Fault delineation using earthquake energy as a source for pre-stack depth imaging

[1] The San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, central California, has been drilled through a fault segment that is actively deforming through creep and microearthquakes. Creeping is accommodated in two fault strands, the Southwest and Central Deforming Zones, embedded within a damaged zone of deformed shale and siltstone. During drilling, no pressurized fluids have been encountered, even though the fault zone acts as a permeability barrier to fluid circulation between the North American and Pacific plates. Microstructural analysis of sheared shales associated with calcite and anhydrite-bearing veins found in SAFOD cores collected at 1.5m from the Southwest Deforming Zone, suggests that transient increases of pore fluid pressure have occurred during the fault activity, causing mode I fracturing of the rocks. Such build-ups in fluid pressure may be related to permeability reduction during fault creep and pressure-solution processes, resulting in localized failure of small fault zone patches and providing a potential mechanism for the initiation of some of the microearthquakes registered in the SAFOD site.

Low-temperature deformation in calcite veins of SAFOD core samples (San Andreas Fault) — Microstructural analysis and implications for fault rheology

The San Andreas fault accommodates 28-34 mm yr(-1) of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The combination of these measurements of fault core strength with borehole observations yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

In July 2002 we installed a vertical array of seismometers in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH). The bottom of this 32 level, 1240 m long array of 3‐ components is located at a depth of ∼2100 m below ground. Surface‐explosion and microearthquake seismograms recorded by the array give valuable insights into the structure of the SAFOD site. The ratios of P‐ and S‐wave velocities (Vp/Vs) along the array suggest the presence of two faults intersecting the PH. The Vp/Vs ratios also depend on source location, with high values to the NW, and lower ones to the SE, correlating with high and low creep rates along the SAF, respectively. Since higher ratios can be produced by increasing fluid saturation, we suggest that this effect might account for both our observations and their correlation with the creep distribution.

The Silurian LongMaXi gas shale in the Sichuan basin is an emerging play in China. It experienced multiple major tectonic evolutions with uplifting and burying sequences in its geological time. As expected, this shale could develop tremendous small-scale discrete natural fractures (DNF) like many other notable North American gas shale plays. But only sparse DNFs were observed from several borehole images in a shale gas field. This paper presents a case study for characterizing complex multiscale natural fracture systems of the gas shale. In the study, natural fracture systems were classified as four types: large-scale faults, medium-scale fracture systems (including microfaults and crushed zones or fracture corridors), small-scale DNFs, and micro-scale fissures per their correspondingly scale-associated geological, geophysical, and petrophyscial features and responses. An optimized ant-tracking approach was implemented to track seismic abnormities and discontinuities as indicators of subseismic medium-scale fractures. Key components of this approach include optimized seismic gathers' angle-range, optimal scanning directions and angles, parameter adjustment criteria, and a comprehensive QC process. The QC process, which considers regional geological and tectonic evolution, fracture mechanical mechanism under strike-slip stress regime, and observations from all available resources, was implemented on each identified medium-scale fracture zone to ensure its reliability. This paper has more focuses on characterizing the medium-scale fracture system because of which can be a unique feature of LongMaXi gas shale compared to its North American counterparts. Characterizing of this medium-scale fracture system considers factors including fracture development mechanism and fracture pattern under strike-slip condition, structure control and curvature tendency, and intensity and amplitude of seismic discontinuity and abnormities. The study shows such medium-scale fractures are widely distributed across the study field. Complexities encountered while drilling and stimulation such as heavy mud loss or screen-out due to unexpected high leak-off rate had strong correlation with them. Two principle orientations of the medium-scale fractures were identified and further validated by outcrop observations and microseismic events observed from hydraulic fracturing monitoring. The study suggests that such medium-scale fractures may be mainly controlled by two major tectonic events during Himalayan movement because their principle orientations are almost exactly following the directions of two major tectonic movements. In combination with all types of natural fractures as defined, 3D multiscale natural fracture model and Geomechanical Earth model were established, which are two keystones for drilling quality and completion quality for further applications. These medium-scale fractured or crushed zones along with large-scale faults and small-scale DNFs across this shale gas field could significantly impact on efficiency and effectiveness of development. In particular, to understand how hydraulic fractures interact with such medium-scale fractures will be essential.

