CHALLENGES IN LAND SEISMIC DATA ACQUISITION

9.1 CHALLENGES IN LAND SEISMIC DATA ACQUISITION

With the continuous development of petroleum exploration, seismic prospecting on land is mainly operated in the more and more complicated mountain, desert, Gobi, loess plateau and swamp areas. The difficulties for seismic exploration include the very complicated near surface and subsurface structures, the difficult data acquisition, too much coherent and random noise, and the seismic data with poor S/N ratio and resolution. However the exploratory development in these areas has a high requirement on the precise of seismic data.

The issue is how to deal with such complicated sedimentary basins with rich oil and gas. The basic issue is of penetration of energy through such complicated near surface and receiving the same after reflection. The tools that we have at our disposal are the choice of energy source & receivers and the pattern of their deployment. This is iterated through modeling studies till we are close to what we desire.

The major challenges in land seismic data acquisition comes from

a)      Complicated near surface

b)     Complicated subsurface structures

c)      Seismic Acquisition in difficult terrain in the form of mountains, desert, swamp areas etc.

d)   Statics in Complicated Surface Area

e)       Too much coherent and random noise

f)       Migration issues

a) Complicated near surface structure.

Firstly, The near surface layer is a loosen structure with an extremely thick weathering layer and a poor receiving and shooting condition; secondly, It is difficult to accurately establish a near surface model with a violently undulating surface in variable lithology that is weathered seriously in great different degrees. So static correction is a tough problem; thirdly, the scatter interference is serious and the S/N ratio is quite poor with old strata outcrop area, dip subterranean formation and crisscross network of gullies distributed everywhere.

b) Complicated near surface structures.

Firstly, the complicated overthrust nappe makes the velocity model very complex; secondly, the developed fractures and steep dipping layers result in a complicated seismic wave field which makes seismic imaging very difficult. The processing of a data having complicated near surface structures is equally difficult and complex.

Solution and Application

Aiming to the technical difficulties faced in inland exploration areas, we have spent a lot of energy to do research on acquisition technique, static correction technique, noise elimination technique and migration imaging technique etc. Through the research work in the past few years, we have achieved a good result of the seismic exploration in complicated area, and many favorable hydrocarbon structures and fields have been discovered, which has promoted the progress of oil and gas exploration technology effectively.

c)   Seismic Acquisition Technology in Complex Area

i)   Target-oriented Recording Geometry Designing Technology.

Due to the complicated underground structure in the complex area and in order to obtain an appropriate recording geometry, we make the designing on the basis of the forward model method in the following procedure: Firstly, build a geological model based on the existing interpretation result; secondly, implement analog shooting in the geological model; and finally design acquisition parameters for recording geometries in different sections in accordance with reflection information received in different shooting sections

ii)  Application Technology of Satellite Remote Sensing Data.

In complex areas, the surface condition is extremely complicated. There are various landforms and physiognomies such as fault scarps, gulches and deserts that make the acquisition very difficult; in addition, the near surface structure is also very complicated. There exist areas with different shooting and receiving conditions such as stratum outcropping areas, desert areas covered with extremely thick sand and gravel accumulation areas in the hills. How to lay out and survey the shooting and receiving points and try to improve the shooting and receiving conditions in these kind of areas are of great importance. In the complex areas, due to interference by various factors such as traffic, transportation and visual field, it is hard to avoid blindness although we have made a complex survey in the field and optimized the shooting and receiving points indoor with large-scale relief maps. In view of this point, we lay out the physical points indoor directly on the high-resolution satellite sensing data in accordance with topographic and geomorphic features; then the survey team can operate with a definite object in the field, which can make the surveying and layout work fulfilled more scientifically and effectively; in the meantime, satellite data also plays an important role in controlling and laying out the surface investigation points, selecting test points and operation organization and management.

iii)  Composite Recording Geometry Designing Technology

In an area where the surface condition is very complicated, the points designed theoretically may not be totally used or not be used due to numerous obstacles distributed within the area; however, since the shooting and receiving conditions are very poor in some sections, it may decrease the data quality when operating with the normal recording geometry. In view of this point, we adopt the composite recording geometry designing technology. In this way, we have avoided the obstacles or the sections with poor shooting conditions to the greatest extent (figure 2) so as to insure the acquisition quality.

