Calculation of the magnetic gradient tensor from total field gradient measurements and its application to geophysical interpretation

Geophysics ◽  
1988 ◽  
Vol 53 (7) ◽  
pp. 957-966 ◽  
Author(s):  
J. Bradley Nelson
Keyword(s):  
Signal To Noise Ratio ◽  
Depth Estimation ◽  
Vertical Gradient ◽  
Rate Of Change ◽  
The Body ◽  
Total Field ◽  
Gradient Tensor ◽  
Field Gradients ◽  
Contour Plots ◽  
Gradient Measurements

The very low inherent noise levels of superconducting quantum interference device (SQUID) sensors have led to proposals for the use of airborne SQUID magnetic gradiometers as geophysical interpretation tools. The quantity measured by such systems will be the gradient tensor, the spatial rate of change of the vector components of the magnetic field. By contrast, existing airborne gradiometers measure the spatial rate of change of the magnitude of the total field. This work describes a technique whereby the gradient tensor can be calculated from measurements of either the vertical or horizontal total field gradients throughout a plane. The signal‐to‐noise ratio of the calculated tensor components is essentially the signal‐to‐noise ratio of the original total field gradient measurements. The resulting tensor components may be upward or downward continued with standard techniques. Two advantages of using the tensor gradients instead of the total field gradients have been determined. Because the tensor components are not a function of the direction of the Earth’s field, contour plots do not suffer the skewing problems that total field or vertical gradient plots do. Thus, tensor gradient contour plots may be easier to interpret or may enhance the information obtained from total field or vertical gradient maps. In addition, the dipole‐tracking algorithm developed by Wynn et al. (1975) has been shown to be quite successful in determining the depth and horizontal location of block‐shaped bodies. The error in depth estimation is a strong inverse function of the ratio of the closest point of approach to largest dimension of the body. However, if the smallest separation is more than twice the largest dimension of the body, errors in depth estimation are less than 10 percent. Because the tensor components are calculated on a horizontal plane, they can be upward continued to meet this condition.

GRADIENT MEASUREMENTS IN AEROMAGNETIC SURVEYING

Geophysics ◽  
1965 ◽  
Vol 30 (5) ◽  
pp. 891-902 ◽  
Author(s):  
Peter Hood
Keyword(s):  
Optical Pumping ◽  
Vertical Gradient ◽  
Horizontal Distance ◽  
Total Field ◽  
Highly Sensitive ◽  
Geophysical Information ◽  
The Difference ◽  
Gradient Measurements ◽  
First Vertical Derivative ◽  
Depth Of Burial

The recent development of highly sensitive magnetometers, such as the optical‐pumping varieties, has made feasible the measurement of the first vertical derivative of the total field (∂ΔT/∂h) in aeromagnetic surveys. This is accomplished by using two sensitive magnetometer heads separated by a constant vertical distance, and recording the difference in outputs. The effect of diurnal is thus eliminated in the resultant differential output, and this is an especially desirable feature in northern Canada where the diurnal variation is usually much greater than is found in more southerly magnetic latitudes. Moreover, steeply dipping geological contacts in high‐magnetic latitudes are outlined by the resultant zero‐gradient contour. It is also possible to obtain the depth of burial of the contact from the graph of (∂ΔT/∂h) versus (x∂ΔT/∂x) where x is the horizontal distance measured from the contact. Similar quantitative interpretations may be made for the point pole and dipole. The data reduction necessary to produce a vertical‐gradient map is much simpler than with the total‐field case because no datum levelling is necessary. Since the aircraft track will be available from the main compilation it is only necessary to plot the resultant vertical‐gradient values on the track map and contour. Thus, two maps will be obtained for little more than the price of one but with a greatly increased gain in geophysical information concerning the geometry of the causative bodies. Actually, a first‐derivative map is difficult (and therefore costly) to produce by any other means. The measurement of the vertical gradient would appear to be the main advantage to using hundredth‐gamma magnetometers in aeromagnetic surveys, since those types presently in service are sensitive enough for the effective delineation of total‐field anomalies.

