CS Pipe to Soil Natural Potential Value

Are you sure the soil at the pipe maximum depth is equally as dry as the surface sand?

How are you measuring this?

Is it possible that you are actually measuring Cathodic polarisation shift as that is normally around 100mV for a well protected structure? The reaction of cathodic protection currents with the electrolyte in contact with the pipe produces a shift in the negative direction of the pipe to electrolyte potential. A value of 100 mV is a good sign that the pipeline is reacting as a cathode and that corrosion is reduced or removed.

Otherwise:-

Are you using a proper reference electrode?

Are you wetting the ground around the electrode, as good soil contact at this point is essential for the test?

Do you have any passive sacrificial anodes or impressed current ground beds attached and turned on?

Are you conducting an instant off or corrosion coupon test?

You should be aiming for a value around -850mV for the test, but remember that (if using the most common Cu-CuSO4 reference electrode) this figure includes a constant of -520mV for the solution potential between the Cu and CuSO4 in the reference electrode. If you are measuring with some other form of reference cell, ie CEL, AgCl, or none at all, then that number would be different, but in general terms you're looking for a minimum 300mV cathodic voltage shift and 100mV cathodic polarization shift.

If you are unable to understand or implement these requirements then you need the services of a qualified corrosion engineer.