Gravity Variations Research Articles

Isostatic Residual Gravity Anomaly Grid of Onshore Australia

SUMMARY Gravity anomalies from geological features in the upper crust are masked by large amplitude long wavelength gravity variations from isostatic roots. An isostatic residual (IR) gravity anomaly grid of onshore Australia has been produced by Geoscience Australia which has these long wavelength features removed. This gravity map reveals more clearly the density distributions of geological interest within the upper crust. The depth to mantle model and subsequent isostatic corrections were produced using a modified version of the USGS program AIRYROOT provided by Intrepid Geophysics. Geoscience Australia’s 2009 Bathymetry and Topography Grid was used to calculate the depth to crustal bottom following the Airy-Heiskanen crustal-root model. The isostatic corrections were then applied to the complete Bouguer anomalies to produce the Isostatic Residual Gravity Anomaly Grid of Australia.

Application of Gravity and Radon Studies to Delineate the Concealed Section of the Khisor Thrust

Radon concentration, a geochemical parameter of naturally occurring noble gas and potential relative gravity field variation, has been studied in the current investigations. These investigations were carried out along and in proximity to the Khisor Thrust in the Trans Indus ranges marking the southern boundary of Bannu basin in Pakistan. The studies were aimed to demark the concealed section of the thrust fault beyond termination of its surface signature. The consistent increasing pattern of radon variation while approaching the approximate location of fault and decrease while moving farther from it suggests the existence of a fault to the subsurface, which should be verified through gravity studies. Variation of radon concentration between 2.5 and 4.1 kBq/m3 was observed on or near the fault zone in exposed and concealed segments, whereas the lower bound variation of 1.1–2 kBq/m3 was encountered on points moving away from fault zone. This variation of radon concentration promisingly supported the idea of the existence of a fault zone beyond the cessation of surface signature. The argument was further validated through the relative gravity variation studies following the radon concentration. The Bouguer anomaly was calculated along all 16 profiles cross-cutting the fault zone. The regional gravity mapped along 231 points of investigation showed a variation between − 90 and 8 mGal. The residual gravity highlighting the effects of upper crust disturbance because of under-thrusting has shown anomalous values between − 11 and 18 mGal. The results when contoured have shown a good congruence with those received from radon concentration contrasts. Conclusively, it can be perceived that these two nondestructive and less laborious techniques work in composite usefully to delineate the unseen earthquake potential source.

A preliminary geophysical analysis of gravity time-variation in Daejeon

A gravity time series slightly longer than a month acquired by using a Burris gravimeter at Daejeon, Korea has been analyzed. After removal of linear drift of the gravimeter, the observed data was found to closely match with the body tide prediction except; (i) small phase differences of each main tidal constituents and (ii) existence of long period component in the observed data. The r.m.s. observed gravity tide variation was 62.4 μGal. The overall g-factor was found as 1.165. The values of g-factor for the four main tidal constituents; M{nt2}, S2, O1, K1, were found as 1.166, 1.172, 1.168, and 1.142 respectively. The r.m.s. value of ocean tidal loading effect on gravity at Daejeon was 0.95 μGal during the time span. Both the rainfall and the atmospheric pressure change have induced gravity variations of amplitude of about 10 to 1 μGal. The long period component, which is clearly found in the observed spectrum, while not existent in the prediction, is possibly caused by the local ground water variation. It is desirable to have a few number of permanent gravity stations operating in Korean peninsula.

