<p>This phenomenom is called <strong><a href="http://www.glossary.oilfield.slb.com/en/Terms/p/pull_up.aspx" rel="noreferrer">velocity pull-up</a>.</strong> The seismic waves that reflected off the top of Layer 2 beneath the carbonate layer have also travelled through the carbonate. Because carbonate (at least in this case) has a faster P-velocity than the surrounding shale, waves that have travelled through the carbonate will return to the surface receivers in a shorter amount of time, relative to waves that only travelled through the shale. Thus the carbonate/Layer 2 interface appears at an earlier two-way-time relative to the Layer 3/Layer 2 shale interface. </p>

CC BY-SA 3.0
2016-04-21T01:59:19.287Z
Cheers for adding link, kwinkunks.
stevej
CC BY-SA 3.0
2016-04-20T02:29:17.387Z
2016-04-20T16:44:45.163Z
2016-04-20T16:44:45.163Z
stevej
<p>Carbonate reefs usually appear as homogeneous bodies surrounded by sediment layers in seismic reflection data, but in the figure below the high amplitude reflection from the top of sediment (layer 2  at 0.3-0.34 seconds) is not horizontal yet the geological interface is horizontal.  </p>

<p>Can anyone explain why? I appreciate any input, thanks.<br>
<a href="https://i.sstatic.net/UJpCi.png" rel="noreferrer"><img src="https://i.sstatic.net/UJpCi.png" alt="Seismic profiles"></a></p>

<geophysics><geology><seismology><carbonate>
Carbonate reef not horizontal in seismic section?
Don Sangione
# Carbonate reef not horizontal in seismic section?

(Asked by: username_0 on 2016-04-20)

Carbonate reefs usually appear as homogeneous bodies surrounded by sediment layers in seismic reflection data, but in the figure below the high amplitude reflection from the top of sediment (layer 2 at 0.3-0.34 seconds) is not horizontal yet the geological interface is horizontal.

Can anyone explain why? I appreciate any input, thanks.  
[![Seismic profiles](https://i.sstatic.net/UJpCi.png)](https://i.sstatic.net/UJpCi.png)

**Tags:** geophysics, geology, seismology, carbonate


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## 1 answers

### Answer by username_1 on 2016-04-20. Score: 12

This phenomenom is called **[velocity pull-up](http://www.glossary.oilfield.slb.com/en/Terms/p/pull_up.aspx).** The seismic waves that reflected off the top of Layer 2 beneath the carbonate layer have also travelled through the carbonate. Because carbonate (at least in this case) has a faster P-velocity than the surrounding shale, waves that have travelled through the carbonate will return to the surface receivers in a shorter amount of time, relative to waves that only travelled through the shale. Thus the carbonate/Layer 2 interface appears at an earlier two-way-time relative to the Layer 3/Layer 2 shale interface.

> **Comments:**
> **username_1**: Cheers for adding link, kwinkunks.  

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