REALIZATION OF SHARP CUT-OFF FREQUENCY CHARACTERISTICS ON DIGITAL COMPUTERS

It was found in Part I of this paper that approximating the sharp cut-off frequency characteristic best in a mean square sense by an impulse response of finite length M produced a characteristic whose slope on a linear frequency scale was proportional to the length of impulse response, but whose maximum overshoot of ±9% was independent of this length (Gibbs' phenomenon). Weighting functions, based on frequency tapering or arbitrarily chosen, were used in Part II to modify the truncated impulse response of the sharp cut-off frequency characteristic, and thereby obtain a trade-off between the value of maximum overshoot and the sharpness of the resulting characteristic. These weighting functions, known as apodising functions, were dependent on the time-bandwidth product Mξ, where 2ξ, corresponded to the tapering range of frequencies. Part III now deals with digital filters where the number 2N–1 of coefficients is directly related to the finite length M of the continuous impulse response. The values of the filter coefficients are taken from the continuous impulse response at the sampling instants, and the resulting characteristic is approximately the same as that derived in Part II for the continuous finite length impulse response. Corresponding to known types of frequency tapering, we now specify a filter characteristic which is undefined in the tapering range, and determine the filter coefficients according to a mean square criterion over the rest of the frequency spectrum. The resulting characteristic is dependent on the time bandwidth product Mξ= (N–1/2)ξ up to a maximum value of 2, beyond which undesirable effects occur. This optimum partially specified characteristic is an improvement on the previous digital filters in terms of the trade-off ratio for values of maximum overshoot less than 1%. Similar to the previous optimum characteristic is the optimum partially specified weighted digital filter, where greater “emphasis is placed on reducing the value of maximum overshoot than of maximum undershoot”. Such characteristics are capable of providing better trade-off ratios than the other filters for maximum overshoots greater than 1/2%. However these filters have critical maximum numbers 2.N_C–1 of coefficients, beyond which the resulting characteristics have unsuitable shapes. This type of characteristic differs from the others in not being a biassed odd function about its cut-off frequency.     

数字地震仪传递函数幅频特性及其应用的分析

数字地震资料是以一种无量纲的“数字数”(Counts)形式记录的，但在数字地震资料的应用中，却需要实际的物理量；而“数字数”与实际的物理量联系的桥梁是数字地震仪传递函数。根据这一实际需要，分析了数字地震资料的产生过程、数字地震仪传递函数的幅频特性以及“数字数”与数字地震仪传递函数的关系。这对我们的实际工作具有一定的现实意义。     

根据数字滤波器的频率响应优选数字处理方法

本文根据数字滤波器压制干扰的原理,指出利用数字滤波器的频率响应、优选数字处理方法和确定最佳滤波参数的具体原则。对于那些未给出频率响应函数或周期选择性函数的数字处理方法,可以直接根据计算方法的权系数算出它们的频率响应。计算递归滤波器的频率响应式子,也可以用来计算其它非递归滤波器的频率响应。本文利用重力固体潮汐分析中计算中心点的零点飘移值若干方法的系数和它们的周期选择性函数,分别计算了它们的频率响应,从两者的数值上比较,结果是一致的。本文还计算了目前大家常用的一些数字处理方法,如五日均值、一阶差分、二阶差分、多点数字平滑等方法的频率响应值。并分析了它们的处理效果和方法的局限性。还根据最佳数字滤波器和DAI数字滤波器的频率响应曲线,说明选择数字滤波器和确定最佳滤波参数的具体方法。     

长春地震台宽频带数字地震波形频谱特性的初步分析

依据长春地震台数字测震资料,计算了不同震中距地震波形的振幅谱,并对主要震相的频率特性进行了初步分析。     

数字磁变仪的设计与实现

本文介绍了石英光电磁变仪及其数字化采集，记录的设计和实现，并讨论了数字磁变仪系统在地磁台站的应用结果。     

安徽省数字化地电场特征初步研究

本文基于安徽省现有的蒙城和嘉山数字化地电场数据,辅以其他地区一些典型地电场台站的数据,通过日变形态、变幅、相关性和频谱分析等方法对数字化地电场的基本特征进行研究,发现多台地电场变化既具有普遍的相似性又各有差异,体现了地电场的全球性和区域性。     

