A PNEUMATIC SEISMIC ENERGY SOURCE FOR SHALLOW WATER / MARSH AREAS*

Non dynamite seismic energy sources have shown only limited success in difficult shallow-water or marsh prospect areas, and none have applicability to both environments. Recently, a method has been devised to adapt a marine pneumatic source for operation in abrasive surroundings. A system composed of four of these modified sources together with associated emplacement and retrieval mechanisms has been developed and has proven to be a reliable and efficient seismic energy source for both shallow-water and marsh applications.     

PNEUMATIC ACOUSTIC ENERGY SOURCE*

In recent years considerable work has been done to devise a satisfactory non-dynamite seismic system that would replace dynamite in offshore areas. Prior to the advent of digital recording and processing, the non-dynamite sources have generally not provided the depth of penetration or the resolution required for satisfactory seismic interpretation. More recent developments in non-dynamite offshore marine sources include adaptation of the Vibroseis from a land unit to a marine unit, and adaptation of the Dinoseis unit from a land to a marine unit. The SUE (Seismic Underwater Explorer) system is a thermodynamic non-dynamite source utilizing a mixture of propane and oxygen detonated in a special chamber approximately 15 feet below the water surface. This source gives penetration to more than 4 sec in areas typified by Gulf of Mexico type geology and shows deeper penetration than had previously been obtained by dynamite along the western United States in areas with 20 lb charge limitations. A pneumatic source, the airgun, has been in production use in the United States since June 1966. This non-dynamite source provides an intriguing amount of versatility and can be expanded to provide additional energy as necessary to obtain the penetration desired. Tests using systems comprised of from eight to twenty-three airguns show penetration in excess of 5 seconds in many areas. Power spectra comparisons both in amplitude and frequency content demonstrate that this is a controlled source generating a controlled seismic wavelet and a controlled frequency spectrum that can be tailored to fit requirements of particular areas. Sample sections obtained in the Gulf of Mexico and the Pacific Ocean offshore California show adequate penetration to 5.0 seconds reflection time. Quantitative measurements with the airguns demonstrate the effect of:

• 1 Variation of the number of guns in the system;
• 2 Shaping the frequency spectrum by using different sizes of airguns in the system;
• 3 Effects on signal-to-noise ratios as a result of stacking several small energy sources together;
• 4 Reproducibility of the initial pulse wavelet from shot to shot.

The improvement in record quality as a result of advanced digital processing with non-dynamite sources is comparable to that obtained with dynamite sources. Non-dynamite sources make additional improvements possible where high source multiplicity is advantageous. Excellent dynamic correlations yield accurate velocity control as well as definitions of apparent velocities attributable to multiples and primary-to-multiple amplitude relationships. Non-dynamite sources are being used more and more extensively in offshore exploration. The advent of digital recording and processing provides a means for improving depth of penetration and resolution of many non-dynamite sources.     

A NEW PARAMETER FOR AEROMAGNETIC SURVEYING*

The author proposes the parameter Δ²/ΔT¹ for possible application in aeromagnetic surveying making use of already available gradiometer systems equipped with sensors separated vertically.     

SOURCE ARRAY SCALING FOR WAVELET DECONVOLUTION*

A seismic source array is normally composed of elements spaced at distances less than a wavelength while the overall dimensions of the array are normally of the order of a wavelength. Consequently, unpredictable interaction effects occur between element and the shape of the far field wavelet, which is azimuth-dependent, can only be determined by measurements in the far field. Since such measurements are very often impossible to make, the shape of the wavelet—particularly its phase spectrum—is unknown. A theoretical design method for overcoming this problem is presented using two scaled arrays. The far field source wavelets from the source arrays have the same azimuth dependence at scaled frequencies, and the far field wavelets along any azimuth are related by a simple scaling law. Two independent seismograms are generated by the two scaled arrays for each pair of source-receiver locations, the source wavelets being related by the scaling law. The technique thus permits the far field waveform of an array to be determined in situations where it is impossible to measure it. Furthermore it permits the array design criteria to be changed: instead of sacrificing useful signal energy for the sake of the phase spectrum, the array may be designed to produce a wavelet with desired amplitude characteristics, without much regard for phase.     

