Strange black pixels scattered on terrain flattened image

Question

I am attempting terrain flattening of the Sentinel-1 images here. However, I am encountering strange black pixels scattered throughout, as shown in the attached figure. I was calculating annual median values after the terrain correction but encountered this issue.

The standalone code for reproducing the issue is available here:

https://code.earthengine.google.com/bdc77c48882e6e9a6f302cec874add26

var ROI = ee.Geometry.Polygon(
        [[[77.7977109177407, 31.763799704742873],
          [77.7977109177407, 29.140781031782804],
          [81.5550351364907, 29.140781031782804],
          [81.5550351364907, 31.763799704742873]]], null, false);

Map.centerObject(ROI, 10);

var toDB = function (img) {
  return ee.Image(img).log10().multiply(10.0);
};

function terrainCorrection(image) {
  var imgGeom = image.geometry();
  var srtm = ee.Image('USGS/SRTMGL1_003').clip(imgGeom); // 30m srtm 
  var sigma0Pow = image;

  // Article ( numbers relate to chapters)
  // 2.1.1 Radar geometry 
  var theta_i = image.select('angle');
  var phi_i = ee.Terrain.aspect(theta_i)
  .reduceRegion(ee.Reducer.mean(), theta_i.get('system:footprint'), 1000)
  .get('aspect');

  // 2.1.2 Terrain geometry
  var alpha_s = ee.Terrain.slope(srtm).select('slope');
  var phi_s = ee.Terrain.aspect(srtm).select('aspect');

  // 2.1.3 Model geometry
  // reduce to 3 angle
  var phi_r = ee.Image.constant(phi_i).subtract(phi_s);

  // convert all to radians
  var phi_rRad = phi_r.multiply(Math.PI / 180);
  var alpha_sRad = alpha_s.multiply(Math.PI / 180);
  var theta_iRad = theta_i.multiply(Math.PI / 180);
  var ninetyRad = ee.Image.constant(90).multiply(Math.PI / 180);

  // slope steepness in range (eq. 2)
  var alpha_r = (alpha_sRad.tan().multiply(phi_rRad.cos())).atan();

  // slope steepness in azimuth (eq 3)
  var alpha_az = (alpha_sRad.tan().multiply(phi_rRad.sin())).atan();

  // local incidence angle (eq. 4)
  var theta_lia = (alpha_az.cos().multiply((theta_iRad.subtract(alpha_r)).cos())).acos();
  var theta_liaDeg = theta_lia.multiply(180 / Math.PI);
  // 2.2 
  // Gamma_nought_flat
  var gamma0 = sigma0Pow.divide(theta_iRad.cos());
  var gamma0dB = ee.Image.constant(10).multiply(gamma0.log10());
  var ratio_1 = gamma0dB.select('VV').subtract(gamma0dB.select('VH'));

  // Volumetric Model
  var nominator = (ninetyRad.subtract(theta_iRad).add(alpha_r)).tan();
  var denominator = (ninetyRad.subtract(theta_iRad)).tan();
  var volModel = (nominator.divide(denominator)).abs();

  // apply model
  var gamma0_Volume = gamma0.divide(volModel);
  var gamma0_VolumeDB = ee.Image.constant(10).multiply(gamma0_Volume.log10());

  // we add a layover/shadow maskto the original implmentation
  // layover, where slope > radar viewing angle 
  var alpha_rDeg = alpha_r.multiply(180 / Math.PI);
  var layover = alpha_rDeg.lt(theta_i);

  // shadow where LIA > 90
  var shadow = theta_liaDeg.lt(85);

  // calculate the ratio for RGB vis
  var ratio = gamma0_VolumeDB.select('VV').subtract(gamma0_VolumeDB.select('VH'));

  return gamma0_VolumeDB;}


var S1 = ee.ImageCollection('COPERNICUS/S1_GRD_FLOAT')
                     .filterDate('2019-01-01', '2020-01-01')
                     .filterBounds(ROI)
                     .filter(ee.Filter.listContains('transmitterReceiverPolarisation', 'VV'))
                     .filter(ee.Filter.listContains('transmitterReceiverPolarisation', 'VH'))
                     .filter(ee.Filter.eq('instrumentMode', 'IW'));

var S1_original = toDB(S1.select('VH').median());
Map.addLayer(S1_original.select('VH'), {min:-30, max:1},'Original'); 

var S1_terrainCorrected = S1.map(terrainCorrection).select('VH').median();
Map.addLayer(S1_terrainCorrected, {min:-30, max:1},'corrected'); 

Answer 1

They are not strange black pixels. You set min value of S1_terrainCorrected image to -30, but the image has values lower than -30. So those lower values pixels are displayed as black. You need to set min value to a lower value, e.g. -50, in the last line:

Map.addLayer(S1_terrainCorrected, {min:-50, max:1}, 'corrected');