRF method. However, the accuracy of the former can be significantly improved if a reasonable zonation is performed

Bayesian evidential learning of soil-rock interface identification using boreholes

Identification of the soil-rock interface of geological profiles has been a challenging task for underground construction because of lack of sufficient borehole data. Traditional spatial prediction methods for geostatistics require subjective assumptions in the functional form of the variograms or covariance models. This study aims to evaluate the uncertainty of the soil-rock interface using the Bayesian evidential learning (BEL) framework without the subjective assumptions. A borehole-intensive site is selected to investigate the impact of borehole number and layout on the estimation of the soil-rock interface. The BEL is further applied to predict the soil-rock interface for metro tunnelling, and the results are validated through geophysical interpretations. The study has shown that BEL can effectively learn the covariance features of the priors. The results underscore the importance of borehole planning in obtaining an optimal reduction in geological uncertainty. Sequential estimation of soil-rock interface can significantly reduce uncertainty in elevations across the site, particularly in areas near the boreholes. To mitigate biases in geophysical interpretation of the soil-rock interface, the utilization of BEL prediction could be beneficial.Ministry of National Development (MND)National Research Foundation (NRF)Submitted/Accepted versionThis work is supported by the National Research Foundation (NRF) of Singapore, under its Virtual Singapore program (Grant No. NRF2019VSG-GMS-001), and by the Singapore Ministry of National Development and the National Research Foundation, Prime Minister’s Office under the Land and Liveability National Innovation Challenge (L2 NIC) Research Program (Grant No. L2NICCFP2-2015-1)

Comparative spatial predictions of the locations of soil-rock interface

The location of soil-rock interfaces (SRIs) may significantly affect the underground construction works, including the design of underground geotechnical structures. The prediction of the location of SRI using limited borehole data is a challenging task. To address this challenge, this paper presents a comparison study of four methods for spatially predicting the SRI elevation, namely the polynomial regression, spline interpolation, one-dimensional spline regression, and a Bayesian-based conditional random field. The consistencies, prediction accuracies, patterns of the predicted curves, and prediction uncertainties for various methods are evaluated. Borehole data from two sites in Singapore are used in the comparative study. The results show that the spline interpolation method produces the least consistent estimation of SRI profiles. The spline interpolation method also has lower prediction accuracies than the other three methods and cannot provide any information regarding the prediction uncertainty. The spatial trend of the geological interface cannot be captured by the polynomial regression method with a relatively high (i.e., 10) order of the polynomial when faults and folds exist. Advantages of the spline regression method over the conditional random field methods include that (i) it provides a clear and explicit spatial trend of the SRI, which well reflects the geological complexity of the sites; (ii) it avoids the cumbersome estimation of random field parametric values, which is a challenging task under the condition of limited data; and (iii) it can differentiate the zones with different prediction accuracies, which cannot be accomplished by the conditional random field method due to limited data. To sum up, the spline regression method produces a simpler and more informative curve of the SRI than the other three methods and thereby is useful as it can guide site investigations to be carried out at geologically uncertain areas to reduce risks, especially for underground construction projectsMinistry of National Development (MND)National Research Foundation (NRF)This research is supported by the Singapore Ministry of National Development and the National Research Foundation, Prime Minister's Office under the Land and Liveability National Innovation Challenge (L2 NIC) Research Programme (Award No. L2NICCFP2-2015-1).

A new Quaternary stratigraphy of the Kallang River Basin, Singapore: implications for urban development and geotechnical engineering in Singapore

