<p>You are looking at low order polynomial approximations of density. Such approximations are only valid over a certain range, typically the range over which the underlying data were collected. Applying the polynomial to a value well outside that range, which is what you are doing, is quite simply invalid. &quot;Garbage in, garbage out.&quot;</p>
<p><strong>Do not extrapolate.</strong></p>

CC BY-SA 4.0
2023-10-31T08:22:20.303Z
2023-10-31T08:22:20.303Z
David Hammen
<p>This is the example density depth functions for various sedimentary sections world wide. But someof the sections marked with red, due to their last term, as i increase the value of z(depth) they become negative. But density can never be negative? What could be other possible reason for <a href="https://i.sstatic.net/KIOwa.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KIOwa.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2023-10-30T06:13:04.380Z
What's the source of the table? It could just be a unit problem. For example, if you input the depth in kilometers you'll get more reasonable, positive values until a greater depth. The density seems to be in g/cm3 instead of kg/m3, so you never know... The unit of $z$ is not mentioned in the caption but could be in the main text.
Jean-Marie Prival
CC BY-SA 4.0
2023-10-30T15:30:05.630Z
Those a local, second order fits to data. They cannot be valid globally (i.e. for all z). Search in the text what the limitations on the validity of the fit in z is.
AtmosphericPrisonEscape
CC BY-SA 4.0
2023-10-31T06:54:23.823Z
Mark, you need to get in the habit of giving additional explanation of where these tables you keep asking about are from.  This has been a consistent problem with your questions.  Context is important to asking worthwhile questions.  Otherwise it's only answering the question for you, and without key information often.  -1
JeopardyTempest
<geology><geophysics>
What is the reason behind the density being negative value
mark22
# What is the reason behind the density being negative value

(Asked by: username_0 on 2023-10-30)

This is the example density depth functions for various sedimentary sections world wide. But someof the sections marked with red, due to their last term, as i increase the value of z(depth) they become negative. But density can never be negative? What could be other possible reason for [![enter image description here](https://i.sstatic.net/KIOwa.png)](https://i.sstatic.net/KIOwa.png)

**Tags:** geology, geophysics

> **Comments:**
> **username_1**: What's the source of the table? It could just be a unit problem. For example, if you input the depth in kilometers you'll get more reasonable, positive values until a greater depth. The density seems to be in g/cm3 instead of kg/m3, so you never know... The unit of $z$ is not mentioned in the caption but could be in the main text.  
> **username_2**: Those a local, second order fits to data. They cannot be valid globally (i.e. for all z). Search in the text what the limitations on the validity of the fit in z is.  
> **username_3**: Mark, you need to get in the habit of giving additional explanation of where these tables you keep asking about are from.  This has been a consistent problem with your questions.  Context is important to asking worthwhile questions.  Otherwise it's only answering the question for you, and without key information often.  -1  

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## 1 answers

### Answer by username_4 on 2023-10-31. Score: 3

You are looking at low order polynomial approximations of density. Such approximations are only valid over a certain range, typically the range over which the underlying data were collected. Applying the polynomial to a value well outside that range, which is what you are doing, is quite simply invalid. "Garbage in, garbage out."

**Do not extrapolate.**


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