OPTIMUM DIGITAL FILTERING AND INVERSE FILTERING IN THE FREQUENCY DOMAIN*

Two distinct filters are developed in the frequency domain which represent an attempt to increase the resolution of fine structure contained in the signal whilst keeping the expected filtered noise energy within reasonable bounds. A parameter termed the White Noise Amplification is defined and used together with a measure of the deconvolved pulse width in order to provide a more complete characterisation of the filters. Each of the two main types of frequency domain filters discussed varies in properties with respect to a single adjustable parameter. This may be contrasted with a time domain Wiener filter which in general has three variables: length, delay and an adjustable noise parameter or weight. The direct frequency domain analogue of the Wiener filter is termed a gamma-Fourier filter, and is shown to have properties which span the range from those of a spiking filter with zero least square error at one extreme, to those of a matched filter at the other extreme of its variable parameter's range. The second type of filter considered—termed the modulated Gaussian filter—is similarly shown to be a perfect spiking filter at one extreme of its parameter range, but adopts the properties of an output energy filter at the other extreme.     

无锡市数字遥测地震台网建设的体会

无锡市数字遥测地震台网是按照无锡市防震减灾十年目标纲要建设的，目前已正式投入运行。我局综合各方专家意见和资料，对台网的组成，台站的布局、选择、建设取得了一些经验。总结建设的体会，供其它台网建设时参考。     

PRINCIPLES OF DIGITAL WIENER FILTERING*

The theory of statistical communication provides an invaluable framework within which it is possible to formulate design criteria and actually obtain solutions for digital filters. These are then applicable in a wide range of geophysical problems. The basic model for the filtering process considered here consists of an input signal, a desired output signal, and an actual output signal. If one minimizes the energy or power existing in the difference between desired and actual filter outputs, it becomes possible to solve for the so-called optimum, or least squares filter, commonly known as the “Wiener” filter. In this paper we derive from basic principles the theory leading to such filters. The analysis is carried out in the time domain in discrete form. We propose a model of a seismic trace in terms of a statistical communication system. This model trace is the sum of a signal time series plus a noise time series. If we assume that estimates of the signal shape and of the noise autocorrelation are available, we may calculate Wiener filters which will attenuate the noise and sharpen the signal. The net result of these operations can then in general be expected to increase seismic resolution. We show a few numerical examples to illustrate the model's applicability to situations one might find in practice.     

青藏高原北部地区地震频度短期异常特征研究

在青藏高原北部地区活动地块划分的基础上,以地块为研究单元,对1970年以来的小震频度进行了全时空扫描,以研究其短临时间特征．结果表明：虽然受地域和地质构造的影响,各地区小震频度的短临异常与5级地震的对应率不相同,但绝大多数地区的小震频度正、负异常的起始时间、最大(最小)值时间和结束时间均出现在中强地震前的l～6个月．对结果统计检验,本区R≥R97.5％的合格率为69％,内、边邻地区合计R值合格率为94％．     

新疆地下流体数字化改造项目中的几个特点

新疆地下流体数字化改造只对温泉、库尔勒和乌什3个观测站进行了改造.这3个观测站所处的环境和条件相差很大,各有特色,在改造中没有统一的模式,而是根据各自的特点结合新疆的实际情况,采取了观测室与水井分离、用水位仪观测流量、扩频微波传输和同孔多项综合观测等措施进行了改造.实践表明这些措施的技术和方法在地下流体台网建设中有较大的参考和推广价值.     

山东形变固体潮数字化资料的干扰因素分析

利用山东省数字化形变观测记录到的固体潮资料,分析了气压、雷电、洞室装修、仪器故障及环境等干扰因素对数字化形变固体潮记录的影响,总结了1次地震前的异常特征,为利用数字化资料及时识别干扰、准确捕捉短临地震异常提供了可参考的实例。     

BASIC PRINCIPLES OF TWO-DIMENSIONAL DIGITAL FILTERING *

The geophysicist involved in the analysis of two-dimensional data should have an understanding of the two-dimensional finite Fourier transform and the mechanics of two-dimensional filtering. Frequency aliasing must be considered when working with sampled data. In two dimensions it is advantageous to consider aliasing in terms of the overlap of the repeating spectra inherent in the finite Fourier transform. Two-dimensional filtering can be performed as a transient convolution in the space domain, as cyclic convolution utilizing the frequency domain or as the multiplication of polynomials using the z-transform. If the “edge” effects are removed, the results of the three methods are identical.     