Structural interpretation of gravity, topography and seismicity

Abstract Global geophysical observations constrain all theories of terrestrial dynamics. We jointly interpret EGM2008 gravity, RET2014 topography and the Global Centroid Moment Tensor database from a structural point of view. We hypothesize that lateral variations of gravity and topography reflect the scale-dependent competence of rocks. We compare the spectral and spatial characteristics of the observed fields with structural predictions from the mechanics of differential grade-2 (DG-2) materials. The results indicate that these viscoelastic materials are a powerful tool for exploring dynamic processes in the Earth. We demonstrate that the known spectral range of Earth's gravity and topography can be explained by the folding, shear banding, faulting and differentiation of the crust, lithosphere and mantle. We show that the low-amplitude long-wavelength bias apparent in the disturbance field can be explained by perturbations to Earth's overall ellipsoidal shape, induced by internal slab loading of the mantle. We find by examining the directional isotropy of the data that the zonal energy in Earth's gravity disturbance is maximized about an axis coincident with the shape-perturbation minimum. The symmetry of tectonic features about this axis, extending from eastern Borneo to Brazil, and its coincidence with the equator suggest the coupling of current plate motions to true polar wander.

Effect of Gravity Variation on the Growth of Wheat and Guinea Corn Seedlings

Effect of Gravity Variation on the Growth of Wheat and Guinea Corn Seedlings

Postglacial Uplift: Record in the Gravity Field and in Neogene–Quaternary Structures

Abstract The history of Quaternary glaciation and postglacial uplift in Fennoscandia is considered in relation to the surface topography, gravity, and number of glacial deposits and is compared to the respective processes in North America. The surface topography and the gravity field are correlated over the Fennoscandian region as a whole and for two reference areas of South Norway and the Kola Peninsula. The gravity field is composed as free-air and Bouguer gravity anomalies using modern global models based on satellite data. The impact of glaciation may be responsible for zoned patterns of both topography and gravity. The glaciation centers of Norway are marked by uplifts reaching 2470 and 1500 m and by circular Bouguer gravity lows of ≤ –200 mGal, which correspond to residual crust thickening. The gravity patterns of the Kola Peninsula and Norway consist of circular and polygonal anomalies caused by both postglacial rebound and rock density variations. The general uplift and related extension of the crust led to the formation of fractures of different sizes, from 100–200 km long and 1–2 km deep fjords in Norway to 1–2 km long and 30–20 m deep local fractures on the Kola Peninsula. The gravity field of central North America is characterized by a generalized map of free-air anomalies within the limits of glaciation and a map of free-air anomalies compiled from the global database. Glaciation and its records in the surface topography and gravity patterns are described in more detail for Wisconsin State (USA), where extension fractures are similar to those on the Kola Peninsula. The models explaining the correlation between postglacial uplift and gravity variations can be further updated using seismic and tectonic data, as it was shown previously for the case of Finland.

Effect of processing methods and storage time on physical characteristics of palm oil

The effect of processing methods and the storage time on the physical characteristics of palm oil was determined. Samples of palm oil were produced using four processing methods (Hot Extraction; Cold Extraction; Hot Extraction with Red Sand and Mechanized Extraction), packaged in plastic containers and stored at ambient temperature for 0 to 3 month(s). There was significant increase in the following physical characteristics of the palm oil samples due to the storage time: cloud point (21.13 to 24.13oC), melting point (39.54 to 41.88oC) and moisture content (0.068 to 0.390%). However, significant decrease occurred in the following characteristics: smoke point (195.30 to 186.3oC), flash point (252.37 to 244.62oC), fire point (314.87 to 306.37oC) and refractive index (1.45 to 1.37). The effects of the four processing methods on the physical characteristics of palm oil were determined. Processing methods had no significant variations in specific gravity (0.902 to 0.905) and fire point (309.62 to 313.25oC) while significant variations occurred in cloud point (22.38 to 23.56oC), smoke point (188.38 to 195.50oC), melting point (40.16 to 41.63oC), flash point (240.75 to 268.50oC), moisture content (0.08 to 0.37%) and refractive index (1.39 to 1.44). Mechanized extraction method of palm oil was observed as the best method because its palm oil had the least specific gravity (0.902±0.006), least cloud point (22.38 ±1.75oC), highest smoke point (195.50±1.41oC), least melting point (40.16±1.03oC), highest flash point (218.50±3.67oC), highest fire point (313.25±2.96oC), least moisture content (0.08±0.02%) and highest refractive index (1.44±0.01). Similar study on the effect of processing methods and storage time on chemical properties of palm oil is recommendable.

