We investigated the crustal architecture of the Parnaíba basin of NE Brazil by analysing receiver functions along a c. 600 km long transect crossing the central portion of the basin. The transect consisted of nine broadband stations interspaced at c. 70 km distance recording continuously for a period of 15 months, with the goal of improving our understanding of the origin and evolution of this large cratonic basin. Our results show that crustal thickness varies between 39 and 45 km along the transect, gradually thickening towards the depocentre, and that bulk Vp/Vs ratios vary between 1.70 and 1.78. The crust can be divided into a 2.0–3.5 km thick layer of low-velocity sediments, a 15–20 km thick upper crust (3.5 < Vs < 3.6 km s−1) and a 18–22 km thick lower crust (3.7 < Vs < 3.8 km s−1). Near the depocentre, where the crust is thickest, the bottom 10–12 km of the crust are characterized by fast S-velocities (4.0–4.2 km s−1). Our findings confirm that stretching of the lithosphere is minimal and compatible with flexural subsidence. However, loading from a thick, high-density layer of mafic intrusive rocks pervading the lower crust – as recently proposed for the basin – is found to be inconsistent with our bulk Vp/Vs ratios and lower crustal S-velocities. Flexural bending by a deeper load, perhaps related to deep mantle convection, seems more plausible.
The EGS Collab project, supported by the US Department of Energy, is performing intensively monitored rock stimulation and flow tests at the 10-m scale in an underground research laboratory to address challenges in implementing enhanced geothermal systems (EGS). Data and observations from the field tests are compared to simulations to understand processes and build confidence in numerical modeling of the processes. We have completed Experiment 1 (of 3), which examined hydraulic fracturing in a well-characterized underground fractured phyllite test bed at a depth of approximately 1.5 km at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. Testbed characterization included fracture mapping, borehole acoustic and optical televiewers, full waveform sonic, conductivity, resistivity, temperature, campaign p- and s-wave investigations and electrical resistance tomography. Borehole geophysical techniques including passive seismic, continuous active source seismic monitoring, electrical resistance tomography, fiber-based distributed strain, distributed temperature, and distributed acoustic monitoring, were used to carefully monitor stimulation events and flow tests. More than a dozen stimulations and nearly one year of flow tests were performed. Quality data and detailed observations were collected and analyzed during stimulation and water flow tests using ambient temperature and chilled water. We achieved adaptive control of the tests using real-time monitoring and rapid dissemination of data and near-real-time simulation. More detailed numerical simulation was performed to answer key experimental design questions, forecast fracture propagation trajectories and extents, and analyze and evaluate results. Data are freely available from the Geothermal Data Repository. Experiment 2 examines the potential for hydraulic shearing in amphibolite at a depth of about 1.25 km at SURF. This site has a different set of stress and fracture conditions than Experiment 1. The Experiment 2 testbed consists of nine subhorizontal boreholes configured in two fans of two boreholes which surround the testbed and contain grouted-in electrical resistance tomography, seismic sensors, active seismic sources and distributed fiber sensors. A "five-spot" set of test wells that extends from a custom mined alcove includes an injection well and four production/monitoring wells. The testbed was characterized geophysically and hydrologically, and three stimulations have been performed using the Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP) tool to measure strains, and a new strain quantifying tool (downhole robotic strain analysis tool -DORSA) was deployed in a monitoring hole during stimulation. Real-time data were broadcast during stimulations to allow real-time response to arising issues.
We assess the performance of the joint inversion of receiver functions (RF) and surface-wave dispersion in the characterization of the sedimentary package comprising the Parnaíba basin. This procedure is routinely utilized in passive-source crustal studies to retrieve S-wave velocity variations with depth, and has seldom been used with higher-frequency datasets to investigate fine sedimentary structure. The Parnaíba basin is a Paleozoic cratonic basin composed of five supersequences, accumulating ∼3.5 km of sedimentary rocks interbedded by Late Cretaceous diabase sills. The dataset used for this research was acquired between 2015 and 2017 through deployment of 10 short-period and one broadband seismic stations distributed along an approximately 100-kilometer-long linear array in the center of the basin. The deployment was carried out under the Parnaíba Basin Analysis Project, a multi-institutional and multidisciplinary effort funded by BP Energy do Brasil. High-frequency RFs (f<4.8 Hz) were calculated from deconvolution of teleseismic P waveforms (30°<Δ<90°) after rotation into the great-circle path, whereas high-frequency dispersion curves (0.25–2 Hz) were obtained through multiple filter analysis of empirical Green’s functions developed from cross-correlation (ZZ component) and stacking (six months) of time–frequency-normalized ambient seismic noise recordings. S-wave velocity–depth profiles down to ∼5 km depth were developed through an iterative, linearized joint inversion approach. Comparison to independent active-source seismic profiles overlapping with our passive-source seismic line reveals the inverted velocity models successfully retrieve sedimentary thickness (top of the Cambrian), sedimentary velocity structure, and depth to the Cenozoic sedimentary sequence. In addition, high-velocity zones at depths ranging from 1.5 to 2.5 km are observed in the inverted velocity–depth profiles, which are interpreted as due to the Late Cretaceous sills interbedding the basin’s sedimentary rocks. The relative low cost of our approach makes it ideal for basic characterization of relatively unknown sedimentary basins.
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