Subsurface Stations Research Articles

Real-Time Offshore Coastal Acoustic Tomography Enabled With Mirror-Transpond Functionality

Real-time offshore coastal acoustic tomography (CAT) with mirror-transpond functionality is the most recent feature added to CAT to transfer travel-time data obtained in an offshore subsurface station to a shore station in real time, forming a round-trip travel time by original transmission and subsequent mirror transmission. The minimum unit, composed of one land and one offshore system, was operated to validate the performance of CAT with mirror-transpond functionality (MCAT) in the feasibility experiment, which was carried out in April 2017 in the Nekoseto Strait of the Seto Inland Sea, Japan. The root-mean-square error (RMSE) of the hourly mean range-average currents calculated from the original–original and original–mirror reciprocal travel-time pairs was 0.05 ms–1, significantly smaller than the variation range of the semidiurnal tidal current (0.2–0.3 ms–1) dominant at the experimental site. The RMSE for the residual current was also significantly smaller than its variation range. This study suggests the feasibility of using a compact MCAT array composed of one land and three offshore stations.

Tackling mass redistribution phenomena by time-dependent GRACE- and terrestrial gravity observations

Time variable gravity field models derived from the satellite mission GRACE have been demonstrated to be consistent with water mass variations in the global hydrological cycle. Independent observations are provided by terrestrial measurements. In order to achieve a maximum of reliability and information gain, ground-based gravity observations may be deployed for comparison with the gravity field variations derived from the GRACE satellite mission. In this context, the data of the network of superconducting gravimeters (SG) of the ‘Global Geodynamics Project’ (GGP) are of particular interest. This study is focused on the dense SG network in Central Europe with its long-term gravity observations. It is shown that after the separation and reduction of local hydrological effects in the SG observations especially for subsurface stations, the time-variable gravity signals from GRACE agree well with the terrestrial observations from the SG station cluster.Station stability of the SG sites with respect to vertical deformations was checked by GNSS based observations. Most of the variability can be explained by loading effects due to changes in continental water storage, and, in general, the stability of all stations has been confirmed.From comparisons based on correlation and coherence analyses in combination with the root mean square (RMS) variability of the time series emerges, that the maximum correspondence between the SG and GRACE time series is achieved when filtering the GRACE data with Gaussian filters of about 1000km filter length, which is in accordance with previous publications.Empirical Orthogonal Functions (EOF) analysis was applied to the gravity time series in order to identify common characteristic spatial and temporal patterns. The high correspondence of the first modes for GRACE and SG data implies that the first EOF mode represents a large-scale (Central European) time-variable gravity signal seen by both the GRACE satellites and the SG cluster.

Rock mechanical investigations of seismic events in a deep longwall coal mine

Seismic events, i.e., fractures in rock that radiate detectable seismic waves, are a worldwide phenomena associated with mining and reservoir exploitation. Longwall mining in German coal mines often causes seismic events, which can be felt at surface where they can lead to a considerable annoyance of residents and sometimes even lead to minor damages of buildings and infrastructure. Strong seismic events are in a large part responsible for a more and more decreasing community acceptance of mining. The mines therefore have to investigate the local conditions, which cause the generation of seismic events, to find ways of avoiding strong mining induced seismic events. This paper describes the research on mining-induced seismicity around a specific active longwall at 1100 m depth under several previously mined longwall panels. Seismic events are recorded by a local network featuring 15 surface and six subsurface stations. Location and fault plane solutions show the occurrence of at least three different event types, which are characterized by distinct modes of failure. Underground core drilling and an extensive rock testing program are used to build up a rock mechanical model featuring a multi-layer strata for use with numerical analyses. The numerical model is able to explain the different failure mechanisms as fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses facilitates the evaluation of the results from seismology approaches, and makes future analysis of longwall-related seismic events possible.

A Very Close Look at a Moderate Earthquake near Sudbury, Ontario

A shallow moderate earthquake of MN (Nuttli magnitude) 4.1 occurred near Sudbury, Ontario, at 07:22:55 UTC (02:22:55 local time) on 29 November 2006. The event was located 6 km from Lively, Ontario, and was induced by mining activity at Inco's Creighton mine. Inco's in-mine monitoring system, and its subsequent visual inspection, places the event at 46.478 N 81.203 W, at a depth of 2.35 km. This event offers us a rare close-up view of a moderate earthquake, because it was recorded at close distances by five three-component broad-band POLARIS (Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity; http://www.polarisnet.ca) seismographic stations that are installed above and within the Creighton mine. Two stations are located on the surface, near the epicenter, while three subsurface stations are at depths of 1 to 2 km. The closest station to the event is within the Sudbury Neutrino Observatory (SNO) lab, which is 2 km beneath the surface and about 300 m above the event. The event caused no damage to the SNO lab and only minor damage to the mine workings; there were no injuries. The event was widely felt in the Sudbury, Ontario, region, as expected based on instrumental ShakeMaps (http://www.shakemap.carleton.ca) and confirmed by Internet intensity reports (http://www.earthquakescanada.ca). The mainshock was followed by two significant aftershocks at 2:36 (MN 2.0) and 2:38 (MN 3.1) local time and by many smaller aftershocks. In addition to the near-source records, this event was well-recorded regionally at distances from 20 km to hundreds of kilometers by the seismographic stations of the Canadian National Seismographic Network (CNSN) and POLARIS. Figure 1 shows the location of the Lively event in relation to regional seismographic stations; the nearest regional station is SUNO, 22 km away. In this paper we provide a preliminary examination of the recorded ground motions from this …

