Reply by the author to N. C. Steenland

Geophysics ◽  
1984 ◽  
Vol 49 (3) ◽  
pp. 311-311
Author(s):  
Sigmund Hammer

Dr. Steenland’s principal criticism arises from an unfortunate overstatement, in my paper, of the precision and anomaly resolving power of the Carson Airborne Gravity method. This criticism is well deserved. My calculation of the probable error of an airborne gravity measurement was based on many thousands of Δg gravity differences at grid‐line intersections, but it made the implicit assumption that the two reported gravity values at each grid intersection were independent. This is incorrect because the grid system of intersection differences is used for controls in the data processing. A realistic value for the probable error of an airborne gravity measurement is of the order of 1 mgal (standard deviation of 1.5 mgal). The associated resolving power for gravity anomalies, above this magnitude, is of the order of 2 to 3 miles (3 to 5 km) at flight speed of 50 knots. Smaller anomalies may be resolved at lower speeds.

2018 ◽  
Vol 17 (1) ◽  
pp. 7-15
Author(s):  
Niraj Manandhar ◽  
Shanker K.C.

Gravimetric geoid plays the important role in the process of local/regional geoidal undulation determination. This approach uses the residual gravity anomalies determined by the surface gravity measurement using the gravimeter together with best fit geopotential model, with the geoid undulations over the oceans determined from the method of satellite altimetry. Mass distribution, position and elevation are prominent factors affecting the surface gravity. These information in combination with geopotential model helps in satellite orbit determination, oil, mineral and gas exploration supporting in the national economy. The preliminary geoid thus computed using airborne gravity and other surface gravity observation and the accuracy of computed geoid was likely at the 10-20cm in the interior of Nepal but higher near the border due to lack of data in China and India. The geoid thus defined is significantly improved relative to EGM –08 geoid.


Geophysics ◽  
1984 ◽  
Vol 49 (4) ◽  
pp. 477-477
Author(s):  
Sigmund Hammer

Mr. Pearson’s Discussion was received after answers by the author had been submitted to prior Discussions by Steenland and Herring. The essential criticisms of all three writers stem from two significant errors in the paper under discussion, namely (1) overstatement of the precision and resolving power of the Carson airborne gravity method based mainly on the 1981 test survey; (2) inadvertent errors in the dimensions of some of the data on Figure 8. An early attempt to publish a correction for item (1) (Hammer, 1982) has been delayed and is not yet in print. A revised Figure 8, redrawn to proper scale, is included as Figure 1 above in the Reply to Herring’s Discussion. Public criticism for my errors in this definitive paper on airborne gravity is useful.


2013 ◽  
Vol 341-342 ◽  
pp. 999-1004
Author(s):  
Wei Zhou ◽  
Ti Jing Cai

For low-pass filtering of airborne gravity data processing, elliptic low-pass digital filters were designed and filtering influences of the elliptic filter order, upper limit passband frequency, maximal passband attenuation and minimal stopband attenuation were studied. The results show that the upper limit passband frequency has the greatest effect on filtering among four parameters; the filter order and the maximal passband attenuation have some influence, but instability will increase with larger order; the effect of the minimal stopband attenuation is not obvious when reaching a certain value, which requires a combination of evaluation indicator accuracy to determine the optimal value. The standard deviations of discrepancies between the elliptic filtered gravity anomaly with optimal parameters and the commercial software result are within 1mGal, and the internal accord accuracy along four survey lines after level adjusting is about 0.620mGal.


1927 ◽  
Vol 17 (1) ◽  
pp. 72-93 ◽  
Author(s):  
H. T. Cranfield ◽  
D. G. Griffiths ◽  
E. R. Ling

1. 670 samples of the mixed milk from 15 herds were analysed, and the average percentages of total ash, soluble ash, insoluble ash, lime and phosphoric acid are given.2. Tables showing frequency distributions are also given, with the standard deviation, mean and probable error of mean for each constituent determined.3. Various correlations of these constituents with solids not fat and protein have been prepared, and these correlations are illustrated by graphs.It is observed that the total ash falls with the solids not fat until low values of solids not fat are reached, when the ash content appears to rise. This variation is confirmed by a curve illustrating the variation in ash content of samples of individual cow’s milk. Soluble ash rises as the solids not fat falls, but the insoluble ash shows a reverse variation. Lime and phosphoric acid both fall with the solids not fat.


PEDIATRICS ◽  
1954 ◽  
Vol 14 (5) ◽  
pp. 563-564
Author(s):  

THE WRITTEN examination of January 15, 1954, was taken by 515 candidates, a larger number than in any previous year except 1953 when there were 607 candidates Grades ranged from a lowest mark of 32.0 to a highest mark of 89.5 Inspection of the range resulted in the decision to place the passing mark at 51. On this basis there were 32, or 6.2%, who failed and were therefore ineligible for oral examination. The distribution of the grades earned by the 515 candidates is presented in the form of a histogram. As an aid to visual assessment of the nature of the distribution, a normal frequency curve computed from the mean and standard deviation of the data has been superimposed on the diagram. The distribution of the grades is clearly and impressively skewed to the left, that is, the scores tend to be massed at the high end of the scale and spread out at the low end. An examination of this type is relatively sensitive in the zone of poorer scholarship where the selection of failures is to be made and relatively insensitive in the range of higher scholarship. The intrinsic reliability of the examination has again been assessed by comparing the grade made by each candidate on his odd-numbered questions with that earned on his even-numbered questions. The comparison reveals a "probable error of estimate," P.E.m, of 2.39, a lower figure and therefore a higher degree of reliability than yielded by any previous examination. The Committee is pleased that the increase in reliability was accomplished in spite of the fact that the length of the examination was decreased from 250 grading points in 1953 to 200 grading points.


