{"id":3191,"date":"2022-10-07T15:18:00","date_gmt":"2022-10-07T15:18:00","guid":{"rendered":"https:\/\/www.rockflow.com\/?p=1657"},"modified":"2023-05-02T15:45:18","modified_gmt":"2023-05-02T15:45:18","slug":"depth-conversion-the-geophysical-black-art","status":"publish","type":"post","link":"https:\/\/rockflow.com\/depth-conversion-the-geophysical-black-art\/","title":{"rendered":"Depth Conversion \u2013 the geophysical black art?"},"content":{"rendered":"\r\n

Depth conversion: every seismic interpreter needs to do it, or sidesteps responsibility by using depth migrated seismic directly in depth – not a safe choice as discussed below. Important things depend on depth. Multi-million dollar wells and investment decisions scaled to in-place volumes, but few interpreters claim expertise in depth conversion.<\/p>\r\n\r\n\r\n\r\n

Geophysicists may interpret seismic every week of their working life, but will only depth convert once in a project with months or years separating their efforts. Its no wonder that the majority feel it is a black art that they might never master. Lack of mastery has led to expensive mistakes, and the need for awareness and training is well understood by companies who have lost money or lost opportunity due to poor depth conversions, but can be overlooked by other companies who have avoided that fate\u2026or failed to identify depth conversion as the cause.<\/p>\r\n\r\n\r\n\r\n

Depth conversion can founder on age-old problems, such as \u2018I don\u2019t have enough time\u2019 (projects are always measured in weeks, not days), or \u2018I always do it this way\u2019: a velocity model suitable for the Niger Delta is unlikely to work in North Africa. But even for the diligent worker there are other ways of going wrong, more subtle traps, that he or she can fall into\u2026<\/p>\r\n

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How to go wrong 1: not thinking geologically<\/em><\/h2>\r\n\r\n\r\n\r\n

Depth conversion (and depth migration) takes seismic reflections measured in time and transforms them to measurements in depth using velocity. Velocity depends on the type of sediment deposited, how it has changed over time, and the fluid filling its pores i.e. velocity depends on geology. If your velocity model doesn\u2019t stick to geological rules, you\u2019ll come unstuck.<\/p>\r\n\r\n\r\n\r\n

Let\u2019s take a look at the well-known \u2018Vok\u2019 method \u2013 the one which says that velocity increases at a constant rate proportional to the depth of burial. This is a widely used tool built into some popular software packages, but rarely is help given on how to wield it. Most importantly, velocity is assigned based on its present-day depth, implicitly assumed to be the rock\u2019s maximum depth of burial. This is fine for a subsiding basin, but what if that basin has undergone broad tectonic uplift (e.g. Barents Sea in Norway), or if a salt or shale diapir has caused localised uplift? In many cases the rock does not \u2018decompact\u2019 to its younger state, but largely holds on to the highest velocity it reached at maximum burial. In a fully inverted basin, the shallowest rocks were once the deepest and fastest, so velocity appears to increase with decreasing<\/em> depth, the opposite behaviour to that described by the Vok model.<\/p>\r\n\r\n\r\n\r\n

In one example, from the North Sea Sole Pit Basin, the reservoir at 3km (10,000ft) had been inverted by an estimated 600m (2,000ft), and a depth conversion with a Vok model underestimated velocity so badly that the resulting reservoir depth map was 440m shallow \u2013 that\u2019s over \u00bc mile high!<\/p>\r\n\r\n\r\n\r\n

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