How to Use Check-Shot Wisely

I share my experience in Chevron, correcting hundreds of old check-shots (even some still with dynamite source) as well as new ones, QC Check-shot / VSP in the field, VSP processing.

Why is the Check-shot error?

You probably agree, that almost everyone thinks check-shot data is always right and just use it for depth conversion.

Many people do not realize that there is human error in acquisition and check-shot calculation.

Human error at check-shot acquisition:

• Reading depth on when geophone in borehole stuck for some reasons, depth reading as if deeper than it should be.
• The position of Airgun and hydrophone is inconsistent.

Input Parameter error at check-shot calculation:

• Determination of MSL. Of course it will be different, if we use a different Coordinate System . Bessel? Or ID74? Or WGS84?
• Datum velocity, rock layer velocity between air-gun and datum.
• Water velocity at Gun Pit.

Preferably for QC check-shot, you plot also in Average Velocity vs Two Way Time. If you are QC in Time vs. Depth, it's hard to see the error. Average velocity plot is easier for QC, it can even be used to see velocity anomalies. For example there is an anomaly low velocity due to over-pressure. Please note that Average Velocity = Depth / Time.

Check-Shot Data in WT Block

There are 5 real check-shot data in the block. The structure in the block is quite complex. All wells penetrate the basement. There are in the sub-basin and there are also in the basement high. Of course, it is difficult to determine which check-shot you will use for depth conversion. More importantly, is the check-shot data correct? If we plot in time vs. depth, it is difficult for QC data. Plot average velocity vs time.

Check-Shot Analysis

Assumption of input parameters are correct. The average velocity on the surface is about 6000 -7000 ft / sec. The first data, velocity seems greater than the next data. Is it possible that there is a higher velocity rock? If yes, just use it. If not, just delete the point.

Assumption of input parameters are correct. It is good check-shot

Assumption of input parameters are correct. The well is located in the basement high. The velocity of the rock at the surface is quite high (carbonate). Beneath the carbonate there is a relatively low rock velocity.

Assumption of input parameters are correct. There are 5 data points that make no sense, the average velocity is smaller than 6000 ft / sec. While the water velocity about 4500 ft / sec. I recommend that all five data points be deleted.

Assumption of input parameters are correct. The average velocity on the surface is about 6000 -7000 ft / sec. The first data, velocity seems greater than the next data. Is it possible that there is a higher velocity rock? If yes, just use it. If not, just delete the point

I hope my short information sharing is useful for geoscientist.