THE PHASE PROPERTY OF A FINITE REALIZATION OF RANDOM NOISE AND ITS INFLUENCE ON DECONVOLUTION *

A finite realization of a discrete random noise process may be considered as a one-sided energy signal. Its phase property can then be described by means of the center position. The samples of such a realization are the components of a random signal vector and the center position is therefore a random variable. A statistical analysis shows that the expected value of the center position equals half the time duration of the realization. This implies that the Z-transform of the realization may be expected to have an equal number of poles and zeros inside and outside the unit circle. The standard deviation from the expected value of the center position is shown to depend on the time duration of the realization and on the autocorrelation of the process. It follows that, for processes that can be described by the convolution of a white series and a disturbance wavelet, the center position is independent of the phase property of the wavelet. A conclusion based on these results is that the homomorphic technique of wavelet estimation through cepstrum stacking must give questionable outcomes. Another conclusion is that the super-position of a realization of random noise on a minimum phase wavelet will in general give a mixed phase resulting signal. It is pointed out that schemes for the derivation of deconvolution filters do not take account of this phenomenon.     

辽宁区域地震短波通信传播特性和频率选择探讨

辽宁区域地震短波通信网对电磁波在无线通信中的传播特性及频率选择等问题，没有进行深入的理论研究和探讨。笔者将通信工作的实践经验升至理论的高度，研究探讨辽宁区域地震短波通信传播特性和频率选择办法，达到在短波数据通信中，选择最佳频率，增加沟通率的目的     

中、强震数字记录波形及震相特征分析

本文从地质构造、波列特征、P波初动、S-P到时差、主要震相、最大振幅、震中距等方面着手对沈阳台记录到的中、强震进行分析,力求归纳出国内和国外不同区域的波形和震相特征。     

新疆历史地震前兆模拟资料转入数字化平台的实现

近年来随着数字化技术的不断发展、成熟和稳定,我国的地震前兆观测技术也逐步实现由模拟观测向数字观测转变,地震前兆数字化观测技术日趋成热、稳定,地震前兆数字化观测普及程度越来越高,产出的数字化观测资料已得到越来越广泛的应用。通过介绍新疆数字地震前兆台网中心模拟资料和数字化资料的管理,分析了历史模拟数据转入数字化平台的实现方法,阐述了地震前兆资料进入数字化统一平台的实际应用。     

云南地区4级地震频度异常特征与强震关系研究

对云南地区18次MS≥6强震事件前中、小地震活动过程进行了分析,发现在这些地震发生前1—3年近场区2°×2°范围内4级地震频度基本上出现了显著增强现象,可以用4级地震年累计频度进行定量描述。通过全时空的扫描,确定年累计频度N≥4为异常阈值,该指标通过了置信度97.5%的R值内符检验。并用调整单元对强震附近4级地震活动增强现象给予了物理解释。     

DIGITAL PROCESSING OF REFRACTION DATA STUDY OF FIRST ARRIVALS*

It has been necessary to resort to the use of “long-line” refraction marine operations in certain areas where it proved impossible to eliminate singing from reflection records despite the number and variety of programs at our disposal for this purpose. Experience has shown that manual processing of offshore refraction records takes a disproportionate length of time in comparison with the surveys themselves, and this is incompatible with the requirements for choosing the site of an exploration well. It thus became necessary to find an “industrial approach” to the solution of this processing problem. It was apparent that automatic picking could also facilitate the interpretation of land refraction data, and that in the case of both marine and land work the interpretations would be more accurate when factors were taken into account which could not be considered when working without the aid of a digital computer. For these reasons a set of programs was developed for automatic picking and interpretation of refracted arrivals. The picking itself consists in searching for the maximum values of the normalized cross-correlation functions of the traces with a “model” trace. The first results thus supplied are: “picked” times, intercept times, maximum values of the correlations, and the values of the tie constants between overlapping spreads. Next, the construction of the relative intercept time curves is performed; a statistical analysis of these curves then allows the determination of the offset distance. From these elements, ⊙ either the delay time curve is produced, after ensuring correct reciprocal times by means of additional minor corrections. Such work is carried out in order to enable the geophysicist to gain a sound idea of the quality of the interpretation. To assist in this aim, part of the trace on both sides of the pick is plotted on the final documents. Valid groupings of several traces involving the same amount of refraction data are thus possible. ⊙ or the refractor depth is constructed with the wavefront method, making use of the relative intercept times. Such a procedure, which is normally applied to first breaks, can also be used for later arrivals exhibiting slight interference and should represent an important step towards processing secondary arrivals with high interference. The development of this package, in response to a need which is shared by both SNPA and CGG, is the result of the joint efforts of the Geophysical Group of SNPA's Pau Research Center and CGG's Technical and Scientific Departments.     

周期—频度谱方法在大庆台网数字化地震记录分析中的应用

论述了周期－频率谱方法在分析数字化地震记录中的应用。结果表明该方法对识别各类振动的性质较为有效，所形成的一整套分析流程适用于各种地震数字记录分析。