Providing In-depth Knowledge

Making depth conversions more reliable has become an obsession for a dedicated geophysicist. As a result the Norwegian consultancy Aker Geo now claims to have the very best tool to convert from time domain to depth domain.
This article appeared in Vol. 6, No. 3 - 2009


This Illustration from the Norwegian Sea offshore Norway shows a regional velocity cube intersecting a Base Cretaceous time map. The horizontal velocity slice is at 6,000ms. Two vertical velocity cross sections, running north-south and east-west, are superimposed. The velocity cube incorporates seismic processing velocities and check shots from wells. The well data are shown as vertical lines. Depth conversion is done by multiplication of the time map with the velocity cube where they intersect. The colours denote increasing values (velocity and time) from red to blue. Illustration: Aker Geo "We all know that reliable depth conversions are crucial when doing detailed prospect evaluations. But making reliable depth maps from time maps is also critical when carrying out regional geological studies, as we have seen many times offshore Norway," says Ivar Meisingset with Aker Geo in Oslo, Norway. Since the early 1980's he has concentrated his efforts on improving depth conversions, until it has now become a passion.  

As a result, in 2007, Aker Geo's hiQbeTM depth conversion cube was launched. It turns out that dedication pays off.  

"In all probability we have superior software for depth conversion on the Norwegian continental shelf. This is why we get praise for the work we do," says Erik Sverre Jenssen, managing director of Aker Geo, who has a long and varied background with both Saga Petroleum and Norsk Hydro before he joined Aker Geo last year.  

His high opinion of the velocity cube gets support from Øyvind Husby in Det norske, one of the more than 50 oil companies that are now actively vacuum-cleaning the Norwegian shelf in order to find the last drops, and who was their first customer of this product .  

"We use the Aker depth conversion cube in all our regional projects, and we always compare local high density velocities derived from 3D seismic with the Aker model for QC and sensitivity analysis," Øyvind says.  

Dependant on input and software

Velocity slice at 2,500 ms in the Barents Sea, offshore Norway. This velocity cube includes data from TGS. The main structural elements, like the Hammerfest Basin and the Loppa High, can clearly be seen on this map, as the velocities will be higher at a given time level on a high than in a basin. On a regional scale, velocity fields will always be dominated by the main geological features. Illustration: Aker Geo "The quality of a regional velocity cube largely depends on two factors. Firstly, it is necessary to have high quality input data, as is always the case when interpreting geophysical data, and secondly, the final result will obviously depend on the software used," says Meisingset.  

The input to the depth conversion consists of processing velocities from seismic data and check shots from wells. On the Norwegian continental shelf, including the North Sea, the Norwegian Sea and the Barents Sea, a huge data base is available to all at a nominal cost. It is principally this database that has been used to make the velocity cubes. "This public domain data base has opened the door for the regional velocity cube method, which was previously reserved exclusively for larger oil companies with their own, proprietary data bases. Today, with plenty of released data, the situation is totally different, offering a window of opportunity for Aker Geo," says Meisingset.  

Aker Geo has developed its own software for velocity cube processing. "In addition to the software, we consider our procedures and the skills of our operators as extremely critical when doing depth conversions," says Meisingset. When it comes to quality, he is not willing to compromise, always hunting for the best solution.

Opening the "black box"

The regional velocity cube method has been used by the industry for a long time. The idea is that all velocities available in a given area, it being seismic or well data velocities, are put into a uniform velocity model, then allowing for conversions directly from time to depth.  

Meisingset himself learned the basics when working with Esso more than 25 years ago. Later on, he developed the method further with Norsk Hydro, and continued doing so in Aker Geo, where he has worked since 2004.  

"We start by collecting all the seismic processing velocities into a "stacking velocity" cube. Then we process another cube where the velocities are being calibrated with check shots. It is this calibrated average velocity cube that can be used directly for depth conversions."  

"Depth maps created by converting the time maps with this cube will consequently be correct at all well positions, provided that the seismic reflector interpreted is in agreement with the well tops," Meisingset claims.  

This particular method has been used by the industry for a long time. The idea is that all velocities available in a given area, whether derived from seismic or well data, are put into a uniform velocity model, thereby allowing for conversion directly from time to depth.  

He describes the Aker Geo velocity cube processing method as consisting of 4 main stages. "First, we balance all of the seismic processing velocities against each other; secondly, we get rid of the anomalies and make sure the field is smooth by filtering; thirdly, we correct for anisotropy (the difference between stacking and well velocities); and finally, we make sure we have a good tie to the well data, as they represent our control points."  

"While we use our own software throughout this process, we find it very important for the operator to be in full control all the way. One of the most labour intensive tasks is therefore to QC and correct the input data. We do not normally edit the data before processing because our software is set up not to expect this, but some of the data sets we work with have formatting, sorting or positioning errors, and it pays in terms of quality to find and correct these."  

"In this way, we are second to none when it comes to converting from time to depth," Meisingset says.  

Multiclient business

Aker Geo have developed two business models for this product, purchase and lease.  

Those who wish to purchase the cubes will get them in a standard ASCII format that can be read in most major software packages, meaning that the user can apply his own favourite software for the depth conversion, it being PETREL, Kingdom, RMS, Geocap or any other. "In this way, the customer will save significant time and money compared with making similar quality cubes themselves. Using our velocity cubes also means that more effort can be put into seismic interpretation, producing higher quality time maps," says Meisingset.  

The lease model is a time limited depth conversion service contract, where the customer gets prioritized access to depth conversion services with the cubes at Aker Geo. "Turn-around time is normally less than 24 hours, regardless of the number of time horizons. The customers are happy with this, especially as time gets short towards the end of a licence round study," Meisingset says.  

Aker Geo can also incorporate the customer's own data into the cubes, if it is available, or create velocity cubes with the customer's data for other regions worldwide. The company's own proprietary software gives it the capability to do this in a very time efficient manner and at what is considered to be a competitive price.  

Cooperation with TGS

The depth conversion team of Aker Geo consists of Ivar Meisingset (in front), Jørnar Heggsum Hubred (left) and Abreham Yacob Abreham. Photo: Halfdan Carstens "We strongly believe we have the best software for processing velocities in the industry. Our software represents a paradigm shift for the velocity cube method. It makes it possible for us to deliver a combination of price and quality that our customers find very attractive," Meisingset says without hesitation.  

This view is shared by Jørn Christiansen of TGS. "We have recently entered into a co-operation agreement with Aker Geo in which we are adding our extensive proprietary seismic data base to their public domain data, including all seismic processing velocities from our long offset 2D surveys in Norway and the UK, and also well data from Aceca.  

"Velocity cubes based on this cooperation will be made available through the existing Aker Geo business models, but they will also be used by TGS for depth conversion of seismic data, meaning that we can deliver SEG-Y format seismic data in depth as well as in time," Christiansen says.  

"We have chosen Aker Geo as cooperation partner for this because their software produces the quality of depth conversion that we need for our multi-client seismic data."


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