Why did I get very low negative SNR value of White Gaussian noise I added into a image -

Why did I get very low negative SNR value of White Gaussian noise I added into a image -

i'm using next code add together white gaussian noise 3d synthetic image created. (100*100*100)

sigma = sqrt(10.0^(-snr/10.0)); r=x+sigma*randn(size(x));

i found different ways of adding noise need different range of snr value. eg. if add together noise stack stack (x=image stack), original image become invisible until snr goes downwards -50. when seek add together noise straight away in 3d, (x=3d image). original image become invisible until snr goes downwards -5. i've looked everywhere couldn't found solution of causing this...could please tell me if it's normal have 3d noisy image snr=-50db or -70db? or there way know true snr of noisy images?

image image-processing noise