iv)  High Sampling Acquisition Technique

In the area where both near surface and underground structures are complicated or higher precision data is needed, it has been the problems that geophysicists care about how to improve the S/N ratio and the resolution of seismic data. In terms of this problem, we have introduced small group interval and high sampling rate acquisition technology. The advantage of this method is that the continuity of the relections increases with the number of traces within the migration aperture, which is favorable to the imaging of seismic waves while guaranteeing that the seismic data can be in a higher resolution as well. We use trace and CMP arrays indoors to suppress the noises along the line.

d)  Statics in Complicated Surface Area

i)       Surface Investigation

Since the surface structure is very complicated in the complex area and with a violent fluctuation, it is difficult for the single surface investigation to find out the variation of the surface; therefore, we introduced the multiple surface investigation method. Besides the conventional refraction and logging, we have also adopted the deep-hole logging, VSP, outcrop logging and non seismic near surface investigation methods. On the basis of this, we establish the model comprehensively with the multiple surface results to guarantee the precise of the surface model.

ii) Intermediate Reference Datum Static Correction

Since the surface structure is very complicated in the complex area and with a violent fluctuation, it is difficult for the single surface investigation to find out the variation of the surface; therefore, we introduced the multiple surface investigation method. Besides the conventional refraction and logging, we have also adopted the deep-hole logging, VSP, outcrop logging and nonseismic near surface investigation methods. On the basis of this, we establish the model comprehensively with the multiple surface results to guarantee the precise of the surface model.

iii) Intermediate Reference Datum Static Correction

Generally, the complex area has a complicated surface structure and a big undulation, so what you have to consider firstly is how to select the datum for static correction. Up to now, there are two methods the horizontal datum and the smoothed datum close to surface .When the static corrections are quite big, the datum will influence the precision of statics and the character of reflection hyperbola. What the middle reference datum statics uses is a smoothed plane under the high velocity layer so as to overcome the problems caused by the undulated top surface of high velocity layer and the violently changing lateral velocity.

iv) Tomographic Inversion Static Correction

The tomographic inversion static method is to calculate the static correction amount by building a near surface velocity model in the following method: Firstly, establish the initial geological model assuming that the model is composed of velocity units. Each unit is a constant velocity; the velocities among the units are different. Secondly, forward model the first break time with the given initial velocity and recoding geometry; thirdly, use the difference between the first break time and the real travel time to calculate the correction to the velocity model. After correcting the model, calculate the first arrival travel time. In this way, an iterative process has been built up. The final velocity model is obtained when the difference between the forward travel time and the real travel time is minimized sufficiently.

(e) Denoising of coherent and random noise

 In most of cases, the scattered wave caused by near surface appears to be linear interference on seismic data record, so we developed A fast dip-angle median filtering technology to suppress the linear interferences. The procedure is: 1) frequency halving; 2) identifying interference waves; 3) eliminating the interference waves.

At the same time, aiming to the complex area, especially the area undulates greatly, where side interference and along line interference are grown up, we have also developed a denoising technology by combing the indoor and outdoor technologies. The procedure is: in the process of field acquisition, suppress the interferences along the direction perpendicular to the line by adopting small group interval and enlarging the lateral array length; in the process of indoor processing, suppress the interferences along the line with trace and CMP array.

(f) Migration Based on Undulate Surface

The complicated overthrust nappe and velocity model have a big influence on structure imaging. The velocity difference between the upper side and the down side of overthrust nappe fault is quite big, and velocity reversion will occur generally; as a result, the diffraction time-distance curve coming from down side of fault and the reflection time-distance curve within the CMP gather are different from hyperbola. Generally the overthrust will influence the time domain stack imaging and migration imaging of the reflections from down side of fault. Owing to the different velocity variation, some reflections cannot image well, some can image but with the form distorted to some extent or even with pseudo structures or faults. The correct method should be the depth migration starting from the surface---before migrating the surface, smoothen the actual surface; make accurate velocity analysis; in the same time, restraint the velocities in different sections with the well log data to build an exact velocity model; finally, fulfill pre-stack depth migration starting from the CMP reference datum.