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101.
Interference effects of aircraft on earth's electromagnetic response at very low frequency and low frequency 下载免费PDF全文
Over the last few decades, very low frequency electromagnetics has been widely and successfully applied in mineral exploration and groundwater exploration. Many radio transmitters with strong signal‐to‐noise ratios are scattered in the very low frequency band and low frequency band. Based on experiences gained from ground measurements with the radio‐magnetotelluric technique operating in the frequency interval 1–250 kHz, broadband magnetometers have been used to cover both very low frequency (3–30 kHz) and low frequency (30–300 kHz) bands to increase the resolution of the near‐surface structure. The metallic aircraft as a conductive body will distort the magnetic signal to some extent, and thus it is important to investigate aircraft interference on the electromagnetic signal. We studied noise caused by rotation of an aircraft and the aircraft itself as a metallic conductive body with three methods: 3D wave polarization, determination of transmitter direction and full tipper estimation. Both very low frequency and low frequency bands were investigated. The results show that the magnetic field is independent of the aircraft at low frequencies in the very low frequency band and part of the low frequency band (below 100 kHz). At high frequencies (above 100 kHz), the signals are more greatly influenced by the aircraft, and the wave polarization directions are more scattered, as observed when the aircraft turned. Some aircraft generated noise mixed with radio transmitter signals, detected as ‘dummy’ signals by the 3D wave polarization method. The estimated scalar magnetic transfer functions are dependent on the aircraft flight directions at high frequencies, because of aircraft interference. The aircraft eigenresponse in the transfer functions (tippers) between vertical and horizontal magnetic field components was compensated for in the real part of the estimated tippers, but some unknown effect was still observed in the imaginary parts. 相似文献
102.
State‐of‐the‐art 3D seismic acquisition geometries have poor sampling along at least one dimension. This results in coherent migration noise that always contaminates pre‐stack migrated data, including high‐fold surveys, if prior‐to‐migration interpolation was not applied. We present a method for effective noise suppression in migrated gathers, competing with data interpolation before pre‐stack migration. The proposed technique is based on a dip decomposition of common‐offset volumes and a semblance‐type measure computation via offset for all constant‐dip gathers. Thus the processing engages six dimensions: offset, inline, crossline, depth, inline dip, and crossline dip. To reduce computational costs, we apply a two‐pass (4D in each pass) noise suppression: inline processing and then crossline processing (or vice versa). Synthetic and real‐data examples verify that the technique preserves signal amplitudes, including amplitude‐versus‐offset dependence, and that faults are not smeared. 相似文献
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本文根据2010年1~12月河北及邻区的83个宽频地震仪12个月连续噪声记录, 分析了河北及邻区瑞利面波的群速度频散曲线并反演了主要分区内的典型路径剪切波速度结构。 首先采用多重滤波方法提取了台站对5~50 s的面波群速度频散曲线, 然后用Herrmann线性反演方法反演了剪切波速度结构。 结果表明, 群速度频散曲线及剪切波速度分布特征与地表地质和构造特征表现出较好的相关性, 清晰地揭示了地壳内部的横向速度变化。 在短周期(8~20 s), 拥有较厚的沉积层的平原地区表现为明显的低速特征, 而隆起地区则表现为较高的群速度分布特征: 随着周期的增加(>20 s)速度的特征有所改变, 30 s之后由于受地壳厚度的影响, 两者的速度差异逐渐缩小, 在中下地壳波速度随深度增加而增大。 同时, 研究发现在研究区域内西南方向的噪声源占主导作用。 相似文献
106.
本文提出一种保护断层、裂缝等地层边缘特征的结构自适应中值滤波器,用于衰减地震资料中的随机噪声.基于地震反射同相轴局部呈线型结构的假设,采用梯度结构张量估计地层倾向,分析地层结构的规则程度,在此基础上引入地震剖面中线型和横向不连续性两种结构特征的置信度量.结构自适应中值滤波器根据这两种置信度量调整滤波器窗函数的尺度和形状,根据地层倾角调整滤波器窗函数的方向,从而使得滤波操作窗能够最佳匹配信号的局部结构特征.将本文方法用于合成和实际数据的处理,并与两种常用中值滤波方法进行对比,结果表明,该方法能够更好地解决地震剖面的随机噪声衰减和有效信号保真的问题,在增强反射同相轴的横向一致性的同时有效保持了剖面内的地层边缘和细节特征,显著改善了地震资料的品质. 相似文献
107.