Development of tight oil reservoirs mainly relies on massive hydraulic fracturing, which can generate complex fracture networks in the reservoirs. These highly discrete fracture networks bring great challenges for reservoir simulation. An efficient model applying an unstructured discretization method and automatic differentiation is proposed. The flexibility in unstructured control volume shapes enables the gridding of complex fracture systems. By introducing the concept of half-transmissibility for each grid, transmissibility list including connections of matrix-matrix, matrix-fracture, fracture-fracture is established. Nonlinear flow and transport equation system is solved by a modified Newton’s method, in which the Jacobian matrix is computed by automatic differentiation (AD). Accuracy of the model was validated by performing simulations using a commercial simulator. We implemented our model in several cases with a uniform physical domain (radial model with diameter=1 km) but different fracture properties. Results show that fracture configuration and property have significant impacts on the production. Moreover, our model shows a high efficiency in the densely fractured system with fracture density up to 1018/km2. The novelty of the model is in the ability to represent complex fracture systems individually and explicitly, and in the application of automatic differentiation, which greatly facilitates the model establishment and improves computational efficiency.

Fluid-filled fractures involving kinks and branches result in complex interactions between Krauklis waves-highly dispersive and attenuating pressure waves within the fracture-and the body waves in the surrounding medium. For studying these interactions, we introduce an efficient 2D time-harmonic elastodynamic boundary element method. Instead of modeling the domain within a fracture as a finite-thickness fluid layer, this method employs zero-thickness, poroelastic Linear-Slip Interfaces to model the low-frequency, local fluid-solid interaction. Using this method, the scattering of Krauklis waves by a single kink along a straight fracture and the radiation of body waves generated by Krauklis waves within complex fracture systems are examined.

This paper describes the analyses of the single-station ambient-vibration measurements performed on the Italian accelerometric network to detect site resonance phenomena potentially affecting earthquake recordings. The use of low cost, high quality microtremor measurement can be helpful to discriminate among soil classes, since several classification schemes based on resonance frequencies were proposed in the last decades. Operatively, in the framework of the Italian Strong Motion Database project (DPC-INGV 2007–2009 S4; http://esse4.mi.ingv.it), soil resonance frequencies have been evaluated from more than 200 ambient vibration measurements in correspondence of accelerometric stations included in ITACA (http://itaca.mi.ingv.it/ItacaNet/). The noise recordings have been analyzed using the same numerical protocol in order to standardize the results. Particular attention has been paid to evaluate the quality of measurements and to develop an on-purpose mathematical tool to automatically estimate the peaks in the horizontal-to-vertical spectral ratio (HVSR) curve. The reliability of the resonance frequencies from HVSR has been tested by comparing estimates provided by independent methods (modeling or earthquake recordings). The test confirmed the reliability of the microtremor HVSR for assessing the resonance frequencies of the examined sites.