Satellite-Derived Regional Apparent Thermal Inertia and Gravity for Mapping Different Rock Types in Parts of Banswara, Rajasthan

In this study, utility of satellite derived Apparent Thermal Inertia (ATI) and residual gravity data have been analyzed for delineating the major rock types of Aravalli Supergroup. Density is a common contributing factor responsible for ATI and gravity variations of rock types. In addition to density, thermal inertia is also controlled by specific heat and thermal conductivity. In the present study, MODIS (Moderate resolution imaging spectrometer) day and consecutive night time land surface temperature (LST), albedo data have been processed to derive ATI image for the study area. On the other hand, Bouguer gravity generated from EIGEN6C4 gravity model data is processed to derive the residual gravity over the study area. It has been observed that ATI values of different rock types are influenced by the variations in weathering pattern and forest cover developed over the rock in addition to the thermophysical parameters of rocks. Forest cover and agricultural activity are responsible for shifting the ATI values to higher side for any rocks due to evapotranspiration; which is responsible for reducing day and night land surface temperature difference. ATI pixels( collected from large rock pavements) of different rocks can however, be used to discriminate them from each other. Conjugate analysis of residual gravity and ATI has been proved useful to discriminate different rock types of spatially well distributed rock exposures. It is found that ATI and gravity values are well correlated for the spatially large surface exposures of rocks. In regional scale, low frequency changes of ATI broadly matches with the changes observed on the corresponding spatial profiles of residual gravity with some localized variations. This is due to the fact that shallow surface gravity anomaly is cumulative effect of vertical and horizontal variations of density in contrast to ATI which is more sensitive to near surface density, conductivity and specific heat variations of rocks. Moreover, high frequency variations imprinted on regional ATI spatial profiles are due to the cumulative effect of complex thermophysical properties of surface elements (weathering residuum, surface moisture variations) density variation and intrinsic thermophysical properties of rocks measured at regional scale. High frequency changes of ATI do not correspond well with residual gravity variation.

An Analysis of Gravitational Gradients in Rotated Frames and Their Relation to Oriented Mass Sources

AbstractMany mass sources within the Earth and its fluid envelopes show elongated geometries, aligning with the orientations of plate boundaries and plate motions, coastlines, rivers, and drainage basins for instance. To enhance their identification and separation in global or regional gravity observations and models, a dedicated method based on gravitational gradients analysis is presented here. This approach provides a detailed description of the geographic pattern of the gravity variations, which are accurately mapped thanks to the regular spatial coverage of high‐accuracy satellite data and arise from lateral density changes within the planet. First, gravity gradients are defined at different spatial scales in spherical frames, which are rotated along the radial axis according to the orientation of the source. The sensitivity of these gradients to the mass distribution inside a spherical Earth is described and analytical expressions relating the source to the observable are introduced. Then, the gravity gradients responses at different spatial scales to flat, elementary mass sources located at the surface and at increasing depth are studied. Specifically, the paper investigates how a source width and orientation can be determined, for localized and oscillatory mass anomalies with different width‐to‐length aspect ratios. This theoretical case study aims at providing a basis for the analysis of more complex mass structures, when applying the presented method to static or time‐varying satellite gravity field models. It may help deciphering the nature of the gravity sources by the detection of meaningful geometries and orientations in the gravity field.

