Here is the reference: U.
Persson, On Degenerations of Algebraic SurfacesUlf, MemoirsOn degenerations of the American Mathematical Societyalgebraic surfaces, 1977 Mem. Amer. Math. Soc. 11 (1977), no. 189.
Clemens, C. H. Clemens DegenerationDegeneration of Kähler manifolds, Duke. Duke Math. J. Volume 44, Number 2 (1977), no. 2, 215-290.
I found the reference here http://web.math.ucsb.edu/~drm/papers/clemens-schmid.pdf also worth reading (David
Morrison, David R. Morrison Chapter VI THE CLEMENSThe Clemens-SCHMID EXACT SEQUENCE AND APPLICATIONSSchmid exact sequence and applications, Topics in Transcendental Algebraic Geometrytranscendental algebraic geometry (Princeton, N.J., 1981/1982), 101-119, Ann. of Math. Stud., 106, Princeton Univ. Press, Princeton, NJ, 1984.