6. Fixed Assets

A summary of fixed assets follows:

(Stated in millions)

	2015		2014
Land.........................................................	$	425	$	445
Buildings & improvements..........................................		3,960		3,733
Machinery & equipment............................................		31,885		31,937
Seismic vessels..................................................		850		849
		37,120		36,964
Less: Accumulated depreciation.......................................		23,705		21,568
	$	13,415	$	15,396

The estimated useful lives of Buildings & improvements are primarily 25 to 30 years. The estimated useful lives of Machinery & equipment are primarily 5 to 10 years. Seismic vessels are depreciated over periods ranging from 20 to 30 years.

Depreciation expense, which is recorded on a straight-line basis, was $3.2 billion, $3.2 billion and $3.1 billion in 2015, 2014 and 2013, respectively.