Bit depth is precision

Bit depth is the number of digital bits used to represent amplitude in each seismic sample. Usually 32 in field or newly-processed seismic data, or 8 or 16 in post-processed seismic data.

When seismic data is recorded, four bytes are used to store the amplitude values. A byte contains 8 bits, so four of them means 32 bits for every seismic sample, or a bit-depth of 32. The magnitude of the amplitude values themselves doesn’t mean much on its own. But we want to use 32 bits to record them, at least at the field recording stage, because when a day might cost hundreds of thousands of dollars, we want to capture every nuance of the seismic wavefield, including noise, multiples, reverberations, and hopefully even some signal! We have time during processing to sort it all out.

Black and white photographs
First, it’s important to understand that I am not talking about spatial or vertical resolution, what we might think of as detail. That’s a separate problem which we can understand by considering a pixelated image. Edges are blocky and stair-stepped, small objects disappear. It has poor resolution: it is spatially under-sampled.

But now consider an image with plenty of detail but no precision. Edges are smooth not blocky, and even small objects are reproduced. But, if the bit-depth is very shallow, there may be only four grey values: black, dark grey, light grey, and white. The deficiency this time is in the subtlety of the pixel values: an imprecise image is not smooth in colour space. An image with only four values has a bit depth of two. Each of the two bits that represent each pixel can have a value of 0 or 1, giving 4 combinations altogether: 00, 01, 10, and 11.

Clearly, using fewer bits means that your files will be smaller: reducing the bit depth from 32 to 8 means only one byte is used where there were four, so files will be reduced in size by 75%. The smaller file will load faster, take up less space in memory, and make your disk space go further.

Application to seismic data
All of this applies to your seismic data. Data is almost always delivered from the processor as 32-bit. In the past, people were more concerned about disk space than today, so often reduced the bit depth in their volumes to 8-bit. This means the amplitude values can take 256 discrete sample values. If you believe this is enough precision, and many people think that for interpretation it is, then you can safely use 8-bit volumes for your work. Indeed, some tools, such as Landmark's GeoProbe volume interpretation tool, can only load 8-bit data.

But for seismic analysis, especially amplitude mapping, spectral decomposition, or pre-stack work, 8-bit data is probably not precise enough. Opinions vary, but I'll tell you what I do: I keep my 32-bit volume on disk, but make all derivative volumes and attribute volumes 16-bit. I think 65 536 values is enough, and because of noise and other uncertainties in the data, any precision beyond that is spurious.

On the other hand, if you've made a seismic volume of, say, waveform class (with a smaller number of discrete values), or even porosity (which can only take a small number of realistic values between 0 and about 35), then there is little sense in using more than 8 bits. Your data manager will thank you and your volumes will load faster!

Author biography
Matt Hall, Agile*, Canada   http://www.agilegeoscience.com/

''Matt is a geoscience consultant and new media evangelist in Nova Scotia, Canada. After fifteen years at high-performing oil and gas operators and a software company, Matt formed Agile* in 2010 and has been creating fun things to do ever since. Favourite subjects feature scale, uncertainty, and signal. Guiding philosophies include disruption, iteration, and pragmatism. You can find him masquerading as @kwinkunks on Twitter.''