数学形态滤波是一种新型的非线性滤波方法.介绍了形态学滤波的基本原理,针对大地电磁信号表现出的非线性、非平稳性和非最小相位特性,综合结构元素特征我们提出一种基于数学形态滤波的大地电磁噪声压制方法.为了有效抑制目标信号中的噪声干扰及修正统计偏倚现象,通过选用合理的结构元素及形态开-闭、闭-开组合,将正、负结构元素级联构造组合广义形态滤波器对实测大地电磁信号进行噪声压制.实验结果表明,该方法是切实可行的,有效地剔除了大尺度干扰及基线漂移,较好地还原了大地电磁信号的原始特征,修正了标准形态算子所产生的统计偏倚现象,去噪精度高.该方法计算速度快,具有潜在优势,为矿集区海量大地电磁信号与强干扰的分离提供了一条新的途径,应用前景广阔. 相似文献
108.
基于中国地震科学探测台阵项目一期在南北地震带南段架设的300多个地震台站,利用2011年10月至2012年9月的连续观测记录,采用基于背景噪声互相关函数的面波层析成像技术,研究了青藏高原东南缘的云南地区面波群速度和方位各向异性分布.结果显示,地壳的面波快波方向呈现近南北向,整体表现出围绕东喜马拉雅构造结顺时针旋转的趋势,和地表GPS速度场以及S波分裂的快波方位较为一致.小江断裂东西两侧的快波方位有一定差异.对反映深度大概在下地壳和上地幔顶部的长周期面波,快波方向从近南北向逐渐向北西向过渡,在菱形块体附近,26°N以南,快波方向和红河断裂的走向趋于一致;其他区域相比上地壳的快波方向也有较大变化,这种结果较为支持青藏高原东南缘的云南地区壳幔变形的解耦.在滇西南,澜沧江向东弧形展布区域,中下地壳快波方位呈现局部的圆周旋转趋势,结合该区地震分布规律和应力主轴方向,推测这种现象和块体挤压及旋转具有一定相关性. 相似文献
109.
Ali Gholami 《Geophysical Prospecting》2014,62(6):1389-1405
Compressed Sensing has recently proved itself as a successful tool to help address the challenges of acquisition and processing seismic data sets. Compressed sensing shows that the information contained in sparse signals can be recovered accurately from a small number of linear measurements using a sparsity‐promoting regularization. This paper investigates two aspects of compressed sensing in seismic exploration: (i) using a general non‐convex regularizer instead of the conventional one‐norm minimization for sparsity promotion and (ii) using a frequency mask to additionally subsample the acquired traces in the frequency‐space () domain. The proposed non‐convex regularizer has better sparse recovery performance compared with one‐norm minimization and the additional frequency mask allows us to incorporate a priori information about the events contained in the wavefields into the reconstruction. For example, (i) seismic data are band‐limited; therefore one can use only a partial set of frequency coefficients in the range of reflections band, where the signal‐to‐noise ratio is high and spatial aliasing is low, to reconstruct the original wavefield, and (ii) low‐frequency characteristics of the coherent ground rolls allow direct elimination of them during reconstruction by disregarding the corresponding frequency coefficients (usually bellow 10 Hz) via a frequency mask. The results of this paper show that some challenges of reconstruction and denoising in seismic exploration can be addressed under a unified formulation. It is illustrated numerically that the compressed sensing performance for seismic data interpolation is improved significantly when an additional coherent subsampling is performed in the domain compared with the domain case. Numerical experiments from both simulated and real field data are included to illustrate the effectiveness of the presented method. 相似文献
110.
High thresholds for avoidance of sonar by free-ranging long-finned pilot whales (Globicephala melas)
R. Antunes P.H. Kvadsheim F.P.A. Lam P.L. Tyack L. Thomas P.J. Wensveen P.J.O. Miller 《Marine pollution bulletin》2014
The potential effects of exposing marine mammals to military sonar is a current concern. Dose–response relationships are useful for predicting potential environmental impacts of specific operations. To reveal behavioral response thresholds of exposure to sonar, we conducted 18 exposure/control approaches to 6 long-finned pilot whales. Source level and proximity of sonar transmitting one of two frequency bands (1–2 kHz and 6–7 kHz) were increased during exposure sessions. The 2-dimensional movement tracks were analyzed using a changepoint method to identify the avoidance response thresholds which were used to estimate dose–response relationships. No support for an effect of sonar frequency or previous exposures on the probability of response was found. Estimated response thresholds at which 50% of population show avoidance (SPLmax = 170 dB re 1 μPa, SELcum = 173 dB re 1 μPa2 s) were higher than previously found for other cetaceans. The US Navy currently uses a generic dose–response relationship to predict the responses of cetaceans to naval active sonar, which has been found to underestimate behavioural impacts on killer whales and beaked whales. The navy curve appears to match more closely our results with long-finned pilot whales, though it might underestimate the probability of avoidance for pilot-whales at long distances from sonar sources. 相似文献