High-Resolution Seismic Technique Through Reducing The Influences Of Unconsolidated Sediment

A low-velocity layer, i.e. unconsolidated sediment, is composed of medium whosevelocity is very low. Commonly, near surface is mainly referred to a low-velocity layer. Thecondition of near surface is very important to oil and natural gas seismic exploration. And thecondition of near surface has a critical influence on seismic resolution. On the one hand, theincompact medium of the earth's surface makes impossible for the good coupling betweengeophone and ground. Then the resolution of seismic signal collected from geophone isdepressed. On the other hand, unconsolidated sediment seriously attenuates high-frequencycomponent of seismic data. Therefore, the resolution of seismic data is reduced. In order tosolve the above problems, we mainly focus on the two aspects of geophone-ground couplingand seismic wave attenuation in low-velocity layers by introducing special coupling matchfilter technology, studying seismic wave attenuation laws and attenuation mechanism whenspreading in the low-velocity layers, presenting high frequency compensation methods basedon summaries of background, history, state of development trends of above problems.1. In order to solve the worse coupling of iron spike geophone (common couplinggeophone) and ground, seismic wave detect system (SWDS) was developed by Dong et. al..SWDS is also named special coupling geophone. Compared with the common couplinggeophone, the seismic signal from special coupling geophone has wider frequency bandwidthand higher fidelity. To a large degree, SWDS solve the disadvantage of seismic signal fromcommon coupling of geophone-ground. However, its cost is higher and the efficiency islower compared with the common coupling geophones. To make full use of advantage andovercome deficiency of SWDS, we brought forward special coupling matching filtertechnique based on SWDS.The special coupling match filters was designed through comparing the seismic signalsfrom common coupling geophone and special coupling geophone under the same acquisitioncondition. Then the seismic data from common coupling geophone were processed using thematch filter to increase resolution. Through applying the filter to the shot gathers fromcommon coupling geophones, the shot gathers were nearly equal to that from SWDSacquired. We have applied the special coupling match filters to the data from commoncoupling geophones in a desert area. The results show that frequency bandwidth of seismicdata from common coupling geophones is increased and resolution is improved after usingthe match filters. Now, because the application effect is good, the special coupling matchfilter technology has been commended to apply in the oil and natural gas seismic exploration.2. The few studies on seismic wave attenuation law focusing on near surface wereimplemented. And the results were concluded mainly through measuring rock sample inlaboratories. However, the results from labs don't accord with actual geology conditionusually, we studied seismic wave attenuations by comparisons when considering the cases inlow-velocity layers in desert area of Tarim and in clay area of Daqing of China using a lot ofup-hole or down-hole seismic data.Firstly, we studied the seismic attenuation in sand area. Through analyzing seismic datafrom many kinds of up-hole or down-hole seismic surveys, we find that there are four sandlayers in near surface and they are dry sand layer, moisture sand layer, fully saturated sandlayer above water table, and over saturated sand layer below water table. Because thedifference of seismic data from different blasting depth is mainly aroused by instant losing ofdetonators for common up-hole seismic (detonators blast in logs and geophone is buried onthe earth's surface), we designed various kinds of up-hole or down-hole seismic surveys todestroy the other influence and acquired different seismic wave that are mainly aroused bydifferent spreading distances in unconsolidated sediment. Through analyzing seismic data ofdifferent spreading distances and their spectral, we find that seismic wave was mainlyattenuated in dry sand layer and moisture sand layer in near surface in sand area. Theanalytical results from a special up-hole survey by using normal shot show that the frequencybandwidth of seismic wave can be reduced 20Hz and frequency peak value can be decreased15Hz for the attenuation by a 13m low-velocity layer. Because the medium of unconsolidatedsediment is mainly composed of sands, the attenuation of seismic wave is mainly related tothe thickness of low-velocity layers in lateral.Secondly, we analyzed seismic attenuation in clay area in Daqing of China. Because themedium of different depths in unconsolidated sediment is mainly composed of clay, and theircompact condition is nearly equal, the difference of seismic data from different blasting depthis mainly aroused by their different spreading distances. Therefore, the seismic data fromcommon up-hole survey in this district can be used to analyze attenuation influence ofunconsolidated sediment. In clay area of Daqing, using the up-hole survey data, we analyzedattenuation law of seismic signal when spreading in unconsolidated sediment for the twoaspects of lateral and vertical orientations. The results show that the attenuation is mainlyaroused by a 8m low-velocity layer in near surface. Because the medium of unconsolidatedsediment is composed of yellow clay, gray clay, sand and other rocks, and the components ofmedium change greatly in lateral, the attenuation of seismic wave is related to the thicknessand lithology of unconsolidated sediments in lateral as well. Through comparing the inversefilters of the two districts, we find that the seismic wave attenuation is much higher in Tarimthan in Daqing.The above conclusions can be used to guide the seismic data collection and processing.At the same time, based on the conclusions, high frequency compensation is implementedeasily for spatial changing.3. Based on the detailed analysis on velocity structure of unconsolidated sediments,through adopting linear transitional layer model, we discussed attenuation mechanism ofseismic wave through unconsolidated sediments. Firstly, we analyzed velocity structure ofunconsolidated sediments in sand area and clay area using common up-hole seismic survey indetail. The results show that there are velocity gradient layers in low-velocity layers of sandarea. But in clay area, the phenomenon is infrequent. Secondly, based on the velocitystructure, by introducing linear transitional layer theory, we designed three kinds of modelsand simulated spreading characteristic of seismic wave in sand area. The results show thatvelocity gradient layers can attenuate seismic wave when the thickness is larger. At last,comparing simulation results with practical seismic data from up-hole or down-hole survey,we proved that velocity gradient layer zone is the main factor for seismic wave attenuation inTarim sand area.In the linear transitional layers, because there are many velocity interfaces, seismicwave can be reflected time after time and many reflections and transmissions cause energyattenuated and frequency lost. This study is also shows that elastic medium can attenuateseismic wave when there is a velocity gradient layer in it. For Daqing district, there is novelocity gradient layer in the unconsolidated sediment, thus seismic wave attenuationmechanism is related to the viscous of medium.4. In order to compensate high frequency components of seismic waves attenuated byunconsolidated sediment, we brought forward a high frequency compensation method(named HFC). The unconsolidated sediments were regarded as a system and the inversefilters of the system were computed by the method of Optimum Wiener Filters using theseismic data from up-hole and down-hole survey. And the attenuation was compensated usingthe inverse filters. Through processing the simulation data and practical seismic data, theresults show that the high frequency compensation method is efficient and acceptable.Compared with other high resolution processing methods, HFC has a clear physical meaningbecause the inverse filters are designed using practical up-hole and down-hole seismicsurvey.The processing results on seismic stacked data in sand area and clay area show that thehigh frequency components are increased and the resolution is heightened after compensation.The efficient frequency bandwidth is heightened nearly 30Hz after HFC processing. Throughanalyzing frequency swept profiles before and after the compensation and the comparingwith synthetic seismic record derived by using sonic log in Daqing district, the results showthat the high frequency components increased is believable and reliable. The comparing oftime slices from the seismic data compensated and uncompensated proves that HFC isefficient. The processing results in sand area and in clay area both show that the profilescompensated is better according with practical geologic condition. At the same time, the lowfrequency components are nearly maintained.Moreover, the efficient use in sand area and clay area proves that HFC can compensateseismic wave attenuation aroused by different attenuation mechanisms in unconsolidatedsediment. Since HFC has been applied in the practical seismic data processing and hasacquired a better effect, it has begun to be accepted by some oil and gas companies.