Geodetic Changes Associated with Crustal Deformation on the Andean Backarc of San Juan, Argentina

In this work we have analyzed the results of temporal geodetic variations associated with tectonic deformation on the Andean backarc in southern San Juan province, Argentina. This area is affected by the subduction of the Juan Fernandez Ridge and has the greatest seismic hazard in Argentina. Since 2000, we have monitored the variations of gravity annually with high precision (g4D) in the active deformation belt, around the city of San Juan (31°S to 32°S, 68°W to 69°W). Temporal gradients of g4D have been associated with active structures and interpreted as subsurface mass redistributions as a result of accumulation and release of stress. These data, in addition to the topographic deformations measured by the Global Positioning System (GPS) and the identification of structures in the crystalline basement from the geophysical models, allow us interpret mechanisms of deformation in the contact between the Precordillera and Pampean Ranges in the foreland of San Juan. In this area, after more than a decade of low activity, in 2012 the Tulum Fault System (TFS) was reactivated, generating crustal events of moderate magnitude including an earthquake of Mw 5.0 (January 18, 2012). According to our interpretation, the fault system accumulated elastic deformation for more than 10 years expressed as height increases at gravity stations (decrease of g4D) until the year 2011. The January 18, 2012 earthquake released part of the energy at the same time as a coseismic increase of g4D. This should be reflected in regional tectonic subsidence from the point of view of conventional gravimetric models. However, GPS reoccupation of the benchmarks located in this fault system indicates a generalized increase in altitude of the fault system in the last 15 years. TFS behavior could be assumed as topographic flexural displacements induced by a system of blind faults. We analyzed the time series of a permanent Global Navigation Satellite System (GNSS) station located to the north of the TFS. The GRLS station (31.61oS; 68.32oW) presents slow variation in the vertical and north components. The variation began on December 20, 2011 reaching its maximum amplitude on January 18, 2012. The geodetic variations coincide with the period of seismic reactivation.

Probing Late-type T Dwarf J − H Color Outliers for Signs of Age*

Abstract We present the results of a Keck/NIRSPEC follow-up survey of 13 late-type T dwarfs (T6–T9), 12 of which have unusually red or blue J − H colors. Previous work suggests that J − H color outliers may represent the high-gravity, low-metallicity (old) and low-gravity, solar-metallicity (young) extremes of the late-type T dwarf population. We use medium-resolution Y- and H-band spectroscopy to probe regions of T dwarf atmospheres that are more sensitive to gravity and metallicity variations than the J band. We find that the spectral morphologies of our sample are largely homogeneous, with peak-normalized, Y- and H-band morphologies consistent with spectral standards. However, three objects stand out as potentially old, with overluminous Y-band spectra compared to their respective spectral standards, and a fourth object stands out as potentially young, with an underluminous Y band. Of these four objects, three have been previously identified as potential metallicity/gravity outliers, including the one object in our sample with a normal J − H color. We fit publicly available atmospheric model grids to our spectra and find that the best-fit physical parameters vary depending on the model used. As we continue to probe the characteristics of the late-T population, differences in synthetic spectra of ∼10%–20% in the blue wing of the Y band and ∼45% at 1.65 μm, for the same physical parameters, must be reconciled. Further development and public availability of nonsolar metallicity models is also recommended. Future progress toward deciphering the impacts of gravity, metallicity, and variability in the late-type T dwarf population will also require high signal-to-noise, multiwavelength and multi-epoch photometry and spectroscopy.

The physical characteristics of oil palm trunk and fast growing species veneer for composite-plywood

Oil palm trunk waste and fast growing species has potential to be used as composite-plywood (comply) raw material. Oil palm trunk as core layer and fast growing species as face and back layers of comply. This research focusing on the raw material characteristics of comply made of fast growing species (sengon, afrika, acacia) and oil palm trunk waste. The aim of this research is to determine the physical properties of raw materials to be used as comply. The physical properties (moisture content, specific gravity and thickness variations) were determined based on British Standard (BS) 373:1957 and Japanese Agricultural Standard (JAS) SE 11:2003. The research results showed that bottom and middle parts of oil palm trunk have moisture content and specific gravity values of 47%, 33% and 0.4, 0.7 respectively. Whereas, values of moisture content and specific gravity fast growing species from bottom, middle and upper parts were 54%, 47%, 35% and 0.25, 0.22, 0.24, respectively for sengon wood, 148%, 120%, 66% and 0.39, 0.32, 0.37, respectively for afrika wood, 80%, 79%, 76% and 0.59, 0.57, 0.59, respectively for acacia wood. The moisture content of fast growing species veneer for sengon, afrika, acacia were 27.32%, 32.83%, and 27.67%, respectively. Thickness variation of fast growing species veneer for sengon, afrika, acacia from bottom, middle and upper parts were 2.00, 1.96, 1.95 mm, 2.25, 2.20, 1.95 mm and 2.11, 2.09, 2.09 mm. All the above physical properties were suitable for comply raw material.

