To what extent can we see signals that fall between frequency bins? I have a simple problem I would like to figure out for fourier analysis of seismic data. 

Let us say that we have a signal $z[n]$, of length $N$. If I take its (same size) FFT, I will get $Z[k]$, also of length $N$. 

I know that zero-padding will not improve my resolution, however, what I am trying to figure out is: If I take an $N$ length FFT, is there ANY risk that I will be missing some information, that would have otherwise shown up had I taken a greater than $N$ length FFT instead? 

The case I worry about is if my seismic signal has energy at a frequency that lies between frequency bins. How well would it show up - or not show up - if I do not zero-pad? I would not want to miss this signal should it in fact exist. Cheers.
zero-padding will not "improve resolution" in the sense that it would help you separate two peaks in the spectrum that are very close (too close) to each other.

but zero-padding, as well as interpolation in the frequency domain, does improve resolution in the sense that it help you to more precisely estimate the location of an isolated peak in the spectrum.  in fact, zero-padding in the time domain is equivalent to interpolating in the frequency domain using the sinc() function (actually the dirichlet function) as the interpolation kernel.