GRADIENT MEASUREMENTS IN GROUND MAGNETIC PROSPECTING

Geophysics ◽  
1965 ◽  
Vol 30 (3) ◽  
pp. 403-410 ◽  
Author(s):  
Peter Hood ◽  
D. J. McClure
Keyword(s):  
Contact Point ◽  
Vertical Component ◽  
Vertical Gradient ◽  
Horizontal Distance ◽  
Total Field ◽  
Gradient Measurements ◽  
Ground Magnetic ◽  
Vertical Contact ◽  
First Vertical Derivative ◽  
Depth Of Burial

The development of electronic magnetometers, i.e., the proton‐precession and fluxgate instruments, for use in ground magnetic surveys has permitted the measurement of the first‐vertical derivative of the total field, or of the vertical component of that field, with negligible addition to the total cost of the survey. The gain in information is, however, significant. Curves for the vertical gradient over a vertical contact, point pole, and finite dipole are presented. The vertical contact is outlined by the zero contour for the vertical gradient of the vertical component, and the depth of burial is half the horizontal distance between the positive and negative maxima. The depth of burial of the point pole and finite dipole is approximately equal to the horizontal distance between the negative half‐maximum points on the vertical‐gradient curves.

scholarly journals Batch spawning facilitates transfer of an essential nutrient from diet to eggs in a marine fish

2013 ◽  
Vol 9 (5) ◽  
pp. 20130593 ◽  
Author(s):  
Lee A. Fuiman ◽  
Cynthia K. Faulk
Keyword(s):  
Fatty Acids ◽  
Essential Fatty Acids ◽  
Maternal Diet ◽  
Short Interval ◽  
Rate Of Change ◽  
The Body ◽  
Sciaenops Ocellatus ◽  
Diet Shift ◽  
Spawning Site ◽  
Batch Spawning

Fatty acid composition of eggs affects development, growth and ecological performance of fish embryos and larvae, with potential consequences for recruitment success. Essential fatty acids in eggs derive from the maternal diet, and the time between ingestion and deposition in eggs is ecologically important but unknown. We examined the dynamics of diet–egg transfer of arachidonic acid (ARA) in the batch-spawning fish, red drum ( Sciaenops ocellatus ), by measuring ARA concentrations in eggs after a single diet shift and during a period of irregular variations in diet. ARA concentrations in eggs changed within 2–16 days of a diet shift. The rate of change was proportional to the magnitude of the shift, with no evidence of equilibration. These results are not consistent with eggs being assembled entirely from accumulated body stores. The immediate source of ARA in eggs appears to be the recent diet. We propose that batch spawning produces rapid diet–egg transfer of ARA because it removes large amounts of fatty acids from the body and prevents equilibration. The immediacy of the diet–egg connection suggests that spawning migration combined with short-interval batch spawning may have evolved to take advantage of nutrients critical for offspring survival that are available at the spawning site.

scholarly journals Snow Depth Estimation on Slopes Using GPS-Interferometric Reflectometry

Sensors ◽  
2019 ◽  
Vol 19 (22) ◽  
pp. 4994
Author(s):  
Haohan Wei ◽  
Xiufeng He ◽  
Yanming Feng ◽  
Shuanggen Jin ◽  
Fei Shen
Keyword(s):  
Time Series ◽  
Snow Depth ◽  
Water Cycle ◽  
Signal To Noise Ratio ◽  
Plate Boundary ◽  
Depth Estimation ◽  
Tilted Surface ◽  
Global Water Cycle ◽  
Gps Antenna ◽  
Good Agreement