<p>In seismology, new sensing technologies are currently emerging that can measure ground motion beyond the conventional seismic translation measurements. In particular, rotational motion sensors record an additional 3 components of ground motion and thus provide access to additional information about the seismic wavefield. </p><p>So far, most studies of rotational ground motion are mainly based on recordings of earthquakes or active sources. In this study, we push the limit towards the very weak motions associated with ocean-generated ambient seismic noise. Our aim is to show the potential of using these measurements in the context of ambient noise interferometry. </p><p>We use recordings from two ring lasers in Germany: the `G-Ring' at the Wettzell Geodetic Observatory, and `ROMY' at the Fürstenfeldbruck Observatory near Munich, at a distance of approximately 160 km. These are the most sensitive instruments to date which offer a local, direct measurement of rotational ground motion. </p><p>We demonstrate that the sensitivity of the Wettzell instrument has been sufficiently improved to detect Love waves in the primary microseismic frequency band. Both the G-Ring and ROMY ring lasers are also capable of detecting Love waves in the stronger secondary microseismic band. This latter frequency range is used to test the possibility of performing noise interferometry with rotational records. </p><p>The first results of rotational noise interferometry between the two ring lasers are promising. The correlation waveform is verified by comparison with interferometry carried out with co-located seismometer data at both locations, as well as with numerical simulations. </p><p>In conclusion, we show that ambient noise interferometry is in principle feasible using real rotational recordings of ocean-generated noise. This proof of concept study forms a first step towards noise interferometery of 6-component displacement data. </p>

The solar radiation or insolation at a specific locality on the Earth's surface is determined by the distance of this point from the sun and by the angle with which the radiation hits the earth's surface. The theory that variations in the earth's orbit determine the amount of radiation and therefore the climate which is the Milankovitch Cycle. In non‐marine lacustrine rift basin, the changes of climate can be recorded by the palaeontology, and it also plays an important role in controlling lacustrine fluctuations. The lacustrine fluctuations are indicated by coastal onlap, which is the landward or lake‐ward onlap of non‐marine coastal deposits. In this paper, based on borehole data, wireline logs, seismic datasets, biostratigraphic data and palaeontology, the lacustrine fluctuation curve can be reconstructed from the onlap of lacustrine deposits in non‐marine sequences. The durations and magnitudes of these changes can be used to construct charts showing cycles of the relative rising and falling of lake level. In addition, the changes of lake level can also be reflected by the palaeontology such as pollen. The living conditions and distributions of pollen can be a good indicator for climate changes and lacustrine fluctuations in an a non‐marine setting. The calculation of the Milankovitch Cycle can be determined by the GR log in the study area which keeps a record of cycle changes. All climatic changes and lacustrine fluctuations are mainly resulted from the Milankovitch Cycles, and therefore, through the reconstruction of lacustrine fluctuations, climate changes and the Milankovitch Cycle, the correspondences among them can be clearly addressed. All the results can contribute a lot in theory research in the study area and lead to a new focus in practical hydrocarbon exploration and exploitation.

In this study, we present an integrated approach to develop a two-scale fracture model for the Ordovician carbonate fractured reservoir in the Yubei area. The objective of this study is to provide new insights into the impact of fractures on oil production and to recommend drilling strategy. Fractures are characterized by using log data, 3D seismic attribute, borehole image logs and reservoir production data. Image logs illustrate that the dominant reservoir fractures trending in the NE direction parallel to the present-day maximum horizontal compressive stress and are consistent with the major fault system. The electrofacies generated with conventional logs are calibrated into different fracture units by using borehole image logs and reservoir dynamic data. The dynamic behavior of the complex matrix and fracture system is characterized for fracture units at wells. For layer bound diffuse fractures, the fracture properties are modeled statistically set by set with borehole image logs, fracture seismic facies volume representing degree of the fracturation and seismic geometric attribute. Fracture lineaments at the corridor scale are extracted directly from edge-enhanced seismic attribute. Fracture lineaments are cleaned and meshed to define their topology relationships to construct a comprehensive DFN (Discrete Fracture Network) model. Fracture networks are characterized deterministically, which constitute a unique fracture model on the reservoir scale. DFN model provides a geological representation how these fractures are spatially organized in the subsurface and how they may enhance or inhibit fluid flow. The final fracture model honored the two scale fracture models. Grid-cell permeability properties are derived from the two-scale fracture model. Based on the dynamic behavior of fracture units, fracture properties are calibrated against the dynamic data. Our results show that NE orientated fractures are open and conductive. The high productivity zones are characterized by high permeability located in the vicinity of faults. Enhanced oil recovery operations could be optimized using production wells oriented at high angles to NE orientated fractures.