Period spacings in red giants

Context. Oscillation modes with a mixed character, as observed in evolved low-mass stars, are highly sensitive to the physical properties of the innermost regions. Measuring their properties is therefore extremely important to probe the core, but requires some care, due to the complexity of the mixed-mode pattern. Aims. The aim of this work is to provide a consistent description of the mixed-mode pattern of low-mass stars, based on the asymptotic expansion. We also study the variation of the gravity offset εg with stellar evolution. Methods. We revisit previous works about mixed modes in red giants and empirically test how period spacings, rotational splittings, mixed-mode widths, and heights can be estimated in a consistent view, based on the properties of the mode inertia ratios. Results. From the asymptotic fit of the mixed-mode pattern of a large set of red giants at various evolutionary stages, we derive unbiased and precise asymptotic parameters. As the asymptotic expansion of gravity modes is verified with a precision close to the frequency resolution for stars on the red giant branch (10−4 in relative values), we can derive accurate values of the asymptotic parameters. We decipher the complex pattern in a rapidly rotating star, and explain how asymmetrical splittings can be inferred. We also revisit the stellar inclinations in two open clusters, NGC 6819 and NGC 6791: our results show that the stellar inclinations in these clusters do not have privileged orientation in the sky. The variation of the asymptotic gravity offset with stellar evolution is investigated in detail. We also derive generic properties that explain under which conditions mixed modes can be observed.

Variability of Body Mass and Urine Specific Gravity in Elite Male Field Hockey Players During a Pre-Olympic Training Camp.

This field-based observational study was designed to examine the intraindividual variation of first morning body mass and urine specific gravity (Usg) in male hockey players (n = 22) during a 10-day training camp. It was also designed to evaluate the prevalence and interrelationship of morning hypohydration and postmatch dehydration using Usg and changes in body mass, respectively. Body mass and Usg were measured upon waking; body mass was also measured before and after matches. Individual means, SD, and coefficient of variation (CV) were calculated for morning body mass and Usg using 3, 6, and 8 days. Daily prevalence for euhydration and postmatch dehydration using morning Usg (<1.020) and changes in body mass (>-2%), respectively, were determined. Measurement of morning body mass and Usg for 3 days had low variability (CV < 1%) with no improvement at 6 or 8 days. Between 36% and 73% of players were considered euhydrated based on morning Usg. Postmatch body mass was reduced >1% in 50-85% of players, with up to 40% experiencing changes >-2%. Postmatch changes in body mass were unrelated to Usg the subsequent morning. These outcomes can be helpful in establishing criteria for detecting meaningful changes in morning body mass and Usg in similar settings, helping to monitor hydration status in elite male athletes. Despite ample fluid availability and consumption, many players experienced hypohydration and dehydration during the camp, indicating that careful monitoring and an individual fluid replacement approach are warranted in these environments.

