Airborne gravimetry: An investigation of filtering

Low-pass filtering in airborne gravimetry data processing plays a fundamental role in determining the spectral content and amplitude of the free-air anomaly. Traditional filters used in airborne gravimetry, the 6 × 20-s resistor-capacitor (RC) filter and the 300-s Gaussian filter, heavily attenuate the waveband of the gravity signal. As we strive to reduce the overall error budget to the sub-mGal level, an important step is to evaluate the choice and design of the low-pass filter employed in airborne gravimetry to optimize gravity anomaly recovery and noise attenuation. This study evaluates low-pass filtering options and presents a survey-specific frequency domain filter that employs the fast Fourier transform (FFT) for airborne gravity data. This study recommends a new approach to low-pass filtering airborne data. For a given survey, the filter is designed to maximize the target gravity signal based upon survey parameters and the character of measurement noise. This survey-specific low-pass filter approach is applied to two aerogravimetry surveys: one conducted in West Antarctica and the other in the eastern Pacific off the California coast. A reflight comparison with the West Antarctic survey shows that anomaly amplitudes are increased while slightly improving the rms fit between the reflown survey lines when an appropriately designed FFT filter is employed instead of the traditionally used filters. A comparison of the East Pacific survey with high-resolution shipboard gravity data indicates anomaly amplitude improvements of up to 20 mGal and a 49% improvement of the rms fit from 3.99 mGal to 2.04 mGal with the appropriately designed FFT filter. These results demonstrate that substantial improvement in anomaly amplitude and wavelength can be attained by tailoring the filter to the survey

Airborne gravity and precise positioning for geologic applications

Airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade. Airborne gravity is especially useful when measured in conjunction with other geophysical data, such as magnetics, radar, and laser altimetry. The aerogeophysical survey over the West Antarctic ice sheet described in this paper is one such interdisciplinary study. This paper outlines in detail the instrumentation, survey and data processing methodology employed to perform airborne gravimetry from the multiinstrumented Twin Otter aircraft. Precise positioning from carrier-phase Global Positioning System (GPS) observations are combined with measurements of acceleration made by the gravity meter in the aircraft to obtain the free-air gravity anomaly measurement at aircraft altitude. GPS data are processed using the Kinematic and Rapid Static (KARS) software program, and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the GPS position solution. Accuracies for the free-air anomaly are determined from crossover analysis after significant editing (2.98 mGal rms) and from a repeat track (1.39 mGal rms). The aerogeophysical survey covered a 300,000 km2 region in West Antarctica over the course of five field seasons. The gravity data from the West Antarctic survey reveal the major geologic structures of the West Antarctic rift system, including the Whitmore Mountains, the Byrd Subglacial Basin, the Sinuous Ridge, the Ross Embayment, and Siple Dome. These measurements, in conjunction with magnetics and ice-penetrating radar, provide the information required to reveal the tectonic fabric and history of this important region

Cardiac Resynchronization With Sequential Biventricular Pacing for the Treatment of Moderate-to-Severe Heart Failure

ObjectivesThe InSync III study evaluated sequential cardiac resynchronization therapy (CRT) in patients with moderate-to-severe heart failure and prolonged QRS.BackgroundSimultaneous CRT improves hemodynamic and clinical performance in patients with moderate-to-severe heart failure (HF) and a wide QRS. Recent evidence suggests that sequentially stimulating the ventricles might provide additional benefit.MethodsThis multicenter, prospective, nonrandomized, six-month trial enrolled a total of 422 patients to determine the effectiveness of sequential CRT in patients with New York Heart Association (NYHA) functional class III or IV HF and a prolonged QRS. The study evaluated: whether patients receiving sequential CRT for six months experienced improvement in 6-min hall walk (6MHW) distance, NYHA functional class, and quality of life (QoL) over control group patients from the reported Multicenter InSync Randomized Clinical Evaluation (MIRACLE) trial; whether sequential CRT increased stroke volume compared to simultaneous CRT; and whether an increase in stroke volume translated into greater clinical improvements compared to patients receiving simultaneous CRT.ResultsInSync III patients experienced greater improvement in 6MHW, NYHA functional class, and QoL at six months compared to control (all p < 0.0001). Optimization of the sequential pacing increased (median 7.3%) stroke volume in 77% of patients. No additional improvement in NYHA functional class or QoL was seen compared to the simultaneous CRT group; however, InSync III patients demonstrated greater exercise capacity.ConclusionsSequential CRT provided most patients with a modest increase in stroke volume above that achieved during simultaneous CRT. Patients receiving sequential CRT had improved exercise capacity, but no change in functional status or QoL

New standards for reducing gravity data: The North American gravity database

The North American gravity database as well as databases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising procedures for calculating gravity anomalies, taking into account our enhanced computational power, improved terrain databases and datums, and increased interest in more accurately defining long-wavelength anomaly components. Users of the databases may note minor differences between previous and revised database values as a result of these procedures. Generally, the differences do not impact the interpretation of local anomalies but do improve regional anomaly studies. The most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conventionally used geoid or sea level. Principal facts of gravity observations and anomalies based on both revised and previous procedures together with germane metadata will be available on an interactive Web-based data system as well as from national agencies and data centers. The use of the revised procedures is encouraged for gravity data reduction because of the widespread use of the global positioning system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with North American and national databases. Anomalies based on the revised standards should be preceded by the adjective “ellipsoidal” to differentiate anomalies calculated using heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid

Airborne Remote Sensing of Wave Propagation in the Marginal Ice Zone

Airborne scanning lidar was used to measure the evolution of the surface wave field in the marginal ice zone (MIZ) during two separate wave events in the Beaufort Sea in October 2015. The lidar data consisted of a 2‐D field of surface elevation with horizontal resolutions between 17 and 33 cm, over a swath approximately 150‐220 m wide, centred on the ground track of the aircraft. Those data were used to compute directional wavenumber spectra of the surface wave field. Comparison with nearly collocated buoy data found the lidar and buoy measurements to be generally consistent. During the first event, waves travelling from open water into the ice were attenuated by the ice. The low spectral spreading and k7/4 spectral dependence of the attenuation was consistent with dissipative models that treat sea ice as a highly viscous fluid floating on a less viscous ocean. Upper‐ocean eddy viscosities calculated using that model were found to be significantly lower than those from previous work. The second event was in off‐ice winds and cold temperatures, allowing measurement of the wave fetch relation in ice‐forming conditions. The wave growth rate was found to be slightly higher than previous measurements under unstable atmospheric conditions without ice formation. Comparison with WAVEWATCH III model output highlighted the importance of accurate ice information and fine geographic computational resolution when making predictions near the ice edge. Finally, the very short scales over which the wave field was observed to evolve in the MIZ are discussed

Controlled Defects in Semiconducting Carbon Nanotubes Promote Efficient Generation and Luminescence of Trions

We demonstrate efficient creation of defect-bound trions through chemical doping of controlled sp³ defect sites in semiconducting, single-walled carbon nanotubes. These tricarrier quasi-particles luminesce almost as brightly as their parent excitons, indicating a remarkably efficient conversion of excitons into trions. Substantial populations of trions can be generated at low excitation intensities, even months after a sample has been prepared. Photoluminescence spectroscopy reveals a trion binding energy as high as 262 meV, which is substantially larger than any previously reported values. This discovery may have important ramifications not only for studying the basic physics of trions but also for the application of these species in fields such as photonics, electronics, and bioimaging