  Growing digital archives and improving algorithms for automatic analysis of
text and speech create new research opportunities for fundamental research in
phonetics. Such empirical approaches allow statistical evaluation of a much
larger set of hypothesis about phonetic variation and its conditioning factors
(among them geographical / dialectal variants). This paper illustrates this
vision and proposes to challenge automatic methods for the analysis of a not
easily observable phenomenon: vowel length contrast. We focus on Wolof, an
under-resourced language from Sub-Saharan Africa. In particular, we propose
multiple features to make a fine evaluation of the degree of length contrast
under different factors such as: read vs semi spontaneous speech ; standard vs
dialectal Wolof. Our measures made fully automatically on more than 20k vowel
tokens show that our proposed features can highlight different degrees of
contrast for each vowel considered. We notably show that contrast is weaker in
semi-spontaneous speech and in a non standard semi-spontaneous dialect.