PHYSICAL ASPECTS OF THE “AIRPULSER” AS A SEISMIC ENERGY SOURCE*

During the last few years the airpulser, or air gun, has become very common as an energy source for marine seismic surveys. This paper describes the physical processes which take place during the operation of the pulser and develops theoretical results concerning the energy and frequency of the radiated signal and the amplitude decay of the secondary bubble pulses. The theory takes into account the presence of the airpulser itself which is assumed to be a rigid sphere within the bubble of released air. The theoretical results are combined and compared with measurements made of the pressure within the airpulser, the acceleration of the body of the pulser, and the amplitude and frequency of the signal radiated into the surrounding water. A formula for calculating the bubble frequency is given and a diagram made of the energy partition between mechanical losses, radiated energy, etc. Finally, a comparison is made of the energy release from the airpulser with that from TNT.     

FIXED SOURCE SYSTEM IN A CONDUCTIVE ENVIRONMENT *

Measuring systems with a fixed source fed with alternating current at sound frequency are used almost exclusively at the present time for a quick search for conductive zones in an environment with a high specific electrical resistivity. Owing to this fact it is useful to measure in a gradient configuration, i.e. to compare the quantities of the corresponding parameters of the field in two relatively close points oriented in the direction of the assumed change of the field. In this way we can obtain a suppression of the impact of the wider environment, since, in principle, we record the first derivative, i.e. the amount of the change only. The present article points out some possibilities of the application of the system with a fixed source in non-magnetic environment with arbitrary resistivity.     

WAVELET DECONVOLUTION USING A SOURCE SCALING LAW*

We present a new method for the extraction and removal of the source wavelet from the reflection seismogram. In contrast to all other methods currently in use, this one does not demand that there be any mathematically convenient relationship between the phase spectrum of the source wavelet and the phase spectrum of the earth impulse response. Instead, it requires a fundamental change in the field technique such that two different seismograms are now generated from each source-receiver pair: the source and receiver locations stay the same, but the source used to generate one seismogram is a scaled version of the source used to generate the other. A scaling law provides the relationship between the two source signatures and permits the earth impulse response to be extracted from the seismograms without any of the usual assumptions about phase. We derive the scaling law for point sources in an homogeneous isotropic medium. Next, we describe a method for the solution of the set of three simultaneous equations and test it rigorously using a variety of synthetic data and two types of synthetic source waveform: damped sine waves and non-minimum-phase air gun waveforms. Finally we demonstrate that this method is stable in the presence of noise.     

ELECTROMAGNETIC DEPTH SOUNDING USING A SOURCE ORIENTATION MODE*

Electromagnetic depth sounding using source orientation as the sounding variable provides advantages in instrumental simplicity and operational flexibility when compared with conventional modes of electromagnetic sounding. The ease with which the technique may be simulated in a scale model permits its application to sounding over localized structures. The theoretical approach to interpretation is at present limited to structures which approximate a perfectly conducting half space. However, scale model tests suggest that the perfect conductor theory may be applicable to many localized structures.     

新型抗震耗能剪力墙结构的振动台试验与分析

本文提出了抗震耗能剪力墙结构的新型式，新型剪力墙的墙板中人为地设置了竖缝，并在竖缝中设置各种型式的耗能装置来控制结构的动力性能，如：结构的侧向刚度、延性、自振频率等，为了验证这一设想，本文进行了两个１０带层竖缝剪力墙模型的振动台试验，通过试验数据分析和结构非线性动力反应计算分析对两个模型的抗震性能进行了研究。     