Snow is one of the most critical sources of freshwater, which influences the global water cycle and climate change. However, it is difficult to monitor global snow variations with high spatial–temporal resolution using traditional techniques due to their costly and labor-intensive nature. Nowadays, the Global Positioning System Interferometric Reflectometry (GPS-IR) technique can measure the average snow depth around a GPS antenna using its signal-to-noise ratio (SNR) data. Previous studies focused on the use of GPS data at sites located in flat areas or on very gentle slopes. In this contribution, we propose a strategy called the Tilted Surface Strategy (TSS), which uses the SNR data reflected only from the flat quadrants to estimate the snow depth instead of the conventional strategy, which employs all the SNR data reflected from the whole area around a GPS antenna. Three geodetic GPS sites from the Plate Boundary Observatory (PBO) project were chosen in this experimental study, of which GPS sites p683 and p101 were located on slopes with their gradients up to 18% and the site p025 was located on a flat area. Comparing the snow depths derived with the GPS-IR TSS method with the snow depth results provided with the GPS-PBO, i.e., GPS-IR with the conventional strategy, the Snowpack Telemetry (SNOTEL) network measurements and gridded Snow Data Assimilation System (SNODAS) estimates, it was found that the snow depths derived with the four methods had a good agreement, but the snow depth time series with the GPS-IR TSS method were closer to the SNOTEL measurements and the SNODAS estimates than those with GPS-PBO method. Similar observations were also obtained from the cumulative snowfall time series. Results generally indicated that for those GPS sites located on slopes, the TSS strategy works better.

scholarly journals Quantifying the effects of spatial resolution and noise on galaxy metallicity gradients

2020 ◽  
Vol 495 (4) ◽  
pp. 3819-3838
Author(s):  
Ayan Acharyya ◽  
Mark R Krumholz ◽  
Christoph Federrath ◽  
Lisa J Kewley ◽  
Nathan J Goldbaum ◽  
...  
Keyword(s):  
High Resolution ◽  
Galaxy Evolution ◽  
Spatial Resolution ◽  
Signal To Noise Ratio ◽  
Low Snr ◽  
Integral Field Spectroscopy ◽  
History Of ◽  
Gradient Measurements ◽  
Integral Field ◽  
Scale Length

ABSTRACT Metallicity gradients are important diagnostics of galaxy evolution, because they record the history of events such as mergers, gas inflow, and star formation. However, the accuracy with which gradients can be measured is limited by spatial resolution and noise, and hence, measurements need to be corrected for such effects. We use high-resolution (∼20 pc) simulation of a face-on Milky Way mass galaxy, coupled with photoionization models, to produce a suite of synthetic high-resolution integral field spectroscopy (IFS) datacubes. We then degrade the datacubes, with a range of realistic models for spatial resolution (2−16 beams per galaxy scale length) and noise, to investigate and quantify how well the input metallicity gradient can be recovered as a function of resolution and signal-to-noise ratio (SNR) with the intention to compare with modern IFS surveys like MaNGA and SAMI. Given appropriate propagation of uncertainties and pruning of low SNR pixels, we show that a resolution of 3–4 telescope beams per galaxy scale length is sufficient to recover the gradient to ∼10–20 per cent uncertainty. The uncertainty escalates to ∼60 per cent for lower resolution. Inclusion of the low SNR pixels causes the uncertainty in the inferred gradient to deteriorate. Our results can potentially inform future IFS surveys regarding the resolution and SNR required to achieve a desired accuracy in metallicity gradient measurements.

Tdpac-Studies of Electric Field Gradients in Amorphous Metallic Systems

1980 ◽  
Vol 3 ◽  
Author(s):  
P. Heubes ◽  
D. Korn ◽  
G. Schatz ◽  
G. Zibold
Keyword(s):  
Electric Field ◽  
Structural Model ◽  
Electric Field Gradient Tensor ◽  
Correlation Technique ◽  
Relative Width ◽  
Electric Field Gradients ◽  
Gradient Tensor ◽  
Field Gradients ◽  
Time Differential ◽  
Metallic Systems

ABSTRACTThe time differential perturbed γ-γ angular correlation technique (TDPAC) is applied to the amorphous metallic systems Ga, Bi, In50Au50 and In80Ag20. The electric field gradient tensor probed by 111Cd nuclei shows a broad probability distribution with a relative width of 0.4 – 0.5 for all systems, as suggested by a continuous random structural model.

3D correlation imaging of magnetic total field anomaly and its vertical gradient

2011 ◽  
Vol 8 (2) ◽  
pp. 287-293 ◽  
Author(s):  
Lianghui Guo ◽  
Lei Shi ◽  
Xiaohong Meng
Keyword(s):  
Vertical Gradient ◽  
Total Field
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