2020 ◽  
Vol 25 (1) ◽  
pp. 65-74
Author(s):  
Hui Qin ◽  
Xiongyao Xie ◽  
Yu Tang ◽  
Zhengzheng Wang

Ground penetrating radar (GPR) is considered an effective tool to detect tunnel lining voids. In this paper, an experimental study was carried out using a physical tunnel lining model to evaluate the performances of different antenna frequencies. We built a 4.2 m–long, 4.2 m–wide, and 2.0 m–high experimental model to simulate the secondary lining, initial lining, and surrounding rock of a tunnel structure. In the model, we created four categories of voids, which are voids in secondary and initial linings, a delamination between the secondary and initial linings, a delamination between the initial lining and sand, and a void buried in the sand, to simulate real cases in tunnel engineering. The GPR wave velocities in the sand and concrete of the model were measured using the reflection method for the calibration of void depth. We employed a commercial GPR system equipped with antennae of different centre frequencies to detect the voids. GPR data were processed using a conventional data processing flow, and the performances of different frequencies were examined. The results show that the 1000 MHz centre frequency GPR is capable of characterizing shallow buried voids in the secondary lining but is not able to penetrate into the initial lining. The 250 MHz centre frequency GPR system is not advised to detect voids in or behind tunnel linings due to its low resolving power for voids of centimetre sizes. The 500 MHz centre frequency GPR system is optimal for void detection because it demonstrated a balanced performance of resolving ability and investigation depth. The findings of this work could be useful references for antenna selection and data processing in real GPR applications.


1965 ◽  
Vol 7 ◽  
pp. 57-64
Author(s):  
Thomas Gehrels

The Wavelength Dependence of Polarization as observed in 32 stars, for which the Henry Draper numbers are given, is shown in figure 1. Details of some of these observations are presented in reference 1.The equipment is now being used with the new 154-cm Catalina reflector of the Lunar and Planetary Laboratory at the University of Arizona. The instrumental polarizations are nearly zero. The data processing and observing techniques have been further improved; the precision is mainly determined by statistics such that the internal probable error in the percentage polarization is ±0.03 percent (±0.0006 magnitude) for a half-hour observation per filter on objects brighter than about 7 magnitudes. The wavelength λ ranges from 0.33 to 0.95 μ covered by seven filters of bandwidth of about 0.05 μ. The wavelength range is being extended to 1.2, 1.6, and 2.2 μ and, with high-altitude ballooning, to 0.28 and 0.22 μ.


Geophysics ◽  
1981 ◽  
Vol 46 (8) ◽  
pp. 1088-1099 ◽  
Author(s):  
Robert B. Rice ◽  
Samuel J. Allen ◽  
O. James Gant ◽  
Robert N. Hodgson ◽  
Don E. Larson ◽  
...  

Advances in exploration geophysics have continued apace during the last six years. We have entered a new era of exploration maturity which will be characterized by the extension of our technologies to their ultimate limits of precision. In gravity and magnetics, new inertial navigation systems permit the very rapid helicopter‐supported land acquisition of precise surface gravity data which is cost‐effective in regions of severe topography. Considerable effort is being expended to obtain airborne gravity data via helicopter which is of exploration quality. Significant progress has also been made in processing and interpreting potential field data. The goal of deriving the maximum amount of accurate subsurface information from seismic data has led to much more densely sampled and precise 2- and 3-D land data acquisition techniques. Land surveying accuracy has been greatly improved. The number of individually recorded detector channels has been increased dramatically (up to 1024) in order to approximate much more accurately a point‐source, point‐detector system. Much more powerful compressional‐wave vibrators can now maintain full force while sweeping up or down from 5 Hz to over 200 Hz. In marine surveying, new streamer cables and shipboard instrumentation permit the recording and limited processing of 96 to 480 channels. Improvements have also been made in marine sources and arrays. The most important developments in seismic data processing—wave‐equation based imaging and inversion methods—may be the forerunners of a totally new processing methodology. Wave‐equation methods have been formulated for migration before and after stack, multiples suppression, datum and replacement statics, velocity estimation, and seismic inversion. Inversion techniques which provide detailed acoustic‐impedance or velocity estimates have found widespread commercial application. Wavelet processing has greatly expanded our stratigraphic analysis capabilities. Much more sophisticated 1-, 2-, and 3-D modeling techniques are being used effectively to guide data acquisition and processing, as direct interpretation aids, and to teach basic interpretation concepts. Some systems can now handle vertical and lateral velocity changes, inelastic attenuation, curved reflection horizons, transitional boundaries, time‐variant waveforms, ghosting, multiples, and array‐response effects. Improved seismic display formats and the extensive use of color have been valuable in data processing, modeling, and interpretation. Stratigraphic interpretation has evolved into three major categories: (1) macrostratigraphy, where regional and basinal depositional patterns are analyzed to describe the broad geologic depositional environment; (2) qualitative stratigraphy, where specific rock units and their properties are analyzed qualitatively to delineate lithology, porosity, structural setting, and areal extent and shape; and (3) quantitative stratigraphy, where anomalies are mapped at a specific facies level to define net porosity‐feet distribution, gas‐fluid contacts, and probable pore fill. In essence, what began as direct hydrocarbon‐indicator technology applicable primarily to Upper Tertiary clastics has now matured to utility in virtually every geologic province. Considerable effort has been expended on the direct generation and recording of shear waves in an attempt to obtain more information about stratigraphy, porosity, and oil and gas saturation. Seismic service companies now offer shear‐wave prospecting using vibrator, horizontal‐impact, or explosive sources. Well logging has seen the acceleration of computerization. Wellsite tape recorders and minicomputers with relatively simple interpretation algorithms are routinely available. More sophisticated computerized interpretation methods are offered as a service at data processing centers.


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