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Disposal of industrial wastewater and activities such as CO2 depend on pressure conditions within deep geologic reservoirs. Injection and storage are also associated with induced seismicity, suggested to result from reservoir compartmentalization and leakage into faults. To understand subsurface pressure conditions within a major regional disposal reservoir, the carbonate Arbuckle Group of Oklahoma, we monitored the water levels in 15 inactive injection wells. The wells were monitored at 30-second intervals, with eight wells monitored since September 2016, and an additional seven from July 2017. All of the wells were monitored until early March 2020. Since 2016, well levels decreased in 3 of the 15 wells (a.k.a. hydraulic head), proportional to near-borehole fluid pressure even considering decreasing regional injection volumes during this period. The well pressures respond to three types of perturbations: (i) gravitational fkuctuations (a.k.a. solid-earth tides) (ii) distal and proximal earthquakes, and (iii) injections into nearby wells. Parameterization of tidal responses illustrates that the near wellbore environments have negative fluid flux (i.e. are leaking). Earthquakes cause differing pressure responses from well to well, with some highly sensitive to proximal events, some to distal events, and some apparently insensitive. Injections have variable impacts in some cases masking tidal and earthquake pressure signals. Collectively, there appears to be a threshold injection rate above which well pressure becomes less sensitive to the volume of injections within 15 km. Multi-scale geological structure and temporal permeability changes are likely controlling the pressure field, indicating leakage of fluids across the system. 

Abstract We perform a search for continuous nearly monochromatic gravitational waves from the central compact objects associated with the supernova remnants Vela Jr. and G347.3. Over 10¹⁸different waveforms are considered, covering signal frequencies between 20 and 1300 Hz (20 and 400 Hz) for G347.3-0.5 (Vela Jr.) and a very broad range of frequency derivatives. The data set used for this first search is from the second observing run of LIGO (O2). Thousands of volunteers donating compute cycles through the computing project Einstein@Home have made this endeavor possible. Following the Einstein@Home search, we perform multistage follow-ups of over 5 million waveforms. The threshold for selecting candidates from the Einstein@Home search is such that, after the multistage follow-up, we do not expect any surviving candidate due to noise. The very last stage uses a different data set, namely, the LIGO O3 data. We find no significant signal candidate for either targets. Based on this null result, for G347.3-0.5, we set the most constraining upper limits to date on the amplitude of gravitational-wave signals, corresponding to deformations below 10⁻⁶in a large part of the search band. At the frequency of best strain sensitivity, near 161 Hz, we set 90% confidence upper limits on the gravitational-wave intrinsic amplitude of $h_0^{90\%}\approx 6.2\times {10}^{-26}$. Over most of the frequency range, our upper limits are a factor of 10 smaller than the indirect age-based upper limit. For Vela Jr., near 163 Hz, we set $h_0^{90\%}\approx 6.4\times {10}^{-26}$. Over most of the frequency range, our upper limits are a factor of 15 smaller than the indirect age-based upper limit. The Vela Jr. upper limits presented here are slightly less constraining than the most recent upper limits of R. Abbott et al., but they apply to a broader set of signals. 

Abstract We answer frequently asked questions (FAQs) about the Hellings and Downs correlation curve—the ‘smoking-gun’ signature that pulsar timing arrays (PTAs) have detected gravitational waves (GWs). Many of these questions arise from inadvertently applying intuition about the effects of GWs on LIGO-like detectors to the case of pulsar timing, where not all of it applies. This is because Earth-based detectors, like LIGO and Virgo, have arms that are short (km scale) compared to the wavelengths of the GWs that they detect ( $\approx {10}^2$–10⁴km). In contrast, PTAs respond to GWs whose wavelengths (tens of light-years) are much shorter than their arms (a typical PTA pulsar is hundreds to thousands of light-years from Earth). To demonstrate this, we calculate the time delay induced by a passing GW along an Earth-pulsar baseline (a ‘one-arm, one-way’ detector) and compare it in the ‘short-arm’ (LIGO-like) and ‘long-arm’ (PTA) limits. This provides qualitative and quantitative answers to many questions about the Hellings and Downs curve. The resulting FAQ sheet should help in understanding the ‘evidence for GWs’ recently announced by several PTA collaborations. 

Abstract We present the results of an all-sky search for continuous gravitational waves in the public LIGO O3 data. The search covers signal frequencies 20.0 Hz ≤f≤ 800.0 Hz and a spin-down range down to −2.6 × 10⁻⁹Hz s⁻¹, motivated by detectability studies on synthetic populations of Galactic neutron stars. This search is the most sensitive all-sky search to date in this frequency/spin-down region. The initial search was performed using the first half of the public LIGO O3 data (O3a), utilizing graphical processing units provided in equal parts by the volunteers of the Einstein@Home computing project and by the ATLAS cluster. After a hierarchical follow-up in seven stages, 12 candidates remain. Six are discarded at the eighth stage, by using the remaining O3 LIGO data (O3b). The surviving six can be ascribed to continuous-wave fake signals present in the LIGO data for validation purposes. We recover these fake signals with very high accuracy with our last stage search, which coherently combines all O3 data. Based on our results, we set upper limits on the gravitational-wave amplitudeh₀and translate these into upper limits on the neutron star ellipticity and on ther-mode amplitude. The most stringent upper limits are at 203 Hz, withh₀= 8.1 × 10⁻²⁶at the 90% confidence level. Our results exclude isolated neutron stars rotating faster than 5 ms with ellipticities greater than $5\times {10}^{-8}\left(\frac{d}{100\mathrm{pc}}\right)$within a distancedfrom Earth andr-mode amplitudes $\alpha \ge {10}^{-5}\left(\frac{d}{100\mathrm{pc}}\right)$for neutron stars spinning faster than 150 Hz.