A RECTANGULAR LOOP SOURCE OF CURRENT ON A TWO-LAYERED EARTH*

Using the concept of reciprocity and the known solution for the electric field of a vertical oscillating magnetic dipole source placed over a two-layered halfspace, an integral expression for the vertical magnetic field produced by a horizontal rectangular loop, carrying an oscillating current and placed on the surface of the same halfspace, is deduced. This expression is such that it can be evaluated by a combination of straightforward numerical integration and digital linear filter techniques. Displacement currents everywhere in space are neglected. Fields both inside and outside the loop are presented as sounding curves. A little over a decade of frequencies (100–3000 Hz) appears to be the minimum requirement and two decades (100–10000 Hz) are sufficient for most situations. Parametric and geometric sounding curves both show good resolution of subsurface layering. Phase shows better resolution than amplitude. Similarly, resolution is higher when conductivity increases with depth than when it decreases. Besides being useful for the electromagnetic depth sounding for layered earth structures, such computations can be applied to determine normal corrections to Turam observations, whenever a rectangular loop of finite size is used as a transmitter.     

关于一种新型横波震源（SH型）的探讨

实现横波（SH波）勘探的关键问题是震源。现有的SH波震源--扭力振动器的缺点是辐射方向性不好,能量集中在水平方向,而在垂直方向辐射的能量几乎为零,这样的震源对于反射波勘探是不利的。 本文从理论上提出用垂直力矩构成横波震源。理论推导结果（高频渐近解）表明,该震源在方位角θ=90°方向上辐射纯SH波,SH波能量辐射集中在垂直方向,而在水平方向几乎为零。显然,这样的震源对于反射波勘探十分有利。 文中还采用Cagniard-deHoop回路积分技术计算了波场的精确解及最大振幅随离源角的变化,结果与解析解一致.     

THE POOR MAN'S SEISMIC SOURCE: A COMPUTER GAME *

Basic requirements for a seismic reflection system using a moderate-power controlled source are discussed. General considerations and computer modelling show promise for geophysical prospecting, especially at low penetration and very high noise level. For many applications, ranging from mineral and water prospecting, to civil and military engineering, and even archeology, a relatively cheap, small and portable device of this kind might be more convenient than conventional explosive sources or sophisticated controlled sources of the vibroseis type. The use of a simple, low frequency periodic signal facilitates rapid data processing procedures and suggests that the mechanical generator will be very simple and cheap.     

关于一种新型横波震源（SH型）的探讨

实现横波（SH波）勘探的关键问题是震源。现有的SH波震源——扭力振动器的缺点是辐射方向性不好,能量集中在水平方向,而在垂直方向辐射的能量几乎为零,这样的震源对于反射波勘探是不利的。 本文从理论上提出用垂直力矩构成横波震源。理论推导结果（高频渐近解）表明,该震源在方位角θ=90°方向上辐射纯SH波,SH波能量辐射集中在垂直方向,而在水平方向几乎为零。显然,这样的震源对于反射波勘探十分有利。 文中还采用Cagniard-deHoop回路积分技术计算了波场的精确解及最大振幅随离源角的变化,结果与解析解一致.     

A NEW PARAMETER FOR THE INTERPRETATION OF INDUCED POLARIZATION FIELD PROSPECTING (time-domain) *

In a previous paper it has been shown that we can relate the transient IP electric field E_p , existing in a rock after a step wave of polarizing current, with the steady-state current density J_ss during the current step wave as follows: E_p =ρ' J_ss . This relation may be interpreted as a generalized Ohm's law, valid in linear cases, in which ρ’(fictitious resistivity) is defined as the product of the true resistivity ρ with the chargeability m. Supposing E _p=— grad U_p and applying the divergence condition div J_ss = o, one can, for a layered earth, obtain a general expression for the depolarization potential U_p as a solution of Laplace's equation ?²U_p= o. Since the mathematical procedure for the solution of this last equation is identical to that used in resistivity problems, we propose now the introduction of an apparent fictitious resistivity ρ'_a (defined as the product of the apparent resistivity ρ_a with the apparent chargeability m_a) as a new parameter for the interpretations of IP soundings carried out over layered structures with a common electrode array. The most general expression of ρ'_a as a function of the electrode distance turns out to be mathematically identical to the general expression of ρ'_a. Therefore it is possible to interpret a ρ'_a field curve using the same standard graphs for resistivity prospecting with the usual method of complete curve matching. In this manner a great deal of work is saved since there is no need to construct proper m_a graphs for the interpretation of IP soundings, as it has been done up to now. Finally some field examples are reported.     

