October 2011

OBJECTIVE       

Related to our commitment to feed Geophysics UI Students and UISC AAPG members’ needs, this event has some objectives to reach to, and those are:

      To study the basic concepts of AVO

      To improve the knowledge of students about AVO method in seismic data analysis

      To practice the applied geophysical concept by doing AVO Modeling and Analysis

      To improve ability in using Matlab for AVO analysis.

The latest activity which has conducted by American Assoaciation of Petroleum Geologists Student Chapter of University of Indonesia is a short course entitled Basic Amplitude Variation Offset (AVO) Analysis by Mr. Paulus Tangke Allo, a geophysicist from Montd’or Petroleum, as the guest lecturer. In this course, we are taught about the basic theory of AVO analysis and also the most basic things that  related to AVO analysis.

Seismic interpretation process has some stages that has to be done in order to get the good quality data which can be interpretated. The stages are divided into the basic stage to the advance stage. Data loading & QC, well to seismic tie, horizon and fault picking, and understanding geology are some parts of basic stage. Meanwhile, AVO, seismic inversion, seismic atibutes, and spectra decomposition in Amplitude Analysis part is the advance stage. Based on the lesson that was given by Mr. Paulus, any amplitude analysis should be geologically

advance stage. Data loading & QC, well to seismic tie, horizon and fault picking, and understanding geology are some parts of basic stage. Meanwhile, AVO, seismic inversion, seismic atibutes, and spectra decomposition in Amplitude Analysis part is the advance stage. Based on the lesson that was given by Mr. Paulus, any amplitude analysis should be geologically consistent with the structural or stratigraphy interpretation in order to obtain the best quality result that has been expected. As we could see in this figure below.

There are two kinds of data that we would get from the real reservoir characterization techniques . They are CDP gathers and Zero Offset Sections (migrated section). The difference between these two datas is about being stacked or yet. The CDP gathers is data which has not been stacked and Zero Offset Section is data which has been stacked.

If the data is Zero Offset section , then next step we do could be doing attribute analysis and DHI analysis, doing AI Inversion or using Geostatistical Inversion. In other hand, if the data is CDP gathers, the next steps coud be doing Partial stack or AVO analysis. And the result of both techniques is Geophysical Reservoir Model.

Basically, AVO is the analysis of amplitude behaviour as the function of offset. AVO is not a new thing. We can use this analysis to help us in determining lithology and fluid variation in less compacted and low velocity rocks formation. And as the development of this AVO analysis, now we can use this analysis to determine lithology in harder and less porous rocks where stacked amplitude may be meaningless.

As we told before, AVO analysis have one workflow technique that helps us to reach what we expected. They are Data DC & editing, well to seismic tie, finding Petrophysical  signal, forward modelling and fluid subtitution  ( in case of fluids diversity, thickness, and porosity ), seismic data conditioning (near vs far offset), and extracting AVO attributes.

 

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OBJECTIVE

                                                                                                 

Related to our commitment to feed Geophysics UI Students and UISC AAPG members’ needs, this event has some objectives to reach to, and those are:

      Learn the concept  of seismic acquisition

      Learn the concept of Seismic Field Operation in marine and land

      Practice in determining geometry either on marine or land seismic acquisition

American Association of Petroleum Geologists (AAPG SC UI) as a part of its responsibility in education, held a short course entitled “Seismic Acquisition” presented by Mr Averrouz Mustavan from PSG. This short course aims participants to understand fundamental of seismic acquisition on-shore and off-shore.

This Seismic Acquisition course covered briefly all the process included in exploration seismology. From the geological data, the seismic acquisition itself, data processing, interpretation, exploration drill, and at last the reservoir modeling. As Mr Averrouz field of expertise is the off shore seismic acquisition, the course gave more information on marine seismic acquisition.

The source of the off shore seismic acquisition, at  its early technology are the dynamites, but as times goes on, unwanted environmental effect, forced new technology to generate the source to be introduced, which is the hydrophone. One of the latest technology used in off shore is the Geostreamer. It offers full removal of seismic wavelet, and higher frequency unaliased.

Mr Averrouz also explain about the technique used for the source. The multi azimuth, narrow azimuth and wide azimuth. Recently, the multi azimuth is the most popular technology used worldwide in marine seismic acquisition. There are several reason to this. First, it is the target  illumination which is never been uniform, and caused the structure can not be seen. Second, its ability to sampled in all direction without aliasing, an also to overcome the multiples which behave differently in different azimuth.

Elaborating more about the seismic processing, the speaker who is also an alumni of Bandung Institute of Technology, introduced the participants with the ghost problems. The basic understanding of this problems require comprehensive view of the how the sensor streamer records the wavefield. A sensor streamer records two wavefields recorded :

  1. Upgoing Field
  2. Reflected down-going wavefield = ghost

Geophysicists are wondering what are the effect of  the ghost to the data processing. Field experience and experiments showed that the ghost  adversely affects our seismic recording that it : a. limit the resolution’

               b. notches in frequency spectra

Solving the ghost problems, geophysicists comes out with couples of improvements , by using two sensors, (hydrophone and verticle particle velocity sensor). The  Hydrophone plays it roles to record the pressure wavefield, record ghost with reversed polarity, and its characteristic is insensitive to direction. While the geosensor works on recording the vertical velocity wavefield, record ghost with unchanged polarity, and it is sensitive to direction.

This solution is packed into the instrument called GeoStreamer. Geostreamer has significant benefits. Some of them are : (1) It will be no receiver ghost (which result in easier to interpret and the record will represent earths reflectivity. (2) Frequency notches are eliminated or in other words in broadband data , it improves resolution, improves penetration, estimat Q directly, an the reliable inversion &Q results

  1. Operational Efficiency (single solid streamer, single tow depth, quiet environment and expanded weather window)

The seismic aqusition off shore , needs cooperation from all the team in the ship. Besides, scientific coordination of Geophysicists, geologists, geochemists, reservoir engineers , the harmony interpersonal of the scientist and the ship crew that made all the marine acquisition process run properly.

Concluding the course, Mr Averrous explain about the development of the Marine seismic acquisition lately, with its advanced technology of increasing numbers of streamers used for effectiveness of acquisition and the better imaging of the sub surface of the sea floor used by the PGS.

 

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