New results on the gravity monitoring (2014\u20132017) of Soultz-sous-For\xeats and Rittershoffen geothermal sites (France)

This article presents the study of the mass redistribution associated with the geothermal energy exploitation of the Soultz-sous-Forêts and Rittershoffen plants by microgravity monitoring in the period 2014–2017. The two plants are located in the eastern part of France in the Rhine Graben. This rift is characterized by thermal anomalies. The Soultz-sous-Forêts enhanced geothermal system is a demonstration site producing 1.7 MWe thanks to three wells 5 km deep. The Rittershoffen geothermal plant is used to produce heat (24 MWth) with two wells of 2 km depth. The most recent production episodes at Soultz-sous-Forets and Rittershoffen began on 24 June 2016 and 19 May 2016, respectively. Each summer, since 2014 for the Soultz-sous-Forêts network and since 2015 for the Rittershoffen network, gravity measurements have been taken with a Scintrex CG5 gravimeter in order to calculate the gravity variation compared to a reference station and a reference time. The stability of the reference station at the Soultz-sous-Forêts plant was investigated by repeated absolute gravity measurements from the FG5#206. Gravity ties with the gravity observatory of Strasbourg were also performed to compensate for the absence of superconducting gravimeter at the in situ reference station. Precise leveling was undertaken simultaneously to each gravity survey showing that vertical ground displacement is lower than 1 cm; hence, this leads us to consider that the detected gravity changes are only due to Newtonian attraction. We do not detect any signal at the Rittershoffen network in the investigated period. After the beginning of production, we noticed a small differential signal at the Soultz-sous-Forêts network, which is spatially associated with the injection and production wells’ positions. Furthermore, the maximum gravity value appears in the same area as the induced seismicity related to the preferential paths of the geothermal fluid. However, a simple model based on a geothermal reservoir of cylindrical shape cannot explain the observations in terms of amplitude.

Language resources to negotiate historical thinking in history classroom interactions

This article examines different patterns of language resources that both teacher and students use to incorporate other voices in history secondary-level classroom interactions. The study particularly centers on the analysis of interpersonal and ideational linguistic resources used by teachers and students that contribute to the inclusion of historical evidence in the discourse, in combination with the building of semantic waves (Maton, 2014, 2016) through the variation of semantic density and semantic gravity. The analysis shows that when teachers and students use a strong semantic gravity (SG+), they also have a tendency of employing a more heteroglossic-oriented discourse, whereas when teachers and students use a strong semantic density (SD+), their discourse tends to be oriented to more monoglossic choices in language (Martin & White, 2005). The language resources chosen by teachers and students in their pedagogical interactions impact how historical thinking is constructed in the classroom; particularly regarding the space, these more or less specialized and nonspecialized language resources provide for the incorporation of epistemic and axiological sources, which are necessary to build history as an interpretative social science through different levels of abstraction.

A new and homogeneous metallicity scale for Galactic classical Cepheids

We gathered more than 1130 high-resolution optical spectra for more than 250 Galactic classical Cepheids. The spectra were collected with the optical spectrographs UVES at VLT, HARPS at 3.6 m, FEROS at 2.2 m MPG/ESO, and STELLA. To improve the effective temperature estimates, we present more than 150 new line depth ratio (LDR) calibrations that together with similar calibrations already available in the literature allowed us to cover a broad range in wavelength (5348 ≤ λ ≤ 8427 Å) and in effective temperature (3500 ≤ Teff ≤ 7700 K). This gives us the unique opportunity to cover both the hottest and coolest phases along the Cepheid pulsation cycle and to limit the intrinsic error on individual measurements at the level of ~100 K. As a consequence of the high signal-to-noise ratio of individual spectra, we identified and measured hundreds of neutral and ionized lines of heavy elements, and in turn, have the opportunity to trace the variation of both surface gravity and microturbulent velocity along the pulsation cycle. The accuracy of the physical parameters and the number of Fe I (more than one hundred) and Fe II (more than ten) lines measured allowed us to estimate mean iron abundances with a precision better than 0.1 dex. We focus on 14 calibrating Cepheids for which the current spectra cover either the entire or a significant portion of the pulsation cycle. The current estimates of the variation of the physical parameters along the pulsation cycle and of the iron abundances agree very well with similar estimates available in the literature. Independent homogeneous estimates of both physical parameters and metal abundances based on different approaches that can constrain possible systematics are highly encouraged.