The Quaternary stratigraphy of many coastal areas in Southeast Asia is poorly understood. Developing a detailed framework is important as many coastal cities are built on these unconsolidated coastal-marine sediments. This study reviews the current understanding of Quaternary deposits in the Kallang River Basin, Singapore, using 161 boreholes to create 14 cross-sections and a 3D geological model. The dataset is augmented with a ~38.5 m long sediment core obtained from Marina South (1.2726°N, 103.8653°E), and a previous record from Geylang (1.3137°N; 103.8917°E), to provide age constraints and stratigraphic reference. A new Quaternary stratigraphic framework for Singapore is presented here, constrained by new radiocarbon and Optically Stimulated Luminescence (OSL) dating, and identify palaeo-features that record the geomorphic and sedimentary evolution of the basin. Fluvial deposits of Pleistocene age are uncomfortably overlain by littoral/tidal sands and subsequently marine clay possibly during Marine Isotope Stage (MIS) 5e (~125 ka BP). Subsequent subaerial exposure and weathering of the marine clay during the last interglacial produced a ‘stiff clay’ layer. The stiff clay is unconformably overlain by Holocene transgressive sands and peats from ~9.5 ka BP, followed by marine clays with a maximum basal age of ~9.2 ka BP. Regressive sandy/peat units were subsequently deposited beginning ~6 ka BP. Here the new Quaternary stratigraphic framework for Singapore is presented, as well as provide important constraints on the regional sea-level history and geomorphological evolution of Singapore's southern coast from MIS 5e to present. The work also highlights the complexities of geoengineering work in such subsurface terranes

Ductile and brittle deformation in Singapore: a record of Mesozoic orogeny and amalgamation in Sundaland, and of post-orogenic faulting

Singapore bedrock geology is dominated by late Permian to Triassic arc magmatism and a genetically related, essentially Middle to Upper Triassic, marine to fluvial volcano-sedimentary inner forearc succession. These Mesozoic strata are deformed into a pattern of NE-translated ductile–brittle deformation structures during the latest Triassic to earliest Jurassic collision and amalgamation of the Sibumasu continental block with the southern part of the Sukhothai Arc. The subduction-related magmatic complex represented in Singapore by the granitic to gabbroic plutons of the Bukit Timah Centre likely acted as a backstop to thrusting at this time. Collisional tectonics drove progressive shortening and steepened earlier-formed inclined asymmetrical folds, culminating in the regional-scale development of a non-coaxial, NE-vergent and NE-facing, fold and thrust system. In Singapore, the Murai Thrust and Pasir Laba Thrust are identified as major elements of this system; both are associated with SW-dipping thrust-imbricate duplex slices. Two distinct early Cretaceous (Berriasian and Barremian) sedimentary successions overstep these collisional tectonic structures. An array of mostly NE–SW and ENE–WSW trending faults and fractures acts as an important control on bedrock unit distribution across Singapore and are most likely generated by Cenomanian dextral shear stress. That stress locally reactivated faults initiated during orogeny, or even earlier. Knowledge of the geotechnical impact of these structural features is critical to both future development and ongoing management of the subsurface in Singapore

The igneous rocks of Singapore: new insights to Palaeozoic and Mesozoic assembly of the Sukhothai Arc

Six plutons of granitic to gabbroic rocks have been recognised in the poorly exposed ground of north and east Singapore using new data from borehole cores and field observations. Five of the plutons were emplaced sequentially during the period 285–230 Ma, and these have been grouped within a single parent unit (Bukit Timah Centre) in a new lithodemic framework for Singapore. These plutons record the development of early Permian to Triassic arc-related magmatism towards the southern end of the Sukhothai Arc system of Thailand, Peninsular Malaysia and Indonesia. Compositional trends in the plutons record decreasing crustal contribution as the arc matured. The volcanosedimentary succession underlying southwest Singapore records contemporaneous deposition in the forearc basin, and includes pyroclastic units of sufficient thickness and extent to be assigned ‘member’ status in a new lithostratigraphical framework for Singapore. The largest pyroclastic unit, which is >150 m thick, developed as volcanic activity peaked at c. 242 Ma and is correlated with a large (probably caldera-forming) eruption of one of the Permo-Triassic plutons. The composition, typology and age of the Bukit Timah Centre plutons confirm their affinity with Eastern Belt intrusions of the Eastern Province (one of three granitoid provinces in Southeast Asia), and support a widely accepted model that Eastern Province granitoids formed in an Andean-type setting as Palaeo-Tethys crust descended beneath the Indochina–East Malaya block. Widespread development of hydrothermal-tuffisite in the Permo-Triassic intrusions is correlated tentatively with rapid uplift following slab breakoff, when arc activity ceased in the Singapore region in the interval c. 230–205 Ma. A sixth pluton, much younger than (and unrelated to) the Bukit Timah Centre, was emplaced in northeast Singapore in the Upper Cretaceous