CrossRail: Scope, background and feasibility

Proposals for an east–west railway linking existing railways across central London have been examined on many occasions over the last 100 years, culminating in the present CrossRail scheme, recommended by the Central London Rail Study in 1989.1 CrossRail, linking counties east and west of London with the City and the West End, will be operated by suburban trains up to twelve cars in length carrying a quarter of a million passengers a day. Twin tunnels will be built beneath central London from west of Paddington to Bethnal Green with five new subsurface stations at key locations. Outside of London, trains will run mainly on existing lines to Reading, Aylesbury and Shenfield with new and extended platforms where required. This first paper outlines the scope of the project, the historical background, initial engineering feasibility work and preparation for the Parliamentary Private Bill.

CrossRail: Scope, background and feasibility

Proposals for an east–west railway linking existing railways across central London have been examined on many occasions over the last 100 years, culminating in the present CrossRail scheme, recommended by the Central London Rail Study in 1989. 1 CrossRail, linking counties east and west of London with the City and the West End, will be operated by suburban trains up to twelve cars in length carrying a quarter of a million passengers a day. Twin tunnels will be built beneath central London from west of Paddington to Bethnal Green with five new subsurface stations at key locations. Outside of London, trains will run mainly on existing lines to Reading, Aylesbury and Shenfield with new and extended platforms where required. This first paper outlines the scope of the project, the historical background, initial engineering feasibility work and preparation for the Parliamentary Private Bill.

CrossRail: Ventilation of the central London tunnels

The ventilation of the two 9 km long central London CrossRail tunnels and the five new subsurface stations required an extensive and intense period of design development and research. The high powered trains, with up to 12 cars and an ultimate service of 32 trains per hour will be significant generators of heat. The large number of people to be handled will require major investment to ensure their comfort and safety in both normal and emergency situations. This paper reviews the development of the design of the ventilation system, including the thermal and aerodynamic effects arising from train operations and the means of ensuring the safety of passengers, staff and emergency service personnel in the unlikely event of a fire.

CrossRail: Ventilation of the central London tunnels

The ventilation of the two 9 km long central London CrossRail tunnels and the five new subsurface stations required an extensive and intense period of design development and research. The high powered trains, with up to 12 cars and an ultimate service of 32 trains per hour will be significant generators of heat. The large number of people to be handled will require major investment to ensure their comfort and safety in both normal and emergency situations. This paper reviews the development of the design of the ventilation system, including the thermal and aerodynamic effects arising from train operations and the means of ensuring the safety of passengers, staff and emergency service personnel in the unlikely event of a fire.

High frequency (1–100 HZ) noise and signal recorded at different depths in a mine, northwest Adirondacks, NY

Seismograms of noise recorded simultaneously at the surface and at two subsurface stations (335m and 945m below surface level) show that high frequency (1–100 Hz) background noise is reduced at the subsurface stations. Seismometers at all three stations are well coupled to hard crystalline bedrock. Most of the noise reduction occurs between the surface and 335m depth. Between 1 and 3 Hz, seismic noise correlates well with wind speed at all levels. Above 3 Hz, wind speed up to 8 m/sec does not influence high frequency noise levels at 335m or 945m depths. At the surface, however, high frequency noise is associated with wind speeds in excess of 2 m/s. A site resonance is characteristic of the surface spectra, but is not characteristic of the subsurface sites. Spectral signal‐to‐noise (S/N) ratios, measured for S‐waves from a regional earthquake (Δ = 530 km, mb = 4.1), are roughly equal from 1–10 Hz at all depths. Above 10 Hz, however, S/N is greater at the subsurface stations with S/N greater than 1 up to 30 Hz for the surface station, ≈ 50 Hz for the 335m station, and ≈ 70 Hz for the 945m station. Results indicate the advantage of employing high frequency sensors sufficiently below the surface to reduce the level of noise and to improve the S/N ratio in the high frequency range. Such an advantage could be critically important for nuclear test detection and discrimination.

Close-in ground motion from the MILROW and CANNIKIN events

abstractMotion of the ground induced by the CANNIKIN and MILROW nuclear explosions beneath Amchitka Island was observed at vertical arrays of subsurface stations in borings offset 90 m from the shot holes and at surface stations at distances ranging from surface zero to about 10 km. Acceleration, particle velocity, and displacement time histories were obtained: vertical components only in the vertical arrays and three orthogonal components at surface stations.MILROW results indicate Rayleigh waves in records from a horizontal range of one-half shot depth and beyond. Analysis showed the initial spall depth near 110 m, with later spallation below 150 m and between 30 and 90 m. Surface data indicate spallation extended laterally to four times shot depth. CANNIKIN data were trucated at about 1.8 sec by power circuit malfunction. Peak motion was recorded at all but the most remote CANNIKIN station before loss of power. Comparison of CANNIKIN data with those from MILROW indicate greater peak motions from the more energetic CANNIKIN explosion, but scaled peak motions from these events are similar.