The modern idea of the geological structure of oil deposits of the Nizhnechutinskoye field is presented. The uniqueness of the field lies in the shallow depths of productive deposits. The oil-saturated layer is 100 m. The shallow depths of productive formations led to low formation temperature and pressure. Under such conditions, oil is inactive, characterized by high viscosity, and dissolved gas is practically absent. The features of the formation of the sedimentary cover in the area of the field predetermined the block structure, while the rocks of the productive horizon are characterized by a complex system of vertical fractures, which greatly complicates the process of oil recovery. A distinctive feature of the studied section of the field is the presence of effusive rocks underlying the main productive stratum and overlapping the lower productive layer. The article systematizes the results of field trial operation, which revealed a number of complicating factors, which is associated primarily with the uniqueness of the geological structure of the studied deposits. Despite the solid history of studying the Nizhnechutinskoye field, which has almost 300 years, given the negligible depth of productive deposits, oil production at this field still remains a difficult task.

‘The making of magma’ discusses the formation of magma. Volcanism is driven by prodigious amounts of heat from deep inside the Earth that partly melts rock to form magma, which accumulates and evolves deep underground, and may drive powerful eruptions that expel ash and lava on to the surface, creating a volcano. This process can be studied by inferences based upon volcano behaviour, seismic information, chemical tracers in volcanic materials, experimentation, and other clues. From enormous, diffuse networks deep in the Earth, the magma rises and gathers together in the crust as a complex system of interconnected fractures, reservoirs, and conduits. These magma reservoirs can be detected by seismic imaging.

The geophysical working group of the University of Genoa conducted a field experiment to analyze site amplification effects in Ripabottoni, a village in the Molise region of Italy. We used both noise and earthquake recordings, combined with detailed geologic and geotechnical surveys, to define site amplification phenomena. The site effects determination was obtained using the Nakamura technique and the H/V spectral analysis applied to earthquake recordings. The results were validated by applying a one-dimensional simulation code. The computed spectral ratios point out three different typologies of site effects: the southern sector of Ripabottoni is characterized by an absence of local amplification phenomena; the central sector of the village shows a local amplification phenomena with a fundamental frequency of 4–6 Hz; and the northeastern side of the village shows a site response at a fundamental frequency between 2–3 Hz.

Seismic data denoising is a critical component of seismic data processing, yet effectively removing erratic noise, characterized by its non-Gaussian distribution and high amplitude, remains a substantial challenge for conventional methods and deep-learning (DL) algorithms. Supervised learning frameworks typically outperform others, but they require pairs of noisy data sets alongside corresponding clean ground truth, which is impractical for real-world seismic data sets. In contrast, unsupervised learning (UL) methods, which do not rely on ground truth during training, often fall short in performance when compared with their supervised or traditional denoising counterparts. Moreover, current unsupervised DL methods fail to address the specific challenges posed by erratic seismic noise adequately. This paper introduces a novel zero-shot unsupervised DL framework designed specifically to mitigate random and erratic noise, with a particular emphasis on blended noise. Drawing inspiration from Noise2Noise (N2N) and data augmentation principles, we develop a robust self-supervised denoising network called robust Noiser2Noiser. Our approach eliminates the need for paired noisy and clean data sets as required by supervised methods or paired noisy data sets as in N2N. Instead, our framework relies solely on the original noisy seismic data set. Our methodology generates two independent recorrupted data sets from the original noisy data set, using one as the input and the other as the training target. Subsequently, we use a DL-based denoiser, denoising convolutional neural network, for training purposes. To address various types of random and erratic noise, the original noisy data set is recorrupted with the same noise type. Detailed explanations for generating training input and target data for blended data are provided. We apply our network to synthetic and real marine data examples, demonstrating significantly improved noise attenuation performance compared with traditional denoising methods and state-of-the-art UL Codes are available on https://github.com/Ji-seismic/N2N_deblending.