## 2 Methods
### GPR Data Acquisition  
The GPR survey was conducted at the Batu Ferringhi site in November 2014. Coastal plains separated by buildings surround the geomorphology of the site. The application of ground penetrating radar (GPR) is common within the geological, environmental, glaciological, engineering and construction and forensic science fields. It is a method that uses radar signals to visualize the subsurface on a monitor. It started with the early use of electromagnetic signals and continuous-wave transmission to locate remote buried items [31]. As the technology developed, the electric properties of polar ice sheets were observed by radio echo-sounding, which was a pulsed technique [32; 33]. It is then improved by incorporating the transmission and reception of pulses by antennas with paired frequencies; the first antenna with a frequency of 500 MHz and below, and the second, 500 MHz and above. The higher the frequency transmitted into the ground, the shorter the wavelength which reduces the depth of the investigation survey but the higher the resolution. It is suitable for archaeological surveys and concrete inspection [31]. However, for engineering and geological studies, a low-frequency antenna is recommended to produce a more profound penetration but lower resolution [13; 31]. In the present study, the GPR profiles were obtained using a 250-MHz shielded antenna. The maximum penetration depth is 2 m. Four survey lines were arranged perpendicular to each other like a grid. The length of the survey lines for Line 1, Line 2, Line 3, and Line 4 were 75, 76, 25.5 and 23.5 m, sequentially. Line 1 is towards the northeast, while Line 2 is towards the southwest. Line 3 is headed southeast, but Line 4 is headed northwest. The operation setting parameter of the GPR, i.e., the point interval, is 0.05 m for Line 1 and Line 3. As for the point intervals for Line 2 and Line 4, they are 0.01 m The positions of these four survey lines, including the starting and ending point for each line, are shown in Figure 2. The acquired data was then transferred from the MALA RAMAC GPR control unit to the computer using the MALA Ground VisionTM software. The data were filtered several steps later for noise and unwanted signal removal [13].  
As for the Persiaran Bayan Indah site, the GPR survey using a 250-MHz shielded antenna was performed in January 2015. The GPR penetration at this site was approximately 5 m. The four lines labeled line 5 to Line 8 stretched perpendicular to each other. Line 5 is towards the southwest, while Line 6 is towards the northeast. Line 7 is headed southeast, and Line 8 is headed northwest. The position of these four survey lines is shown in Figure 3. The point interval for Line 5 and Line 7 is 0.05 m, but it is 0.01 m for Line 6 and Line 8.  
Figure 2: Survey lines of GPR at site 1, Batu Ferringhi, representing natural coastal landform, plotted on Google Earth (2015), Batu Ferringhi, Penang, Malaysia. The coordinates for the initial point of survey lines (\(\bullet\)) is 05\({}^{\circ}\)28\({}^{\prime}\)17.24\({}^{\prime\prime}\) N and 100\({}^{\circ}\)14\({}^{\prime}\)39.00\({}^{\circ}\) E. DigitalGlobe 2015. [[http://earth.google.com](http://earth.google.com)]([http://earth.google.com](http://earth.google.com)) (accessed on 3 February 2015).