更新过程与概率分配模式在潜在震源区发震概率计算中的应用

采用双泊松模式更新过程研究华北的北部地区各级地震的两个发震速率,并对各速率进行检验,最后利用概率分配模式,求出各潜在震源区未来50a的发震概率     

A NEW APPROACH TO VIBROSEIS DECONVOLUTION*

A method is proposed to obviate the shortcomings of conventional deconvolution approaches applied to vibroseis data. The vibroseis wavelet reduces the time domain resolution of the earth's impulse response by restricting its passband. The spectrum of the wavelet is assumed to be a “low quefrency”phenomenon, and hence it can be estimated by low cut cepstral filtering. The wavelet's amplitude spectrum can then be removed by spectral division. By using an approach which is consistent with the principle of maximum entropy, the undetermined portions of the seismogram's Fourier transform can be filled in by autoregressive prediction. The process of initially deconvolving in a restricted passband reduces the enhancement of noise contaminated parts of the spectrum, and the spectral extension scheme increases the time domain resolution of the process.     

THE P400 WATERGUN SOURCE IN THE QUEST FOR IMPROVED RESOLUTION AT ALL DEPTHS*

The low-frequency response of the P400 watergun is an improvement over that of the S80 version. It has been further enhanced by deployment in a vertically staggered array. The notch in the amplitude spectrum at about 35 Hz due to the interference between the precursor and main implosion pulse has been virtually eliminated, and depth notches due to the free surface interface at the upper end of the spectrum have been greatly reduced. The spectral band is thus very broad and well-shaped and corresponds to an even shorter signature. After convolution with the streamer ghost, the amplitude of the implosion pulse from the composite far-field signature is more than 30 times the composite precursor amplitude. Thus the signal approaches the minimum-phase condition. Shot-generated noise scattered back by diffractors located at or near seabed is the single greatest impediment to increased penetration, especially in high resolution surveys. Such noise decays much less rapidly than signal, particularly so with broad band data. Furthermore, back-scattered interference coming from a cone of bearings between 15–45° with respect to the line is particularly damaging since in these directions the stack enhances the interference and confuses the primary velocity analysis. The watergun source patterns are therefore element-weighted to maximize directivity within these critical directions in the frequency band 50–155 Hz. When spatial aliasing is reduced by increased streamer spatial sampling and by discrete anti-alias filters during the record cycle,f-k filtering can be applied to the field records as an effective supplement to the source and receiver patterns without the aliased ‘wrapped around’ noise alignments destroying the upper end of the spectrum of genuine reflectors. Furthermore, increased spatial sampling is a pre-requisite in thef-k-migration process if the valuable higher frequencies of broad band data are to be migrated in steep-dip situations without aliasing. Thus, spatial resolution is a necessary complement to temporal resolution if maximum advantage of the broad-band P400 watergun source is to be realized.     

A NEW APPROXIMATE METHOD FOR DIRECTLY INTERPRETING GRAVITY ANOMALY PROFILES CAUSED BY SURFACE GEOLOGIC STRUCTURES*

For outcropping bodies an approximate direct interpretation of the associated gravity anomaly is generally obtained with the flat plate formula. Results can be significantly improved if the causative body is approximated by a bell shape instead of a flat plate. A set of parameter curves allows the conversion to depth data. The validity of the method is borne out by synthetic models and by field examples in a Nevada valley with Tertiary and in the Los Angeles Basin. The method provides structural definition more accurate than can be obtained with the flat plate formula, particularly in the case of narrow anomalies.     

剪切位错内源的反射法

本文将计算综合地震图所用的反射法,推广于层状介质中的剪切位错内源情况。