## 6 Discussion
In terms of velocity values, the conventional tomogram shows that the local anomalies and the first layer are merged in one layer with irregular bottom and a velocity of 700 m/s to 750 m/s, which is a value between the true velocity of the local anomalies (400 m/s) and the true velocity of the first layer (800 m/s). The second layer at the conventional tomogram has a velocity ranging between 1400 and 1550 m/s, and the third layer shows a velocity of 2500 m/s or greater. The land-streamer tomogram successfully differentiated between the local anomalies and the first layer. The seismic velocity of the local anomalies at the land-streamer tomogram is ranging between 430 and 450 m/s, while the seismic velocity of the first layer is ranging between 850 and 950 m/s. The seismic velocity of the second layer is ranging between 1300 and 1500 m/s. In summary, the conventional tomogram successfully showed the third layer and the correct velocity values of the second and third layers but failed to differentiate between the first layer and the local anomalies, while the land-streamer tomogram successfully showed the local anomalies, first layer, and second layer with correct thickness and velocity values, but was not able to detect the third layer due to its limited depth of penetration. The differences between the conventional and land-streamer results can be explained as follows; the land-streamer raypaths are shorter than those of the conventional surveys, hence, the inversion algorithm will not be affected by the high-velocity values at a deeper depth. However, in the conventional setup case, the long raypaths will be smeared along many velocity cells, some of them are at shallow depths while the others are at deep depths, hence, the effect of the high-velocity values at the deep depths will contaminate the low-velocity values at the shallow depth and vice versa. This phenomenon will decrease the accuracy of the tomogram, especially at shallow depths. One final comparison is made between the land-streamer 1-m receiver-interval data (Figure 10a) and the conventional 1-m receiver-interval data (Figure 10b) after limiting the \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Anomaly**} & \multicolumn{4}{c}{**Center Position**} & \multicolumn{4}{c}{**Width**} & \multicolumn{4}{c}{**Thickness**} \\ \cline{2-10}  & **T \({}^{*}\) (m)** & **C \({}^{*}\) (m)** & **e \({}^{*}\) (\%)** & **T \({}^{*}\) (m)** & **C \({}^{*}\) (m)** & **e \({}^{*}\) (\%)** & **T \({}^{*}\) (m)** & **C \({}^{*}\) (m)** & **e \({}^{*}\) (\%)** \\ \hline A & 50 & - & - & 10 & - & - & 5 & - & - \\ B & 150 & 150 & 0 & 20 & 49 & 145 & 10 & 15 & 50 \\ C & 250 & 250 & 0 & 30 & 60 & 100 & 15 & 21 & 40 \\ D & 350 & 350 & 0 & 40 & 71 & 77 & 20 & 27 & 35 \\ \hline \hline \end{tabular}  
* T \({}^{*}\) is the true model, C \({}^{*}\) is the result from conventional tomogram, and e \({}^{*}\) is the error between T and C.  
\end{table} 
Table 4: Comparing the dimension and position of the four local anomalies shown on the conventional tomogram (Figure 8a) to those shown on the true model (Figure 7). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Anomaly**} & \multicolumn{4}{c}{**Center Position**} & \multicolumn{4}{c}{**Width**} & \multicolumn{4}{c}{**Thickness**} \\ \cline{2-10}  & **T \({}^{*}\) (m)** & **L \({}^{*}\) (m)** & **e \({}^{*}\) (\%)** & **T \({}^{*}\) (m)** & **L \({}^{*}\) (m)** & **e \({}^{*}\) (\%)** & **T \({}^{*}\) (m)** & **L \({}^{*}\) (m)** & **e \({}^{*}\) (\%)** \\ \hline A & 50 & 50 &maximum source-receiver offset of the conventional data to 100 m at both sides of the shot point. The two tomograms show that the four local anomalies can be detected with similar shapes and sizes. However, the conventional tomogram shows better imaging of layer 1, especially between offsets 360 m and 450 m where the land-streamer tomogram shows a wrong low-velocity anomaly, and the conventional tomogram did not show it.