Three-dimensional gravity inversion using automatic regional-residual components separation with application to the Iwaki area in Fukushima Prefecture, Japan

We have developed a simple method for analysing gravity data to define 3D subsurface structures consistent with geological maps and reflection seismic survey results. This new method makes it possible to automatically separate gravity anomalies caused by structures that are the target of analysis (residual gravity) and not subject to gravity anomaly analysis (trend surfaces or regional gravity variations). This separation is achieved by assuming that the gravity anomaly trend surface is extremely smooth prior to analysis. In addition, this analysis can be carried out automatically, so it is possible to use a boot-strap simulation to evaluate the effect that gravity data accuracy and survey point distribution have on the analysis results. Using this newly developed method, we conducted a 3D subsurface structural analysis based on newly obtained gravity survey data in an area around Iwaki City, Fukushima Prefecture. The results of the 3D structural analysis revealed sharp depressions in the basement rock and basin-shaped underground structures along the Yunodake and Yamada faults. Finally, we were able to estimate the influence of survey point distribution and abrupt changes in the structure on the analysis results using a boot-strap simulation of the 3D analysis.

Hybrid gravimetry monitoring of Soultz-sous-Forêts and Rittershoffen geothermal sites (Alsace, France)

We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts and Rittershoffen sites. Soultz-sous-Forêts site is the first enhanced geothermal system demonstration site producing electricity in France. Here a geothermal fluid at 165 °C allows to produce around 1.5 MW thanks to one injection well and one production well 5 km deep. Rittershoffen site is dedicated to an industrial use and it is designed to produce 24 MWth heat power with 2 wells around 2.5 km deep. The most recent production episodes of Rittershoffen and Soultz-sous-Forêts geothermal plants have started respectively on May and June 2016. To study underground mass redistribution, time-lapse relative microgravity measurements have been done since 2014 on a network designed ad hoc for Soultz-sous-Forêts site and since 2015 for Rittershoffen site. After tide and drift correction, double differences are calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. Absolute gravity measurements have been also collected at one of the reference stations. Before the beginning of the production in 2016, the stability of the Soultz-sous-Forêts reference station was monitored through the repetition of absolute measurements and continuous gravity records. In 2016, regular ties between the reference stations and the Strasbourg gravity observatory STJ9 have been done. Several superconducting gravimeters operate continuously in STJ9. Thus, we approach the concept of hybrid gravity. Vertical deformation is also controlled thanks to six continuous GPS measurements: the height changes are less than 1 cm. So we consider that our gravity variations are only due to subsurface mass changes. For the Soultz-sous-Forêts network, we notice significant changes in agreement with the position of the injection and the production wells. The maximum gravity change is 31 ± 8 μGal. On the contrary, we do not detect any similar signal for the Rittershoffen network. A simplistic model using two spherical sources located at 5 and 2 km for Soultz-sous-Forêts and Rittershoffen sites respectively shows negligeable surface gravity changes when taking into account the known injection and production flow rates.

Comparison between seasonal variations in tidal and internal gravity wave activity as derived from observations at Maimaga and Tiksi

Since 2015, simultaneous observations of temperature of the high-latitude mesopause (87 km) have been made at Maimaga (63.04° N, 129.51° E) and Tiksi (71.58° N, 128.77° E) stations. These stations record spectra with Shamrock (Andor) photosensitive infrared spectrographs detecting the OH (3, 1) band in the near-infrared region (about 1.5 μm). We analyze temperature data obtained in observation seasons from 2015 to 2017. Standard deviations of temperature σ from its mean values are taken as characteristics of wave activity at night. We have obtained standard temperature deviations corresponding to internal gravity waves (IGW) (σgw) and tidal waves (σtd). Mean night rotational temperatures of hydroxyl emission almost coincide, and seasonal variations of gravity and tidal waves have a similar form during two seasons of simultaneous observations at Tiksi and Maimaga.